Final project report
A performance assessment of
domestic fridge thermometers
A performance assessment of a representative range of domestic fridge
thermometers was undertaken in a series of scientific experiments to
ascertain the accuracy, resolution and responsiveness of the
thermometers to a change in temperature. This information will be of
use to those communicating to consumers about food waste and food
safety, and those providing tools for sale or use by consumers.
Project code: RBC821-001
Research date: April-July 2009
Date: October 2009
WRAP helps individuals, businesses and
local authorities to reduce waste and
recycle more, making better use of
resources and helping to tackle climate
change.
Document reference: [RBC821-001. Report prepared by Sophie Easteal, Banbury, WRAP]
Written by: R.M.George, R.D.Thorn and G.I.Hooper (Campden BRI)
Front cover photography: Selection of thermometers used in this research.
WRAP and Campden BRI believe the content of this report to be correct as at the date of writing. However, factors such as prices, levels of recycled content and
regulatory requirements are subject to change and users of the report should check with their suppliers to confirm the current situation. In addition, care should be taken
in using any of the cost information provided as it is based upon numerous project-specific assumptions (such as scale, location, tender context, etc.).
The report does not claim to be exhaustive, nor does it claim to cover all relevant products and specifications available on the market. While steps have been taken to
ensure accuracy, WRAP cannot accept responsibility or be held liable to any person for any loss or damage arising out of or in connection with this information being
inaccurate, incomplete or misleading. It is the responsibility of the potential user of a material or product to consult with the supplier or manufacturer and ascertain
whether a particular product will satisfy their specific requirements. The listing or featuring of a particular product or company does not constitute an endorsement by
WRAP and WRAP cannot guarantee the performance of individual products or materials. This material is copyrighted. It may be reproduced free of charge subject to the
material being accurate and not used in a misleading context. The source of the material must be identified and the copyright status acknowledged. This material must
not be used to endorse or used to suggest WRAP’s endorsement of a commercial product or service. For more detail, please refer to WRAP’s Terms & Conditions on its
web site: www.wrap.org.uk
Executive summary
The objective of this study was to survey the range of fridge thermometers
available to UK domestic consumers and assess the performance of a
representative sample in a series of scientific experiments to ascertain their
accuracy, resolution and responsiveness to a change in temperature.
A wide range of domestic fridge thermometers are available to the UK consumer and this study has evaluated the
performance of a representative sample. The majority are either liquid-in-glass, electronic, liquid crystal or
bimetallic thermometers, although specialist devices such as infrared thermometers are also available.
The accuracy of the fridge thermometers tested was generally good, with the majority of devices measuring
fridge air temperatures to within ±0.5°C of the actual fridge temperature (as indicated by a calibrated, rapid
response temperature measurement device). Response times of the fridge thermometers were also determined
and most of the devices assessed indicated the appropriate temperature within 30 minutes, satisfying typical food
safety guidance for chilled foods. It was noted in these tests that some of the fridge thermometers assessed were
not designed to directly measure fridge air temperature. One type had a standard liquid-in-glass thermometer
embedded into a clear liquid; this was designed to slow down the speed of response of the thermometer and be
more representative of the temperature change actually experienced by the food products within the fridge.
Another type used an infrared sensor to directly measure the food product/packaging temperature.
Tests showed that there were no appreciable differences between the majority of fridge thermometers tested
their responsiveness with either increasing temperatures (warming) or decreasing temperatures (cooling). In
both heating and cooling tests, the majority of thermometers took around 20 minutes to reach the temperature
indicated by the calibrated thin-wire reference temperature.
The readability and usability of the thermometers varied considerably. The electronic thermometers could be read
most easily and accurately as the displays showed the temperature clearly and in most cases they could be read
without opening the fridge door (as a sensor is placed in the fridge and the electronic display outside the fridge).
The liquid in glass thermometers were generally considered to be the most difficult to read as they often had to
be looked at closely to determine the reading. The bimetallic strip thermometers generally displayed the reading
clearly with a good scale.
All of the thermometers had scales that spanned a much wider range than is necessary for domestic refrigerator
use, this made the proportion of the scale that was of interest to the consumer (typically 0-10°C) small as a
proportion of the entire range. This consequently made the thermometers more difficult to read and less precise
than they could be.
Repeatability between batches of a single fridge thermometer type was also assessed and it was shown that
results were generally consistent. The 'Coldzone' temperature indicator was used for this test. This is a liquid
crystal device used by the Food Standards Agency (FSA) as a means of promoting food safety in domestic
refrigerators. These assessments showed that there was no appreciable variation between the 10 replicate
devices of the indicator tested. A major advantage of this type of device is that the indication is a clear "yes/no"
whether the fridge is at the appropriate food storage temperature rather than providing a precise temperature
reading as the others did.
The instructions provided to consumers on the packaging of fridge thermometers was often quite variable. Many
fridge thermometers were supplied with no instructions, some had an indication of recommended fridge
temperatures and some provided advice on the temperature ranges recommended for different categories of
food. It is suggested that manufacturers of these devices should provide more detailed information to the
consumer on recommended fridge temperatures; this would help in both improving food safety and reducing food
waste that results from poor temperature control in domestic refrigerators.
The information obtained in this study will be of use to those communicating to consumers about food waste or
food safety, and those providing tools for sale or use by consumer.
Table 1 summarises the results of the testing programme undertaken for this study.
A performance assessment of domestic fridge thermometers
1
Table 1 Summary of the results of the testing programme.
Therm.
number
1
Image
Operating
principle
(manufacturer)
Liquid in glass
(Brannan)
Therm.
price
Accuracy (based on
testing programme)
Scale
Readability
£2.50
Measured temperature
to within ±0.5°C of
the reference
temperature
-40 to +30°C
1°C divisions
70 divisions in
total
1°C error possible if scale
is not viewed straight
(parallax error)
-40 to +30°C
1°C divisions
70 divisions in
total
-40 to +25°C
1°C divisions
65 divisions in
total
-30 to +30°C
1°C divisions
60 divisions in
total
-40 to +40°C
1°C divisions
80 divisions in
total
1°C error possible if scale
is not viewed straight
(parallax error)
2
Liquid in glass
(Brannan)
£2.50
Measured temperature to
within ±1°C of the
reference temperature
3
Liquid in glass
(ETI)
£2.75
Measured temperature
to within ±0.5°C of
the reference
temperature
4
Liquid in glass
(Unbranded)
£3.00
Measured temperature to
within ±1°C of the
reference temperature
5
Liquid in glass
(Endotherm)
£7.23
Measured temperature
to within ±0.5°C of
the reference
temperature. Large
1°C error possible if scale
is not viewed straight
(parallax error)
0.5°C error possible if
scale is not viewed
straight (parallax error)
2°C error possible if scale
is not viewed straight
(parallax error)
Comments
On-pack guidance
suggests fridge
should be set to 35°C
No instructions were
supplied
Large potential error
in reading
thermal lag,
took appreciably longer
than the others to reflect
changes in temperature
(heating and cooling)
A performance assessment of domestic fridge thermometers
2
Therm.
number
6
7
Image
Operating
principle
(manufacturer)
Liquid in glass
(Food Safety
Direct)
Therm.
