Thermometers
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You need to use a thermometer or a temperature sensor if you want to measure
temperature. The table shows some of the different types of thermometers and
temperature sensors available today.
Thermometer
Mercury in glass
Range:
–39 °C to 630 °C
Advantages:
accurate, mercury expands easily and
is easy to see.
Disadvantages:
expensive, dangerous if broken
(mercury is poisonous).
Alcohol in glass
Range:
–117 °C to 79 °C
Advantages:
cheap, can be used in very cold areas.
Disadvantages:
may not be accurate (alcohol sticks
to the glass).
Sensor
Thermistor
This is a resistor and the resistance varies with
temperature. It is used as part of an electric circuit.
Range:
depends on the type of thermistor used.
Advantages:
can give continuous readings, provides
an electrical signal which can be used
by computer equipment.
Disadvantages:
may have limited range, not very accurate.
Thermocouple
Two metals joined in a circuit can generate a small voltage
when there is a difference in temperature between the two ends.
Range:
depends on metals used.
Advantages:
One junction
has to be kept
cheap, gives continuous readings, can
at a constant
measure the temperature of very small objects. temperature.
Disadvantages:
not always very accurate.
V
metal B
metal A
The other
junction is
used to measure
temperature.
Liquid crystal
Some crystals change colour when their temperature changes.
Range:
depends on the types of liquid crystals used.
Advantage:
cheap, easy to read.
Disadvantages:
not very accurate.
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Thermometers (continued)
Different thermometers and sensors measure different temperature ranges. If you
need to measure the temperature, you need to choose the right thermometer or
sensor for the job.
?
1 Many school science departments used to use mercury in glass
thermometers, but now use alcohol in glass instead. Write down two
reasons why you think they changed.
2 An expedition to the Antarctic needs to choose a thermometer to take
with them. Which kind of thermometer would you recommend?
Explain your answer.
3 If you feel ill, your parents or a doctor may take your temperature.
Which kind of thermometer do you think your parents would use?
Explain your answer.
4 Liquid crystals can be made into flexible strips that can be held against
someone’s forehead. Why do you think these thermometers are often used
to take the temperature of young babies.
5 Which kind of sensor could be used to control the temperature inside a
fridge? Explain your answer.
S
knowledge
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