UY1: Measurement Of Temperature

If a physical property varies with temperature, you can use it to set up a scale to measure temperature. Such a property is a thermometric property.

The device that thus measure temperature is called a thermometer.

Thermometric PropertyThermometer
Volume expansion of a gasGas thermometer
Volume expansion of a liquidLaboratory or clinical thermometer
Volume expansion of a solidBi-metallic strip thermometer
Pressure change of a fixed mass of gasConstant – volume gas thermometer
Changes in e.m.f.Thermocouple
Changes in electrical resistanceResistance thermometer or thermistor
Changes in wavelength Pyrometer
Changes in magnetic flux or sound energy Magnetic or sonic thermometer

1) Liquid-In-Bore Thermometer (invented around 1654)

Two Common types:
– Mercury (Melting Point: -39°C, Boiling Point: 357°C)
– Coloured Alcohol: Ethyl Alcohol (Melting Point: -114°C, Boiling Point: 78°C), Methyl Alcohol (Melting Point: -98°C, Boiling Point: 65°C)

How It Works:
The volume of mercury or alcohol in the bulb varies with temperature according to the volume expansion coefficient (β). This volume is read by the position of the liquid meniscus inside a tiny capillary bore.

Calibration typically at two points only:
(i) ice point of water (0°C)
(ii) steam point of water (100°C)

The rest of the scale is then constructed by interpolation.

General Problem:
The discrepancies between thermometers can be large for temperatures far away from their calibration points.

(2) Bimetallic-strip Thermometer

bimetallic strip

Common Types: Invar (Ni-Fe) and steel.

How it works:
The bimetallic strip has two different metal strips welded together. The two metal strips have different thermal expansion coefficients (ɑ). Therefore the bimetallic strip bends as the temperature changes because the two metals expand/contract by different amounts.

(3) Thermocouple

How It Works:
A junction with two dissimilar metals in contact generates a voltage called the thermoelectric voltage which varies with temperature according to the Seebeck coefficient. The thermocouple system has at least two junctions. One junction is the probe junction, while the other junction is the cold junction to be kept at a reference temperature. The voltage difference between these junctions is given by ΔV = SΔT, where S is the Seebeck coefficient. In practical measurement circuits, it is not convenient to use a fixed reference temperature. Hence, a cold-junction compensation is applied by direction sensing the cold-junction temperature by another device, such as a thermistor. Since the probe junction is small and can have a very small thermal mass, it can follow temperature changes rapidly and come to equilibrium quickly.


(4) Resistance temperature detector (RTD) and thermistor

Common Types: Platinum RTD, Si thermistor

How it works:
The electrical resistance of metals (semiconductors) increases (decreases) with temperature. The resistance of platinum changes more uniformly thean most metals and can be used to measure temperatures accurately in the range of -260°C to 630°C. The resistance of semiconductors on the other hand can change by orders of magnitude over small temperature ranges. The resistance is measured by passing a fixed current through it and measuring the voltage drop.

– Very sensitive at low temperatures
– Not very sensitive at high temperatures

(5) Radiation thermometer (IR thermometer)

Common Types:

How It Works:
A radiation thermometer consists of an optical system and detector. The optical system focuses the energy emitted by an object at a distance onto a detector which is sensitive to the radiation. The output of the detector is proportional to the amount of energy radiated by the target object, which is related to the object’s temperature. Infrared radiation thermometers measures in the 0.7 to 20 micrometer wavelength range.

(6) The optical pyrometer

How It Works:
An optical pyrometer consists a telescope, a red glass filter and a small electric lamp bulb connected in series with a variable resistor, an ammeter and a battery (or power supply). The lamp bulb is incorporated into the telescope at its focus so that the filament of the lamp appears superimposed on the image of the hot object whose temperature is to be measured. The current through the filament is adjusted by varying the resistance until it is neither darker (cooler) than the hot object, nor brighter (hotter). Then the filament is of the same temperature as the distant object.

Since no part of the instrument needs to come into contact with the hot object, the optical pyrometer may be used for very high temperature (above 1300°C) measurements.

(7) The constant-volume gas thermometer

Mainly of historic interest. (Extensive studies around 1780 by J. Charles)

How It Works:
The pressure of a fixed volume gas varies directly with temperature according to the gas constant (which is independent of the nature of the gas). The volume of the gas is kept constant by raising or lowering the reservoir B to keep the mercury level at A constant.

Experimental indication of absolute zero
The pressure in the gas thermometer increases linearly with temperature. When this pressure is extrapolated to zero, a universal temperature is obtained independent of the gas used. This corresponds to -273.15°C, and is called absolute zero.
This experiment finding is significant because it indicates that there is a true zero in temperature. Therefore, you only need to define one other fixed point to define the temperature scale. This eventually gave rise to thermodynamic temperature scale.

Next: Temperature Scales

Previous: The zeroth law of thermodynamics

Back To Thermodynamics

Back To Thermodynamics (University Year 1 Physics)

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