price
Accuracy (based on
testing programme)
Scale
Readability
Comments
£2.49
Measured temperature
to within ±0.5°C of
the reference
temperature
-30 to +50°C
10°C divisions
8 division in
total
2-3°C error (parallax error
and estimation of scale)
Large potential error
in reading
Liquid in glass
(Foodsafe)
£10.00
Measured temperature
to within ±0.5°C of
the reference
temperature. Large
-40 to +40°C
1°C divisions
80 divisions in
total
2°C error possible if scale
is not viewed straight
(parallax error)
Large potential error
in reading
thermal lag, took
appreciably longer than
the others to reflect
changes in temperature
(heating and cooling)
8
Liquid in glass
(Chef Aid)
£1.95
Measured temperature
to within ±0.5°C of
the reference
temperature
-30 to +30°C
1°C divisions
60 divisions in
total
1°C error possible if scale
is not viewed straight
(parallax error)
9
Bimetallic
£3.50
Measured temperature to
within ±1°C of the
reference temperature
-30 to +30°C
1°C divisions
60 divisions in
total
0.5°C error possible if
scale is not viewed
straight (parallax error)
No instructions were
supplied
10
Bimetallic (Food
safety Direct)
£2.49
Measured temperature to
within ±1°C of the
reference temperature
-30 to +30°C
10°C divisions
6 divisions in
total
3-4°C error (parallax error
and estimation of scale)
Large potential error
in reading
A performance assessment of domestic fridge thermometers
3
Therm.
number
Operating
principle
(manufacturer)
Bimetallic (ETI)
Therm.
price
Accuracy (based on
testing programme)
Scale
Readability
£1.80
Measured temperature
to within ±0.5°C of
the reference
temperature
-30 to +30°C
1°C divisions
60 divisions in
total
1°C error possible if scale
is not viewed straight
(parallax error)
12
Electronic (Multi
Thermo)
£11.50
-50 to +150°C
0.1°C resolution
0.1°C (possible error
when temperature is
rounded to 1 decimal
place).
13
Electronic
(Digitron)
£26.75
Measured temperature
to within ±0.5°C of
the reference
temperature
Measured temperature
to within ±0.5°C of
the reference
temperature
-30 to +40°C
1°C resolution
1°C (possible error when
temperature is rounded to
0 decimal place).
14
Electronic (ETI)
£6.80
Unable to measure
temperature to within
±1°C of the reference
temperature
-9.9 to +49.9°C
0.1°C resolution
0.1°C (possible error
when temperature is
rounded to 1 decimal
place).
15
Electronic (ETI)
£14.00
Measured temperature
to within ±0.5°C of
the reference
temperature
-39.9 to
+69.9°C
0.1°C resolution
0.1°C (possible error
when temperature is
rounded to 1 decimal
place).
11
Image
Comments
Unable to measure
temperature to
within ±1°C of the
reference
temperature
A performance assessment of domestic fridge thermometers
4
Therm.
number
Operating
principle
(manufacturer)
Electronic (ETI)
Therm.
price
Accuracy (based on
testing programme)
Scale
Readability
£7.50
Measured temperature
to within ±0.5°C of
the reference
temperature
-39.9 to
+69.9°C
0.1°C resolution
0.1°C (possible error
when temperature is
rounded to 1 decimal
place).
17
Infrared surface
(Fluke)
£57.00
Unable to measure
temperature to within
±1°C of the reference
temperature
0.1°C (possible error
when temperature is
rounded to 1 decimal
place).
Unable to measure
temperature to
within ±1°C of the
reference
temperature
18
Liquid crystal
(Coldzone)
£1.58
Coldzone indicators
changed from showing
‘OK’ when the
environment temperature
was in the range 1°C to
2°C to a blank indication
when the environment
temperature was above
3°C to 4°C
-30 to +200°C
0.1°C
resolution. LED
highlights above
60°C and below
4°C
NA
Specification says that
"OK" is displayed at
temperatures less than
+5°C
Risk of fridge being
set at too low a
temperature
16
Image
Comments
A performance assessment of domestic fridge thermometers
5
Contents
1.0
2.0
3.0
4.0
5.0
Introduction .............................................................................................................. 7
Methodology for fridge thermometer assessment .................................................... 8
Results of the fridge thermometer assessment ........................................................ 9
3.1
Survey of the range and types of fridge thermometers available to UK consumers ..... 9
3.1.1 Conclusion from the survey of the range and types of fridge thermometers
available to UK consumers ................................................................................... 14
3.2
Results of the assessment to measure fridge thermometer performance .................. 15
3.2.1 Accuracy of the fridge thermometers ......................................................... 15
3.2.2 Conclusion from the tests to measure accuracy .......................................... 20
3.2.3 Resolution, readability and clarity of the fridge thermometers ...................... 21
3.2.4 Conclusion from the tests to assess resolution, readability and clarity of the
fridge thermometers............................................................................................ 23
3.2.5 Response times of the fridge thermometers to changing fridge temperatures 23
3.2.6 Conclusion of the tests to assess the response times of the fridge
thermometers to changing fridge temperatures ..................................................... 27
3.2.7 Repeatability of selected fridge thermometers ............................................ 28
3.3
An assessment of the guidance given to the consumer ........................................... 29
3.3.1 Conclusions from the assessment of the guidance given to the consumer ..... 34
Conclusions from this study .................................................................................... 35
References ............................................................................................................... 36
Acknowledgements
Campden BRI gratefully acknowledge the financial support and technical guidance provided by WRAP for this
work.
A performance assessment of domestic fridge thermometers
6
1.0
Introduction
Advice from the Food Standards Agency (FSA) states that ensuring a fridge is at the correct temperature
(typically between 0-5oC) is essential to prevent the growth of potentially harmful bacteria in foodstuffs
(Richmond, 1991; FoodSense, 1994). Fridge temperatures also have a key role in minimising food spoilage and
waste (FSA, 2009).
For these reasons it is advisable to use a fridge thermometer to check fridge temperatures. A study by the Food
Refrigeration and Process Engineering Research Centre (FRPERC) reported that over 60% of UK consumers did
not know the appropriate temperature for their fridge to store food safely. It also revealed that over two-thirds
of consumers surveyed did not have a fridge thermometer and, of those that did, the fridge temperature was
checked less than once a month (FRPERC, 2005). More recent research suggests that the number of consumers
knowing the temperature that their fridges should be set to has increased to almost 80% (WRAP, 2009), but this
research and additional research by the FSA 1 reveals that the majority still do not have a reliable method for
determining at what temperature their fridge is actually operating.
Previous surveys of UK domestic fridge temperatures (James and Evans, 1992) showed that fridge temperatures
ranged from 0oC to 12oC, with an average of around 6oC. This study suggested that the most appropriate way of
determining fridge temperatures would be to use a fridge thermometer.
However, there is some uncertainty on the performance and reliability of thermometers sold for domestic use.
Generally, they are relatively low cost devices and there is often little guidance to the consumer on their
appropriate use for effective monitoring of fridge temperature. There are also many different types of fridge
thermometers currently available, including:





electronic digital thermometers;
liquid-in-glass thermometers (mainly spirit-filled, sometimes mercury-filled);
mechanical thermometers (e.g. bimetallic dial types);
liquid crystal device (LCD) thermometers; and
infrared thermometers.
Each of these devices are also available in different formats, for example:






thermometers that measure fridge air temperature;
thermometers that are encased in a food simulant material to mimic temperature similar to that of the food
stored in the fridge;
thermometers that are designed to probe the food;
thermometers that indicate maximum and minimum temperatures;
thermometers that have a scale for direct reading of temperature; and
thermometers that have generic scale markings, e.g. 'safe', 'chilled', 'acceptable' and 'OK'.
The objective of this study was to survey the range of fridge thermometers available to UK domestic consumers
and assess the performance of a representative sample of these thermometers in a series of scientific tests to
ascertain the accuracy, resolution and responsiveness of the thermometers to a change in temperature.
