14.2 Temperature Scales

14.2 Measuring Temperature

Thermometric Properties

Thermometric Property

• A thermometric property is physical property that changes in a predictably with temperature

Examples of Thermometric Properties

1. Density of a liquid.
2. Volume of a gas at constant pressure.
3. Resistance of a metal or thermistor
4. E.M.F. of a thermocouple

Thermometers

Thermometer

• A thermometer is a device used to measure temperature.
• A thermometer makes use of a thermometric property of a given material.

1. Liquid-in-glass Thermometer

Liquid-in-glass Thermometer

• Relies on the change in density (expansion) of a liquid (e.g. alcohol or mercury)

Liquid in glass thermometer

2. Resistance Thermometers and Thermistors

Metal Resistance Thermometer (e.g. Platinum)

• Electrical resistance increases linearly with temperature.

Variation of resistance of different metals with temperature

Thermistors (NTC)

• Resistance changes rapidly and non-linearly over a narrow range of temperatures.
• As the thermistor gets hotter, its resistance decreases.

Thermistor Graph

3. Constant Volume Gas Thermometer

Constant Volume Gas Thermometer

• It is a highly precise instrument that measures temperature by observing the change in pressure of a fixed mass of gas kept at a constant volume.
• Directly measures thermodynamic temperature

Gas thermometer

Principle of Operation

The thermometer relies on Gay-Lussac’s Law, which states that for a fixed mass of an ideal gas at constant volume, the pressure is directly proportional to its thermodynamic temperature: $P\propto T$

Therefore, if you can measure the pressure $p$ of the gas, you can determine its absolute temperature $T$: $T=\text{constant}\times p$

Advantages

1. Directly measures thermodynamic temperature (kelvin).
2. Most accurate thermometer.
3. Operates over a wide range of temperatures.
4. Used to calibrate other thermometers.

Disadvantages

1. Large Thermal Mass / Slow Response Time: The large glass bulb and gas inside take a long time to reach thermal equilibrium with their surroundings.
2. Cannot Measure Rapidly Changing Temperatures: Because of the slow response time.
3. Alters the Object’s Temperature: It cannot be directly used to measure the temperature of small objects. The large thermal capacity of the bulb would draw too much thermal energy away from the object.

Important

• Not just any gas can be used. The gas must approximate and ideal gas as closely as possible.
• As such, helium is the most suitable gas.

4. Thermocouple

Thermocouple

• Consists of two dissimilar metals (different metals) joined at two junctions.
• When there is a temperature difference between the two junctions, an e.m.f is produced.
• One junction is used to measure temperature while the other is at a reference temperature, typically $0^{\circ }\mathrm{C}$ (pure melting ice).

Thermocouple Circuit

Advantages

1. Fast response time.
2. Can measure rapidly changing temperatures.
3. Can measure very high temperatures.
4. Small thermal capacity (does not absorb much thermal energy from object being measured).

Disadvantages

1. Requires a reference temperature.
2. Low sensitivity at certain ranges.