fi1ll
Correct Application of Liquid-in-Glass Thermometers for
Accurate Temperature Measurements in the Clinical Laboratory
Ed. note: Mr. Ween prepared
this Special
Report
at
our request. He is eminently
fitted to do so, being
president
of a company that makes precision
thermometers, a member since 1951 of Committee E-20 on
Temperature
Measurement
of ASTM, a member of the
Standards
Committee of SAMA since 1955, with deep
involvement
in thermometry,
an actively
involved
member of ANSI since 1965, and a member of NCCLS’s
Subcommittee
on Temperature.
The importance of temperature measurement in the clinical laboratory has only recently become universally recognized. This article deals with the role of liquid-in-glass
thermometers in accurately measuring and monitoring
temperatures.
Addftlonal
Standards
ice-points
#{149} National Bureau of
Standard Reference Materials
#{149}SRM 933
#{149}
Keyphrases:
#{149}
SRM 934
Background
Industry has long understood the criticalrole of
temperature and the need to measure itaccurately in
the laboratory (1). Temperature is a prime factor in
quality
control.
Many
societies,
in conjunction
trade associations
with government
and technical
agencies,
are
actively engaged in writing standards
for liquid-in-glass
laboratory
thermometers.
The American
Society for
Testing
Materials,
The Manufacturing
Chemists
Association,
and The Scientific
Apparatus
Makers Association are some of the organizations
involved in this
work. Many of the standards
that are developed
go on
to become National and International
Standards
under
the aegis of The American National Standards
Institute.
Temperature
in the Clinical
perature and itsmeasurement inthe clinical
laboratory,
the National Bureau of Standards,
at the request of the
College of American
Pathologists
and The American
Association
for Clinical
Chemistry,
has recently
(3)
developed
two new Standard
Reference Materials,
Nos.
933 and 934 (Figure
1). A detailed
description
and
statement
of the use of these precision glass thermometers can be found in NBS Special Publication
260-48,
by B. W. Mangum
and J. A. Wise (available
from the
Superintendent
of Documents,
U. S. Government
Printing
Office, Washington,
D. C. 20402, at 604 per
copy)
thermometers.
to be solved
have
become
more
involved
1112
CLINICAL CHEMISTRY,
Vol. 22, No. 7, 1976
NCCLS
Proposed
Standard
PS1-2
They
should
not
be used
in the
day-
same immersion.
In addition
they are relatively
inexpensive.
These are the instruments
that should
be used
routinely
in the lab. They should
be checked
for accuracy with the aid of the SRM from time to time. If nec-
Laboratory
and
complex, temperature
measurement
in turn has become
a more important
component
in the methods
being
developed
to deal with these matters
(2). The need for
standards
has been
acknowledged.
The National
Committee
for Clinical Laboratory
Standards
is actively
engaged in developing
these standards,
and a subcommittee
dealing with temperature
is currently
in full
operation.
Further
to confirm the importance
of tern-
in
to-day work, but rather to calibrateand check the accuracy of the routine thermometers
used in the laboratory.
Thermometers
are now available
that are similar to
the SRM’s in design. Although neither accompanied
by
a Report of Calibration
nor equipped
with auxiliary
scales at 0 #{176}C,
they are of the same length, have the same
scale range and sub-divisions,
and are calibrated
for the
Although
there has always been a concern with temperature
in the clinical laboratory,
until recently very
little had been done in the area of standardization
of
procedures
to measure,
monitor, and control it. As the
problems
and
“Standard
forTemperature CalibrationofWater Baths,
Instruments,and Temperature Sensors.”A copy of this
proposed standard
isavailablefrom NCCLS,
Villanova,
Pa. 19085, at $2.00 per copy.
These SRM’s were developed
as primary
standards
for the clinical field (4). They are available directly from
NBS as well as from commercial
sources catering to the
needs of the clinical laboratory.
Being primary
standards, they are considerably
more costly than ordinary
Fig. 1.
essary, they can be calibrated
over the entire range with
use of the SRM as the standard.
When the SRM’s are used itisof critical
importance
that they be used properly.’Anyone unfamiliar
with the
characteristics
of liquid-in-glass
thermometers
can
actually
multiply,
rather
than reduce, the chance of
errors by improper
handling.
It is of crucial significance
to understand
the function
of the ice-point
(auxiliary
scale at 0 #{176}C
as a reference
point) and to take the icepoint reading
correctly
(4, 5). The ice-point
is incorporated
into the design of SRM 933 and SRM 934 at
considerable
cost, and serves as a means of observing
any minute changes in bulb volume that would change
the indications
of the thermometer.