1
http://www.food.gov.uk/multimedia/pdfs/publicattitudestofood.pdf
A performance assessment of domestic fridge thermometers
7
2.0
Methodology for fridge thermometer assessment
The assessment of the fridge thermometers was done in three stages:
1.
A survey of the range and types of fridge thermometers available to UK consumers was conducted and
examples were purchased for the evaluation phase of the study. The assessment encompassed fridge
thermometers from across the range available to consumers, including replicate samples of one device to
assess the repeatability of the device.
2.
An assessment programme was undertaken to measure fridge thermometer performance, including:




accuracy of temperature measurement;
resolution, readability and clarity of each fridge thermometer;
response times of each thermometer to a change in fridge temperature; and
consistency of fridge thermometer performance.
3.
An assessment was made of the guidance given to the consumer on the packaging, on how to use the fridge
thermometer. Key factors included assessing the presence and accuracy of:





advice on reading the temperature display, e.g. how to read the thermometer accurately and consistently;
advice on interpreting the reading given on the fridge thermometer, e.g. what does the temperature mean?
advice on where to place the thermometer in the fridge;
advice on what temperature the fridge should be set to;
advice on what the consumer should do if the thermometer indicates that the fridge temperature is outside
the recommended temperature range; and
the clarity of the advice given to the consumer.

Thermometer 18 (detailed in Table 2) is not designed to provide a quantitative indication of fridge temperature
and so could not be compared directly with the other fridge thermometers in this study. It is therefore excluded
from the majority of the tests. See section 3.2.7 for details of results for this thermometer.
A performance assessment of domestic fridge thermometers
8
3.0
Results of the fridge thermometer assessment
3.1
Survey of the range and types of fridge thermometers available to UK consumers
Most domestic home-ware or cooking equipment suppliers sell fridge and freezer thermometers. They are also
often advertised in the national press and in various magazines, catalogues and websites. A survey of the types
available via these sources suggested that there are five major types available. These are liquid-in-glass
thermometers, electronic thermometers, bimetallic thermometers, infrared thermometers and liquid crystal
thermometers. Within each category there was a reasonable variation in price, operating principle, size and
readabilty. However, within each type of fridge thermometer available, there was little difference between
different makes of thermometer other than cosmetic changes to reflect branding and packaging.
From the desk-based review of literature and web-based sources, a total of 18 different domestic fridge
thermometers were purchased for this study, representing the range currently available to UK consumers. They
were purchased from a variety of sources, including mail order catalogues, internet suppliers, magazine
advertisements and in store. These included eight versions of liquid-in-glass fridge thermometers, three versions
of bimetallic fridge thermometers, five versions of electronic fridge thermometers, one version of a liquid crystal
fridge thermometer and one version of an infrared fridge thermometer. Full details of the thermometers chosen
for the assessment are given below in Table 2, which summarises the fridge thermometers and their major
features. Images of the thermometers are given in Figures 1-17 below.
Table 2 Domestic fridge thermometers evaluated in this study.
Thermometer
number
1*
2*
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Operating principle
(manufacturer)
Liquid in glass (Brannan)
Liquid in glass (Brannan)
Liquid in glass (ETI)
Liquid in glass (Unbranded)
Liquid in glass (Endotherm)
Liquid in glass
(Food Safety Direct)
Liquid in glass (Foodsafe)
Liquid in glass (Chef Aid)
Bimetallic
Bimetallic (Food safety Direct)
Bimetallic (ETI)
Electronic (Multi Thermo)
Electronic (Digitron)
Electronic (ETI)
Electronic (ETI)
Electronic (ETI)
Infrared surface (Fluke)
Liquid crystal (Coldzone)
Location of sensor part
of thermometer
Bulb exposed to air
Bulb exposed to air
Bulb exposed to air
Bulb exposed to air
Bulb immersed in liquid gel
Bulb exposed in air
Thermometer
price
£2.50
£2.50
£2.75
£3.00
£7.23
£2.49
Bulb immersed in liquid gel
Internal (air)
Inside case
Inside case
Inside case
Penetration probe
Inside case
Inside case
Outside case
Outside case
Food temperature
Air temperature
£10.00
£1.95
£3.50
£2.49
£1.80
£11.50
£26.75
£6.80
£14.00
£7.50
£57.00
£1.58
* Thermometers 1 and 2 were identical. There were both included to test consistency.
A performance assessment of domestic fridge thermometers
9
Figure 1 Thermometer 1 and 2.
Figure 2 Thermometer 3.
Figure 3 Thermometer 4.
Figure 4 Thermometer 5.
Figure 5 Thermometer 6.
A performance assessment of domestic fridge thermometers
10
Figure 6 Thermometer 7.
Figure 7 Thermometer 8.
Figure 8 Thermometer 9.
Figure 9 Thermometer 10.
A performance assessment of domestic fridge thermometers
11
Figure 10 Thermometer 11.
Figure 11 Thermometer 12.
Figure 12 Thermometer 13.
A performance assessment of domestic fridge thermometers
12
Figure 13 Thermometer 14.
Figure 14 Thermometer 15.
.
Figure 15 Thermometer 16.
A performance assessment of domestic fridge thermometers
13
Figure 16 Thermometer 17.
Figure 17 Thermometer 18.
3.1.1 Conclusion from the survey of the range and types of fridge thermometers available
to UK consumers
There appears to be a good range of fridge thermometers available to the consumer and they are available
widely from a variety of sources, such as stores, magazines, national and local press advertisements and
numerous website sources. The prices of fridge thermometers also covers a wide range, from less than £2.00
for the simple LCD indicator type to over £50 for the infrared types. The wide variety of fridge thermometers
available can accommodate different consumer budgets and preferences for design and appearance. However,
such a wide range could cause some consumer confusion particularly given that people will not necessarily know
which fridge thermometer would give the most meaningful information of the temperature of their fridge.
Although five different types of fridge thermometer were chosen for this assessment, the most common fridge
thermometers available to the consumer were either the liquid-in-glass, bimetallic or electronic types.
Consequently, more examples of these thermometers were chosen for the testing programme than the other
types.
A performance assessment of domestic fridge thermometers
14
3.2
Results of the assessment to measure fridge thermometer performance
An assessment was undertaken to assess the performance of the domestic fridge thermometers in a range of
tests designed to simulate actual conditions of use. Four tests were used to assess performance:




accuracy of temperature measurement, as compared with UKAS-calibrated thermometers;
resolution, readability and clarity of each fridge thermometer;
response times of each thermometer to changing fridge temperatures; and
repeatability of fridge thermometer performance.
The fridge thermometers selected for this work were compared with standard reference thermometers which had
been previously calibrated to a national standard (United Kingdom Accreditation Service, UKAS). Campden BRI
operate a thermometer calibration service which is approved to UKAS standards (UKAS Testing Laboratory No.
0407).
3.2.1 Accuracy of the fridge thermometers
For this test, each of the fridge thermometers were placed in a controlled-temperature cold room environment at
a nominal 'chilled' temperature of between +2 and +5°C. The air temperature immediately surrounding each of
the thermometers under test was monitored using thin bare-wire thermocouples (type K, nickel-chromium)
attached to a temperature datalogger (Squirrel datalogger, Grant Instruments Ltd). Prior to the test, the
thermocouples and datalogger had been previously calibrated against a nationally traceable reference
temperature instrument.
