Principles of Thermometry
Any property of a material which changes with temperature can be used to indicate or measure temperature. For example, the expansion of solids, liquids and gases are all used to make thermometers. Other examples are:
an electrical resistance thermometer, in which the resistance of a length of wire changes with temperature;
a thermocouple thermometer, in which junctions between two wires of different metals generate a voltage when the junctions are at a different temperature;
a pyrometer in which high temperatures are judged by comparing the colour of a hot object with a reference colour scale.
The type of thermometer which is chosen for a particular application will be decided by some of the following points:
Does the thermometer work over the range of temperatures required?
Is the thermometer sensitive enough (Can it detect small enough changes of temperature)
How quickly does the thermometer respond?
How small, portable and convenient is the thermometer?
Can the thermometer give continuous reading and be connected to an electrically operated chart-recorder or warning device
How expensive is it?
To fix a temperature scale on a thermometer we chose two easily obtainable temperatures, the temperature at melting point and at the boiling point. These two temperatures and called the lower and upper fixed point. On the Celsius scale the lower fixed point is the temperature of melting pure ice, also known as the ice point. The ice point is fixed at 0 oC. The upper fixed point is the temperature of the steam just above boiling water and is known as the steam point. The steam point is 100 oC. We ca then divide the temperature range between the two fixed points into number of equal parts called degrees.
Absolute zero is the lowest temperature possible, it is -273 oC or 0 K (kelvin). On the Kelvin scale or the absolute scale, one division is called the Kelvin and is exactly equal to one division or degree on the Celsius scale. Therefore the following holds true
Absolute zero 0 K or -273 oC
Ice point 273 K or 0 oC
Steam point 373 K or +373 oC
Sensitivity of thermometers
A thermometer is sensitive if it can detect small changes in temperature. The range of a thermometer determines the temperatures that can be measured. A clinical thermometer has a range of 35 oC to 42 oC that is suitable for measuring human temperature. It's fine tube give 1/5 or 1/10 degree sensitivity, making it easy to detect small changes in temperature.
Thermocouple
If two different metals are joined in an electric circuit and one wire junction is cold and the other is hot, a small electric current is generated in the circuit. The current increases and the temperature difference between the two junctions increases. So if one junction is kept at a fixed temperature such as 0 oC, the other junction can be used as a small probe to measure temperatures of 0oC. The main advantages of the thermocouple thermometers are:
the wire junction can be very small and needs very little heat to warm it up. This means that it responds very quickly to temperature changes and that it can be used in very small or precise locations.
The output of the thermometer is an electrical signal which can be used to operate electrical equipment capable of giving waring of sudden temperature changes or of keeping continuous records of temperatures. By choosing particular pairs of metal temperatures of up about 1500oC can be measured.
Liquid in glass thermometer
The mercury in glass or alcohol in glass thermometer use the cubical expansion of a liquid to measure the temperature. It is designed so that
The liquid is contained in a thin-walled glass bulb to help conduction of heat through the glass to the liquid
the amount of liquid should be small if a quick response is needed because a small quantity takes less time to warm up
The fine tube should be uniform to give even expansion along it
Making the tube finer increases the sensitivity of the thermometer
The space above the liquid is evacuated during manufacture to prevent a high pressure of the trapped air when the liquid expands a lot
The maximum ranges of liquid in glass thermometers depend on the choice of liquid. Mercury is suitable for most purposes but alcohol can be used at a lower before it freezes.
Mercury thermometers are fairly cheap, very easy to use and are portable, but they cannot be used to record temperatures automatically or electrically.