Thermometers and Thermometric Properties
Thermometers and Thermometric Properties
First let the thermometer reach the object temperature. Then convert a measurable property into a temperature reading.
The thermometer must match the object
A thermometer measures its own temperature. Use its reading for the object only after the two have reached thermal equilibrium.
A small sensor has a small . It needs little energy to change temperature, so it responds quickly and changes the object temperature less.
Choose a property that changes with temperature
This measurable change is called a : a physical property that changes with temperature and can be measured.
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| Temperature-measuring method | Thermometric property | What is fixed or known |
|---|---|---|
| Liquid-density method | density of a chosen liquid | liquid and pressure are known |
| Constant-pressure gas thermometer | volume of the gas | pressure is constant |
| Resistance thermometer | electrical resistance of a metal | sensor material |
| Thermocouple | e.m.f. from two different metals | reference-junction temperature |
- A useful property changes by a measurable amount. It gives one clear property value for each temperature in the range you need.
- The change does not have to be . If the graph is not a straight line, use the calibration curve. Do not assume equal scale intervals.
Calibration connects a reading to temperature
means recording the property at known temperatures. The resulting table or graph is then used to find an unknown temperature.
Worked example
Use a linear calibration
A resistance thermometer reads 50 Ω at 0 °C and 70 Ω at 100 °C. It now reads 60 Ω. Find the temperature.
The new reading is halfway between 50 Ω and 70 Ω, so the temperature is halfway between 0 °C and 100 °C.
Why a thermocouple can respond quickly
A thermocouple uses the e.m.f. produced by two different metals as its thermometric property. The e.m.f. depends on the temperature difference between its measuring junction and a reference junction.
A small measuring junction has a small thermal capacity, so it reaches the object temperature quickly. This gives a short response time (9702/41/M/J/24 Q2(b)(ii)–(iii)).