Measurements of each fridge thermometer were taken at 2 minute intervals over a period of c. 30 minutes at the
controlled temperature. For the infrared thermometer, the device was used to measure the temperature of a
container of water, which had been allowed to equilibrate to the temperature of the cold room.
Table 3 shows the results of the 'calibration' on the fridge thermometers under test. The stable temperature
achieved by the calibrated thin wire thermocouple was +3.7°C and the table shows the temperature displayed by
each test thermometer and the difference between the measurements of the test thermometer and the
calibrated reference thermometer.
A performance assessment of domestic fridge thermometers
15
Table 3 Results of the constant temperature test on domestic fridge thermometers.
Thermometer
number
Thermometer
type
Temperature measured
by test thermometer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18*
LIG
LIG
LIG
LIG
LIG
LIG
LIG
LIG
Bimetallic
Bimetallic
Bimetallic
Electronic
Electronic
Electronic
Electronic
Electronic
Infra red
Liquid crystal
4.2°C
4.35°C
3.7°C
2.8°C
4°C
3.8°C
3.65°C
3.95°C
4.45°C
3.05°C
4.15°C
4.07°C
3.8°C
5.12°C
3.8°C
3.92°C
2.54°C
-
Difference between test
thermometer and
reference of +3.7°C
0.46°C
0.61°C
-0.04°C
-0.94°C
0.26°C
0.06°C
-0.09°C
0.21°C
0.71°C
-0.69°C
0.41°C
0.33°C
0.1°C
1.38°C
0.06°C
0.18°C
-1.20°C
-
* results for this thermometer are shown below in Section 3.2.7.
The results in Table 3 show that the fridge thermometers tested were relatively accurate, with 11 of the 17
thermometers in this test measuring temperature to within ±0.5°C of the reference temperature and 15 of the
17 thermometers measuring temperature to within ±1°C of the reference temperature. There were also no
obvious differences in performance between the different classes of fridge thermometer; the liquid-in-glass,
bimetallic and electronic thermometers all performed similarly.
The two fridge thermometers that performed less well in this test were Thermometers 14 and 17. The reasons
for this were thought to be that Thermometer 14 (electronic type) had the electronic temperature sensing
element embedded within a robust plastic enclosure that may have resulted in poor thermal heat transfer.
Thermometer 17 (infrared type) measures temperatures at the surface of a food or food package and,
consequently, the results from this device could not be directly compared with the air temperature measured by
the calibrated reference temperature probe.
The following figures (Figures 18 to 21) show how each of the fridge thermometers under test respond in
comparison to the actual cold room (fridge) temperature, as measured by the calibrated thin wire thermocouple.
This reflects the ability of the fridge thermometers to accurately measure the air temperature within the fridge.
The calibrated thin-wire thermocouple will respond accurately and rapidly to changes in fridge temperature,
including the slight temperature fluctuation of fridge temperature that occurs during normal operation of the
fridge. The ability of the fridge thermometers to follow the indicated calibrated thermocouple temperature gives
an indication of accuracy and speed of response of the fridge thermometer.
A performance assessment of domestic fridge thermometers
16
Figure 18 shows the measurements recorded by the liquid in glass thermometers (Thermometers 1 to 8). The
temperature in the controlled-temperature room, as measured by the calibrated thin-wire thermocouple, was
cycled between 2°C and 5°C. This changing temperature influenced the measurements provided by the fridge
thermometers under test, but due to the much larger dimensions of the fridge thermometers, they responded
much more slowly to changes in temperature than the calibrated thermocouple. There is consequently an
inherent time lag between the temperature measurements and a slight difference in the actual temperatures
measured between the calibrated thermocouple and the fridge thermometers.
Figure 18 Temperatures measured with the liquid-in-glass fridge thermometers.
5.5
Thermometer temperature (°C)
5
Reference
4.5
Thermometer 1
Thermometer 2
4
Thermometer 3
Thermometer 4
3.5
Thermometer 5
Thermometer 6
3
Thermometer 7
2.5
Thermometer 8
2
0
10
20
30
40
Time (minutes) of test
A performance assessment of domestic fridge thermometers
17
Figure 19 shows the measurements recorded by the bimetallic strip fridge thermometers (Thermometers 9, 10
and 11). The temperature in the controlled-temperature room, as measured by the calibrated thin-wire
thermocouple, again cycled between 2°C and 5°C. The bimetallic thermometers used in this test were much
larger and bulkier than the calibrated thin wire thermocouple and consequently they responded much more
slowly to changes in temperature than the calibrated thermocouple. As shown for the previous class of fridge
thermometers, there is an inherent time lag between the temperature measurements and a slight difference in
the actual temperatures measured between the calibrated thermocouple and the fridge thermometers.
Figure 19 Temperatures measured with the bimetallic fridge thermometers.
Thermometer temperature (°C)
5.5
5
4.5
Refrerence
4
Thermometer 9
3.5
Thermometer 10
3
Thermometer 11
2.5
2
0
10
20
30
40
Time (minutes) of test
A performance assessment of domestic fridge thermometers
18
Figure 20 below shows the measurements recorded by the electronic fridge thermometers (Thermometers 12 to
16). As before, the temperature in the controlled-temperature room was cycled between 2°C and 5°C. This
changing temperature influenced the measurements provided by the fridge thermometers under tests, but due
to the much larger dimensions of the fridge thermometers, they responded much more slowly to changes in
temperature. As indicated for the previous class of fridge thermometers, there is an inherent time lag between
the temperature measurements and a slight difference in the actual temperatures measured between the
calibrated thermocouple and the fridge thermometers. This was particularly apparent for fridge thermometer 14
which had the electronic temperature sensing element embedded within a robust plastic enclosure that seemed
to result in a slower and less accurate measurement of fridge temperature, possibly as a result of poor thermal
heat transfer.
Figure 20 Temperatures measured with the electronic fridge thermometers.
Thermometer temperature (°C)
5.5
5
4.5
Reference
Thermometer 12
4
Thermometer 13
3.5
Thermometer 14
3
Thermometer 15
2.5
Thermometer 16
2
0
10
20
30
40
Time (minutes) of test
A performance assessment of domestic fridge thermometers
19
Figure 21 below shows the measurements recorded by the infrared fridge thermometers (Thermometer 17). The
temperature in the controlled-temperature room was again cycled between 2°C and 5°C. This changing
temperature influenced the measurements provided by the infrared thermometer under test. This thermometer
measured the temperature of a small container of water in the fridge and due to the much larger dimensions of
this container compared to the thin wire thermocouple, this thermometer responded much more slowly to
changes in temperature.
Figure 21 Temperatures measured with the infrared fridge thermometer.
Thermometer temperature (°C)
5.5
5
4.5
4
3.5
3
Reference
2.5
Thermometer 17
2
0
10
20
30
Time (minutes) of test
40
3.2.2 Conclusion from the tests to measure accuracy
This test showed that all of the fridge thermometers evaluated showed some variation from the calibrated thin
wire thermocouple. None of the fridge thermometers measured the full extent of temperature change in the
fridge as indicated by the reference device. This was not unexpected, as all the fridge thermometers had a
larger thermal mass than the thin-wire reference temperature probe. However, no particular type of
thermometer stood out as being significantly better or worse than another (N.B. the infrared fridge thermometer
does not directly measure fridge air temperature and so cannot be directly compared with the calibrated
reference temperature probe).
