Principles of Thermometry

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?

Upper and Lower Fixed Points

To set 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 can 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

To convert degree Celcius to Kelvin, use the following equation

Kelvin = degree celcius + 273

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.

Types of Thermometers

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 as 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 greater than 0 ^oC.

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 warning of sudden temperature changes or of keeping continuous records of temperatures.

  • By choosing particular pairs of metal temperatures of up about 1500^oC 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.

Design features

• • 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 is 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 temperature 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.

Clinical thermometers have a constriction just above the bulb so that it prevents the mercury in the stem from flowing back into the bulb before a reading is taken. They have a shorter range (35 ^oC - 42 ^oC) compared to a laboratory thermometer (which may range from -10 ^oC to 110 ^oC) because it is not necessary to have a thermometer to measure a human temperature of 80 ^oC.

Pyrometer

https://en.wikipedia.org/wiki/Pyrometer

A pyrometer is a type of remote-sensing thermometer that uses colour to determine the temperature. The temperature of a surface is determined from the spectrum of the thermal radiation it emits.

Extra reading

http://www.explainthatstuff.com/how-pyrometers-work.html