Each SRM thermometer
is accompanied
by a “Report
of Calibration”
showing the true reading of the ice-point
as well as the
corrections
to be applied at the significant
temperatures
on the main scale. The corrections
stated on the Report
of Calibration
apply if the ice-point reading is taken (in
the manner prescribed)
and found to be in agreement
with that stated in the report. If the ice-point
reading
is found to be higher or lower than stated, all other
readings
will be higher or lower by the same amount.
The temperature
of melting ice isO #{176}C
and is an excellent reference point to use in checking liquid-in-glass
thermometers.
Because
of the critical importance
of the ice-point
reading to the use of the SRM thermometers
the following precautions
should be taken.
1. Use shaved ice made from clear pieces of commercial ice or ice made from distilled water.
2. Place the shaved ice in a carefully
cleansed
and
rinsed Dewar flask. Add enough distilled water to form
a tightly packed slush without floating the ice.
3. Use extreme care in handling
the ice, to prevent
contamination.
4. Rinse the bulb of thermometer
with distilled
water
before insertingitinto the ice bath. Do not touch the
bulb afterrinsingit, because it ispossibleto contaminate the ice with salts from sweat on the hand, resulting
in erroneous
readings.
The question oftenarisesas to how oftenthe ice-point
reading should be taken on a calibrated standard
(SRM’s
933 and 934). The following
can be used as a
guide. The firsttime the standard is to be used, the
ice-point
should be taken in accordance
with the instructions
on the Report of Calibration
that accompanied the thermometer.
A note of the reading should be
made, along with the time and date. Compare
this
reading with that given in the Report of Calibration.
Two weeks later, take another reading, making note of
the time and date, as well as of the reading.
If the
‘To reduce thepossibility of errors (which can be significant) owing
to the improper use of thermometers,
it would be helpful to refer to
some papers dealing
with this subject. NBS Monograph 150, entitled
“Liquid-in-Glass
Thermometers,”
by Jaqueline A. Wise, is available
fromtheSuperintendent
of Documents, the Government Printing
Office, Washington, D. C. 20402 at 60 per copy. A paper entitled
“Care and Use of Liquid-in-Glass Thermometers”,
published in ISA
Transactions
7, No. 2, by the Instrument
Society of America (ref. 1)
is available
from ERTCO
(address below), at $1.00 per copy.
reading
has not changed,
allow about
a month
before
taking another
reading of the ice-point.
Again, make a
note of the time and date as well as of the reading. If a
change occurs, remember
to change the other corrections on the Report of Calibration
by the same amount
and in the same direction.
Should
no change be observed after the first three times the ice-point
is taken,
the time between
ice-point
checks may be prolonged.
On the other hand, if the ice-point
should be found to
be drifting, a shorter interval between ice-point readings
is indicated.
If the standard
is rarely used, the ice-point
reading should be taken each time it is used. Keeping
records of the ice-point
reading will develop a history
of the stability
of the bulb.
An example follows of how a change in the ice-point
reading will effect the other readings on the thermometer.
Original
Corrections
from the
NBS Calibration
Thermometer
reading,
+0.005
25.000
30.000
37.000
Report
#{176}C
Corrections,
-0.005
-0.010
-0.005
+0.005
#{176}C
k later ice-point reading is found to be 0.000 #{176}C;
the new
table would be as follows:
0.000 #{176}C
-0.005
0.000
+0.010
0.000 #{176}C
25.000
30.000
37.000
The thermometers
used by the clinical chemist are
frequently chosen from laboratory supply house catalogs.
In many
instances
they
may
have
been
designed
for use in other applications. Results of two similar tests
run in different laboratories may not always agree. It is
therefore essential, when formulating
a method that
requires the precise use of a thermometer,
that a full
description of the thermometer
be given so that anyone
duplicating
the method will use the same instrument,
in the same manner, and thereby increase the probability of consistent results.
References
1.Ween, S., Care and use of liquid-in-glass
ISA Transactions
7,93 (1968).
2. Bowers,
Proposal
C. N.,Jr.,
Bergmeyer,
for I.F.C.C. Reference
laboratory
H. U., and
Methods
thermometers.
Moss,
D. W., Draft
for Enzymes,
pp.17-18
(1974).
B. W., Standard Reference Materials 933 and 934: The
Bureau of Standards’ Precision Thermometers
for the
clinical laboratory. Clin. Chem. 20, 670 (1974).
4. Mangum, B. W., and Wise, J. A.,Nat. Bur. Stand. Spec. Pubi.
260-48, Description and Use of Precision Thermometers
for the
Clinical Laboratory, SRM 933 and SRM 934 (1974).
5. ref. 1,p95.
3. Mangum,
National
Sidney
(ERTCO) Ever Ready Thermometer
693 Broadway
New York, N. Y. 10012
Ween
Co., Inc.
CLINICAL CHEMISTRY,Vol. 22, No. 7. 1976
1113