All types of fridge thermometer also showed some degree of thermal lag, i.e. they responded to changes in
temperature more slowly than the calibrated thin-wire reference temperature probe. Again, this was expected of
the devices as they have larger thermal mass than the thin-wire reference probe and take longer to respond to
changes in temperature. Indeed, all of the fridge thermometers had some degree of built-in thermal lag - this
would prevent the devices from responding too rapidly to changes in temperature that were not representative
of the temperature within the fridge e.g. on opening the fridge door. A large thermal lag is a key feature of
some of the fridge thermometers under test: thermometers 5 and 7 have a liquid-in-glass thermometer
embedded into a silicon oil, which has the effect of slowing down the temperature response. The intention of
this type of fridge thermometer is not to indicate fridge air temperature, but to indicate the temperature likely to
be experienced by the food within the fridge.
The results of this test suggest that it is not possible to recommend any one particular fridge thermometer, or
even one type of fridge thermometer, over another. These tests highlight the importance of consumers keeping
any thermometer in the fridge for at least 30 minutes before taking a reading. The thermometers are not
A performance assessment of domestic fridge thermometers
20
designed to measure rapidly changing temperatures given this is not, in reality, a requirement of these devices
under normal consumer use.
Testing the accuracy of the thermometers was important, as the accuracy of a device can be described as how
closely the reading on the device corresponds to the measurement scale. This would have to be determined by
averaging the readings of the device over a period of time in a stable temperature environment. The results
show that all of the thermometers performed reasonably well.
3.2.3 Resolution, readability and clarity of the fridge thermometers
In this test, an assessment of the factors which influence how the consumer may read the fridge thermometer
was made. The resolution of the fridge thermometer indicates the temperature range and smallest scale
divisions that the thermometer can read. The readability is the ability to distinguish between individual
increments on the temperature scale, allowing the consumer to measure temperature to e.g. 1°C, 0.5°C or
0.1°C. The clarity of the fridge thermometer is a subjective assessment of the ease of reading the thermometer,
e.g. is the scale clear or does the thermometer need to be taken out of the fridge to be read?
For this test, an assessment was made by Robin Thorn (Head of Thermometer Calibration Services, Campden
BRI). Table 4 indicates the resolution and an assessment of readability of each of the fridge thermometer types
under test. The thermometer scales were assessed and the divisions noted. On some of the thermometers the
scales were very clear and it was possible to estimate a half scale division point between the scale divisions, this
is why some of the resolutions are greater than the divisions on the thermometer.
Table 4 An assessment of thermometer resolution, readability and clarity.
Thermometer
number
Thermometer
type
Scale
(range, scale
divisions)
1
LIG
2
LIG
3
LIG
4
LIG
5
LIG
-40 to +30°C
1°C divisions
70 divisions in total
-40 to +30°C
1°C divisions
70 divisions in total
-40 to +25°C
1°C divisions
65 divisions in total
-30 to +30°C
1°C divisions
60 divisions in total
-40 to +40°C
1°C divisions
80 divisions in total
6
LIG
-30 to +50°C
10°C divisions
8 division in total
7
LIG
-40 to +40°C
1°C divisions
80 divisions in total
Description
Refrigeration
thermometer - LIG
thermometer
encapsulated in
silicon oil
Colour scales
indicated for -18°C
to 0°C, 0°C to 5°C
and 5°C to 63°C
ranges
Foodsafe Fridge
thermometer
Possible error associated
with reading scale
(maximum resolvable
resolution)
1°C error possible if scale is
not viewed straight (parallax
error)
1°C error possible if scale is
not viewed straight (parallax
error)
1°C error possible if scale is
not viewed straight (parallax
error)
0.5°C error possible if scale is
not viewed straight (parallax
error)
2°C error possible if scale is
not viewed straight (parallax
error)
2-3°C error (parallax error and
estimation of scale)
2°C error possible if scale is
not viewed straight (parallax
error)
A performance assessment of domestic fridge thermometers
21
Thermometer
number
Thermometer
type
Scale
(range, scale
divisions)
8
LIG
9
Bimetallic
10
Bimetallic
-30 to +30°C
1°C divisions
60 divisions in total
-30 to +30°C
1°C divisions
60 divisions in total
-30 to +30°C
10°C divisions
6 divisions in total
11
Bimetallic
12
Electronic
13
Electronic
-30 to +40°C
1°C resolution
14
Electronic
-9.9 to +49.9°C
0.1°C resolution
15
Electronic
-39.9 to +69.9°C
0.1°C resolution
16
Electronic
-39.9 to +69.9°C
0.1°C resolution
17
Infrared
18
Liquid crystal
-30 to +30°C
1°C divisions
60 divisions in total
-50 to +150°C
0.1°C resolution
Description
Colour scales for 15°C to 0°C, 0°C to
8°C and 8°C to
30°C ranges
Needle vibrated
with any movement
Food storage/
refrigeration Probe
type temperature
sensor.
Temperature can
be read with fridge
door shut,
temperature
updates slowly.
Fridge
thermometer.
Temperature
sensor within body
of instrument.
Temperature can
be read with fridge
door shut
Temperature can
be read with fridge
door shut
Non-contact food
safety thermometer
-30 to +200°C
0.1°C resolution. LED
highlights above 60°C
and below 4°C
Specification says that "OK" is displayed at
temperatures less than +5°C
Possible error associated
with reading scale
(maximum resolvable
resolution)
1°C error possible if scale is
not viewed straight (parallax
error)
0.5°C error possible if scale is
not viewed straight (parallax
error)
3-4°C error (parallax error and
estimation of scale)
1°C error possible if scale is
not viewed straight (parallax
error)
0.1°C (possible error when
temperature is rounded to 1
decimal place).
1°C (possible error when
temperature is rounded to 0
decimal place).
0.1°C (possible error when
temperature is rounded to 1
decimal place).
0.1°C (possible error when
temperature is rounded to 1
decimal place).
0.1°C (possible error when
temperature is rounded to 1
decimal place).
0.1°C (possible error when
temperature is rounded to 1
decimal place).
n/a
A performance assessment of domestic fridge thermometers
22
3.2.4 Conclusion from the tests to assess resolution, readability and clarity of the fridge
thermometers
As can be seen from Table 4, the readability and usability of the thermometers varied. The electronic
thermometers could be read most easily and accurately as the displays showed the temperature clearly and in
most cases they could be read without opening the fridge door.
The liquid in glass thermometers were generally considered to be the most difficult to read as they often had to
be looked at closely to determine the reading. This may lead to inaccuracies as a consumer may have to remove
the thermometer from the fridge to read it, which in turn may affect the displayed reading. However the slow
speed of response of most of the liquid in glass thermometers means that this is unlikely to be a issue unless the
user takes more than 30 seconds to read the thermometer. The bimetallic strip thermometers generally
displayed the reading clearly with a good scale.
The two thermometers sold by “Food Safety Direct”, (Thermometers 6 and 10) were generally considered to be
the most difficult to read as the scales were very widely spaced and not clear. The same was the case with
Thermometers 5 and 7. Given fridge temperatures should fall between 0-5oC, an error of 2-3oC is significant and
could lead to an incorrect temperature being set.
All of the thermometers had scales that spanned a much wider range than was necessary for domestic
refrigerator use, this made the proportion of the scale that is of interest to the consumer (typically 0-10°C) small
as a proportion of the entire range, this consequently made the thermometers more difficult to read and less
precise than the ideal.
3.2.5 Response times of the fridge thermometers to changing fridge temperatures
The response time of the fridge thermometers to react to step changes in fridge air temperature was determined
through two tests.
Firstly, a warming-up test was conducted. The fridge thermometers were allowed to equilibrate to a fridge
temperature of 5°C for a period of at least 24 hours and then brought into a controlled temperature environment
of 18°C (thought to be a typical kitchen ambient temperature). Secondly, a cooling-down test was conducted
where the fridge thermometers were equilibrated to a room temperature of 18°C and then taken into a fridge
environment of 5°C.
Measurements from each test fridge thermometer were taken at 5 minute intervals throughout the test to
establish the rate of response of each fridge thermometer to the step change in temperature. Results from the
fridge thermometers under test were compared with a calibrated thin-wire thermocouple.
Figures 22 to 25 below show the results of the warming-up tests for the different categories of fridge
thermometer evaluated in these tests. The figures show the temperatures measured by each device in the
chilled environment and the 'step change' as the devices were taken into the room temperature regime. The
results indicated that, as expected, the thin wire thermocouple responded to changes in temperature more
rapidly than any of the fridge thermometers under test, but all the thermometers generally responded well. Two
of the liquid in glass thermometers (Thermometers 5 and 7) have the sensor part of the thermometer
encapsulated within a liquid gel in order to slow down the speed of response. They are marketed as
thermometers which indicate the fridge temperature as it might be experienced by the food and not the fridge
air temperature and therefore took a longer time to respond to changes in temperature.
Some of the electronic thermometers responded very slowly to the changes in temperature, this is due to a
number of factors, including the positioning of the sensing element. On one device (Thermometer 13), the slow
response was due to the slow rate at which the display unit was updated by the sensors.
A performance assessment of domestic fridge thermometers
23
Figure 22 Response of liquid-in-glass fridge thermometers to changing fridge temperatures (warming).
Thermometer temperature (°C)
25
Reference
20
Thermometer 1
Thermometer 2
15
Thermometer 3
Thermometer 4
10
Thermometer 5
5
Thermometer 6
Thermometer 7
0
0
20
40
60
80
Thermometer 8
Time of test (minutes)
Figure 23 Response of bimetallic fridge thermometers to changing fridge temperatures (warming).
Thermometer temperature (°C)
25
20
15
Reference
Thermometer 9
10
Thermometer 10
Thermometer 11
5
0
0
20
40
60
80
Time of test (minutes)
A performance assessment of domestic fridge thermometers
24
Figure 24 Response of electronic fridge thermometers to changing fridge temperatures (warming).
Thermometer temperature (°C)
25
20
Reference
15
Thermometer 12
Thermometer 13
10
Thermometer 14
Thermometer 15
5
Thermometer 16
0
0
20
40
60
80
Time of test (minutes)
Figure 25 Response of infrared fridge thermometer to changing fridge temperatures (warming).
Thermometer temperature (°C)
25
20
15
Reference
10
Thermometer 17
5
0
0
20
40
60
80
Time of test (minutes)
A performance assessment of domestic fridge thermometers
25
Figures 26 to 29 below show the results of the cooling-down tests for the different categories of fridge
thermometer evaluated in these tests. The results indicated that, as expected, the thin wire thermocouple
responded to changes in temperature more rapidly than any of the fridge thermometers under test. Again,
because two of the liquid in glass thermometers (Thermometers 5 and 7) have the sensor part of the
thermometer encapsulated within a liquid gel in order to slow down the speed of response, these thermometers
took appreciably longer (particularly Thermometer 5) than the thin-wire reference temperature probe to cool to
fridge temperature.
Figure 26 Response of liquid-in-glass fridge thermometers to changing fridge temperatures (cooling).
Thermometer temperature (°C)
25
Reference
20
Thermometer 1
Thermometer 2
15
Thermometer 3
Thermometer 4
10
Thermometer 5
5
Thermometer 6
Thermometer 7
0
0
10
20
30
40
Thermometer 8
Time of test (minutes)
Thermometer temperature (°C)
Figure 27 Response of bimetallic fridge thermometers to changing fridge temperatures (cooling).
25
20
15
Reference
Thermometer 9
10
Thermometer 11
5
Thermometer 12
0
0
10
20
30
40
Time of test (minutes)
A performance assessment of domestic fridge thermometers
26
Figure 28 Response of electronic fridge thermometers to changing fridge temperatures (cooling).
Thermometer temperature (°C)
25
20
15
Reference
10
Thermometer 12
Thermometer 13
5
Thermometer 14
Thermometer 15
0
0
10
20
30
Time of test (minutes)
40
Thermometer 16
Figure 29 Response of infrared fridge thermometer to changing fridge temperatures (cooling).
Thermometer temperature (°C)
25
20
15
10
Reference
Thermometer 17
5
0
0
10
20
30
Time of test (minutes)
40
3.2.6 Conclusion of the tests to assess the response times of the fridge thermometers to
changing fridge temperatures
The temperature response trials were carried out to determine how quickly the thermometers responded to a
step change in temperature, whilst it is unlikely that consumers will carry out such a test (if the thermometer is
left in the fridge) it was an important test, as it gives an indication as to how fast the thermometers will respond
to a more gradual change in temperature such as placing a large hot item of food in the fridge (which could
warm up the fridge contents). The project team were looking to see whether there were any really slow or really
fast thermometers - the former may have implications on food poisoning if response times were of the order of a
couple of hours and the latter may have implications if they responded so quickly and changed as soon as the
fridge door was opened.
A performance assessment of domestic fridge thermometers
27
The results of these tests showed that there were no appreciable differences between the majority of fridge
thermometers tested in changing with either increasing temperatures (warming) or decreasing temperatures
(cooling). In both heating and cooling tests, the majority of thermometers took around 20 minutes to reach the
temperature indicated by the calibrated thin-wire reference temperature. The thin wire thermocouple used to
monitor the ambient temperature around the thermometers was considered to change temperature within 10-15
seconds and any apparent lag in response was caused by intervals between measurements.
A difference was seen with Thermometers 5 and 7 which took appreciably longer than the others to reflect
changes in temperature (heating and cooling). As mentioned previously, these thermometers had the active
temperature sensing element (a liquid-in-glass thermometer) embedded in a silicon oil to deliberately suppress
the speed of temperature response. These fridge thermometers are sold as indicators of food temperatures in
the fridge rather than fridge air temperature indicators and therefore are designed to respond more slowly to
temperature change.
The infrared fridge thermometer was the most rapidly responding thermometer in these tests, both in heating
and cooling. The results of this fridge thermometer, which measured the temperature at the surface of a small
container of water, responded as quickly as the thin-wire reference probe.
In order to provide the consumer with a meaningful indication of fridge temperature, it is suggested that the
most useful fridge thermometer would fully respond to changes in temperature within 30 minutes of the
temperature change. This would correspond with the recommended guidelines for food handlers in the
preparation of foods and the ‘maximum time at room temperature: 30 minutes (critical limit 2 hours)’ (Food
Hygiene Handbook, 2005). This timescale would be useful in indicating significant temperature changes within
the fridge that may cause problems in food safety and would avoid reflecting minor changes. Significant
temperature change may occur if, for example, the fridge door was left open for a period of time or if a large,
warm food item was introduced into the fridge which then affected the overall fridge temperature.
The results of this test on the fridge thermometers suggested that the majority of fridge thermometers complied
with this requirement; those fridge thermometers (Thermometers 5 and 7) that did not are deliberately designed
to respond differently.
3.2.7 Repeatability of selected fridge thermometers
The results presented earlier in this report showed that fridge thermometers of the same type could show a
slight difference in performance (e.g. in the accuracy tests). For example, Thermometers 1, 2 and 3 are very
similar in design and construction, but gave temperature measurements that were 0.5°C different in the fridge
environment. Consequently, an assessment of the repeatability between different samples of the same
thermometer was carried out.
In this test, Thermometer 18 (liquid crystal thermometer, 'Coldzone') was used (Figure 30) to ascertain the
variability between different samples of a single type of thermometer. This thermometer does not have a
numerical display, but if the fridge temperature is below +5°C, then the liquid crystal display indicates an "OK"
message to the consumer. These fridge thermometers therefore, do not provide a quantitative indication of
fridge temperature and so could not be compared directly with the other fridge thermometers in this study. This
particular thermometer is used by the FSA as a means of promoting the importance of correct fridge
temperatures in guaranteeing food safety.
A performance assessment of domestic fridge thermometers
28
Figure 30 Coldzone liquid crystal fridge thermometer.
Ten Coldzone fridge thermometers were tested in a controlled temperature environment under a steadily
changing temperature regime. To accomplish this, the indicators were put in a cooled water bath and then the
temperature was monitored and slowly increased from 1 to 5°C. The reference temperature was continuously
monitored using an array of calibrated thin-wire thermocouples placed immediately adjacent to the
thermometers under test.
This test was repeated several times, both in heating (1°C to 5°C) and cooling (5°C to 1°C). After several trials
it was found that all of the Coldzone indicators changed from showing ‘OK’ when the environment temperature
was in the range 1°C to 2°C to a blank indication when the environment temperature was above 3°C to 4°C. The
colour change on these devices took 3 seconds during the warming up test and 4-5 seconds during the cooling
down test. The temperature range of the colour change was not as indicated on the product packaging.
However, it did err on the side of food safety.
It was also noted that the colour change was one-way below the 3-4°C temperature threshold. A further test
was conducted where the Coldzone indicator was placed into a freezer; at temperatures below 0°C all of the
Coldzone indicators still indicated ‘OK’, even though the temperature was lower than that expected in a domestic
fridge.
During the testing it was noted that all of the Coldzone indicators changed at the same temperature with little
difference between the 10 indicators tested.
3.3
An assessment of the guidance given to the consumer
It was important to assess the guidance given on the thermometer and its packaging since this is the key way in
which to ensure consumers understand how to use and read their thermometer. Incorrect use could mean the
thermometer is not showing the correct temperature and/or that the consumer is not aware whether the fridge is
at the correct temperature. Key factors in the assessment included the presence and accuracy of:






advice on reading the temperature display, e.g. how to read the thermometer accurately and consistently;
advice on interpreting the reading given on the fridge thermometer, e.g. what does the temperature mean?
advice on where to place the thermometer in the fridge;
advice on what temperature the fridge should be set to;
advice on what the consumer should do if the thermometer indicates that the fridge temperature is outside
the recommended temperature range; and
the clarity of the advice given to the consumer.
Tables 5-8 give an assessment of the guidance given.
A performance assessment of domestic fridge thermometers
29
Liquid in glass (LIG) thermometers
The LIG thermometers work on the principle of a bulb of liquid (usually a coloured alcohol liquid, sometime mercury), which expands or contracts along a fine bore within a
glass tube upon heating or cooling.
Eight thermometers of this type were purchased for this study. Most precision LIG thermometers that are available have the temperature scale inscribed directly onto the glass
column of the thermometer (these were not included in this research because they are not designed for fridge use). Less accurate versions (such as those purchased for this
study have the temperature scale inscribed onto the plastic case of the device. All of the thermometers purchased for this study had the temperature scale marked on the
case of the thermometer. This makes it more difficult to read and more prone to errors whilst being read.
Table 5 Liquid in glass thermometers evaluated in this study, along with their instructions for use
Thermometer
number
1
2
3
Operating principle
(manufacturer)
Liquid in glass
(Brannan)
Liquid in glass
(Brannan)
Liquid in glass (ETI)
Location of sensor
part of
thermometer
Bulb exposed to air
Thermometer
price
Bulb exposed to air
£2.50
Bulb exposed to air
£2.50
£2.75
Pack instructions in italics and comments
"Avoid the risk of food damage by running your appliance too warm. Avoid
wasting electricity by having your appliance unnecessarily cold.
Refrigerator- Food must be kept between 0 and 5°C
Freezer- Food should be kept below -18°C".
This seems sensible but no guidance on where thermometer should be
placed.
"Hang or place the thermometer on or as near as possible the middle shelf,
towards the back of the fridge. Like food, the thermometer will take a while
to reach the appliance temperature, allow one hour initially. Check that the
fridge is at its correct temperature (between 3 and 5°C), if above this range,
turn the refrigerator thermostat dial up to the next number, repeat until the
optimum temperature is obtained".
4
5
Liquid in glass
(Unbranded)
Liquid in glass
(Endotherm)
Bulb exposed to air
£3.00
Bulb immersed in
liquid gel
£7.23
Good comprehensive advice on use of thermometer. Interesting that a
different temperature range is given here than for no. 11, which is the same
manufacturer, and that the range is between 3 and 5°C, which is higher than
other guidance.
No instructions were supplied with this thermometer.
Positioning and temperature suggestions:
"Fridge or chiller: 1 to 5°C (40°F) Use Velcro to attach to fridge door or side
of fridge.
A performance assessment of domestic fridge thermometers
30
Thermostat control: Leave the ENDOTHERM for 2 hours at coldest location, if
the temperature needs adjusting, turn the thermostat to next unit, repeat if
necessary.
Refrigerator control: Read temperature first thing in the morning. A
progressive rise in temperature over several days indicates refrigeration
failure.
Food Monitoring: Record temperature twice a day."
6
7
Good comprehensive advice on use of thermometer though following the
positioning advice could mean that the coldest point in the fridge may be at
the correct temperature but rest of fridge may be too warm.
No instructions were supplied with the thermometer but markings on case
indicate that fridge should be between 0 and 5°C.
Liquid in glass
(Food Safety Direct)
Liquid in glass
(Foodsafe)
Bulb exposed in air
£2.49
Bulb immersed in
liquid gel
£10.00
"Hang or place the thermometer on the middle shelf, towards the back of the
fridge. Like food, the thermometer will take a while to reach the appliance
temperature. Check that the fridge is at the correct temperature (between 0
and 5°C), if it is above this range, turn the fridge thermostat down until the
optimum temperature is obtained".
Liquid in glass (Chef
Aid)
Internal (air)
£1.95
Accepted temperatures should be:
Fridge. 35 to 40°F (2 to 4°C).
Good advice.
8
Useful guidance based on US regulations, but nothing about where to place
thermometer.
A performance assessment of domestic fridge thermometers
31
Bimetallic strip thermometers
Bimetallic strip thermometers work on the principle that a strip or coil made from dissimilar metals, will bend or curl as the metals expand and contract at different rates with
temperature.
Table 6 Bimetallic strip thermometers evaluated in this study, along with their instructions for use.
Thermometer
number
9
10
11
Operating principle
(manufacturer)
Bimetallic
(Unbranded)
Bimetallic (Food
safety Direct)
Bimetallic (ETI)
Location of sensor
part of
thermometer
Inside case
Thermometer
price
£3.50
No instructions were supplied with this thermometer
Inside case
£2.49
No instructions were supplied the thermometer but markings on case
indicate that fridge should be between 0 and 5°C
Inside case
£1.80
"Hang or place the thermometer on the middle shelf, towards the back of the
fridge. Like food, the thermometer will take a while to reach the appliance
temperature, allow one hour initially. Check that the fridge is at the correct
temperature (between 0 and 5°C, in the green zone), if it is above this
range, turn the fridge thermostat down until the optimum temperature is
obtained".
Pack instructions
Good advice and visual indicator is useful.
A performance assessment of domestic fridge thermometers
32
Electronic thermometers
Electronic thermometers can operate on a variety of principles to sense and display the temperature of an environment. The most common method of operation for low cost
devices is the use of a thermistor, a small sensing element that changes electrical resistance in response to temperature. Electronic devices can be very stable and accurate
but some of the models tested are very slow to respond to changes in temperature.
Table 7 Electronic thermometers evaluated in this study, along with their instructions for use.
Thermometer
number
Operating principle
(manufacturer)
Location of sensor
part of
thermometer
Penetration probe
Thermometer
price
£11.50
Instructions on use were given, but did not contain any guidelines for the
temperatures that a fridge should be operated at, or how the thermometer
should be used.
Instructions on use were given, but did not contain any guidelines for the
temperatures that a fridge should be operated at, or how the thermometer
should be used. The user can set alarms if the temperature is outside a preset
limit.
Instructions on use were given, but did not contain any guidelines for the
temperatures that a fridge should be operated at or how the thermometer
should be used.
Instructions on use were given, but did not contain any guidelines for the
temperatures that a fridge should be operated at, or how the thermometer
should be used.
Pack instructions
12
Electronic (Multi
Thermo)
13
Electronic (Digitron)
Inside case
£26.75
14
Electronic (ETI)
Inside case
£6.80
15
Electronic (ETI)
Outside case
£14.00
16
Electronic (ETI)
Outside case
£7.50
Instructions on use were given, but did not contain any guidelines for the
temperatures that a fridge should be operated at, or how the thermometer
should be used.
17
Infrared surface
(Fluke)
Food temperature
£57.00
(Paraphrased from instructions given). The thermometer is designed to check
that chilled foods are kept chilled and hot foods are kept hot. To assist in this
check, there is an indicator that lights when the foods are below 4°C and
above 60°C.
A performance assessment of domestic fridge thermometers
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Thermo-chromic indicators
Thermo-chromic indicators use liquid crystal displays to indicate temperature, or to provide a warning message when temperatures are above or below a particular
temperature threshold.
Table 8 Thermo-chromic indicators evaluated in this study, along with their instructions for use.
Thermometer
number
18
Operating principle
(manufacturer)
Liquid crystal
(Coldzone)
Location of sensor
part of
thermometer
Air temperature
Thermometer
price
£1.58 each
Pack instructions
Printed on the indicator “If the centre of the circle is green your chiller
temperature is set correctly. If the centre of the circle is Black your chiller
temperature is set too high!”
Clear instructions
3.3.1 Conclusions from the assessment of the guidance given to the consumer
The instructions enclosed with the thermometers ranged from some comprehensive examples to ten of the thermometers that had no indications of how best to use them.
Only eight of the thermometers tested had instructions that gave an indication of what temperature a fridge should be operating at, and these showed a variation in the
temperatures specified. Four of the thermometers were supplied with instructions that specified the position where the thermometer should be placed; however, we are not
aware of any general pattern as to where the coldest point in the fridge is normally located. There may be limited benefit from using a fridge thermometer to measure the
coldest location of the fridge as the warmest location is likely to have more of an impact on food safety.
A performance assessment of domestic fridge thermometers
34
4.0
Conclusions from this study
This study has evaluated the performance of a representative selection of domestic fridge thermometers available
to the UK consumer. A summary of the results is given in Table 1. As mentioned in the introduction, advice from
the Food Standards Agency (FSA) states that ensuring a fridge is at the correct temperature (typically between 05oC) is essential to prevent the growth of potentially harmful bacteria in foodstuffs (Richmond, 1991; FoodSense,
1994). Fridge temperatures also have a key role in minimising food spoilage and waste (FSA, 2009).
The conclusions that can be drawn from this study are:

A wide range of domestic fridge thermometers are available. The majority are either liquid-in-glass,
electronic, liquid crystal or bimetallic thermometers. These are widely available in most supermarkets and
hardware stores and also from mail order sources and through the national press. The retail price of
thermometers ranges from under £2 for the simple liquid crystal or liquid-in-glass types to over £25 for some
electronic types. Given the low price options, fridge thermometers are a very affordable tool for consumers to
ensure their fridge is operating at the correct temperature, that will ensure shelf life is not compromised.

The accuracy of the fridge thermometers tested was generally good, with the majority of devices measuring
fridge air temperatures to within ±0.5°C of the actual fridge temperature.

Some of the fridge thermometers tested were not designed to directly measure fridge air temperature. One
type had a standard liquid-in-glass thermometer embedded into a clear liquid gel; this was designed to slow
down the rate of temperature change of the thermometer and be more representative of the temperature
change actually experienced by the food products within the fridge. Another type used an infrared sensor to
directly measure the food product/packaging temperature within the fridge and responded much more quickly
than the other thermometers.

The readability and usability of the thermometers varied considerably. While the electronic thermometers
could be read easily and accurately (and in most cases they could be read without opening the fridge door)
the liquid in glass thermometers were generally considered to be the most difficult to read as they often had
to be looked at closely to determine the reading. This may lead to inaccuracies as a consumer may have to
remove the thermometer from the fridge to read it, which in turn may affect the displayed reading.
Thermometer 18 is the easiest for a consumer to interpret. This thermometer does not have a numerical
display, but if the fridge temperature is below +5°C, then the liquid crystal display indicates an "OK" message
to the consumer.

All of the thermometers had scales that spanned a much wider range than was necessary for domestic
refrigerator use, this made the proportion of the scale that was of interest to the consumer (typically 0-10°C)
small as a proportion of the entire range, this consequently made the thermometers more difficult to read and
less precise than the ideal. This is something for consideration by fridge thermometer manufacturers.

Repeatability between batches of the same fridge thermometer type was also assessed and it was shown that
results were generally consistent. The 'Coldzone' temperature indicator was used for this test, a liquid crystal
device being used by the FSA as a means of promoting food safety in domestic refrigerators. The testing
showed that there was no appreciable variation between the 10 replicate devices of this indicator tested. It is
assumed that replicates of the other 17 thermometers would also operate consistently.

The instructions provided to consumers alongside the fridge thermometers were limited and quite variable.
Many fridge thermometers were supplied with no instructions, some had an indication of recommended fridge
temperatures and some provided advice on how to position the thermometer in the fridge, but the
instructions for all the thermometers could be vastly improved to ensure that they are used by consumers in
the right way and give accurate readings.
A performance assessment of domestic fridge thermometers
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5.0
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References
Sprenger, R.A. (2005). The Food Hygiene Handbook. Highfield, UK (www.highfield.co.uk).
FoodSense (1994). Keeping food cool and safe: a guide from the Ministry Of Agriculture, Fisheries and Food.
FoodSense, London SE99 7TT, UK.
FRPERC (2005). How cold is your fridge? Food Refrigeration & Process Engineering Research Centre,
University of Bristol. http://www.frperc.bris.ac.uk/home/opinion/items/item0002.htm.
FSA (2009). Food Standards Agency (www.eatwell.gov.uk/keepingfoodsafe/storing).
James, S.J. and Evans, J.E. (1992). The temperature performance of domestic refrigerators. International
Journal of Refrigeration, 15 (5), 313-319.
Richmond, M. (1991). The microbiological safety of food part II. Report of the committee on the
microbiological safety of food. HMSO, London.
WRAP (2009). Reducing Food and Packaging Waste throughout the Chill Chain, not yet published.
A performance assessment of domestic fridge thermometers
36
www.wrap.org.uk/retail