-c- Measuring Temperature

Measuring Temperature

Mercury Thermometer

Mercury Thermometer “Explaining”

calibrating a thermometer

Mercury Thermometer “How it works?”

Thermometric properties

Temperature can be measured by using any physical properties that vary with temperature. Such properties are called thermometric properties. Examples of thermometric properties are

  1. volume of a liquid – liquid-in-glass thermometer
  2. length of solid – bimetallic strip
  3. gas pressure – constant volume gas thermometer
  4. electrical resistance – resistance of a platinum resistor
  5. electromotive force – e.m.f of a thermocouple when the junctions are placed in different temperature

When a liquid is heated, it expands. This expansion may be related to the temperature of the liquid. If the volume of the liquid is calibrated with the temperature, then by using the existing scale, one may obtain the temperature by reading the volume of liquid. All gases exert a pressure on the wall of its container. At certain temperature, the gas exerts a certain pressure. At another temperature, it exerts another pressure. By calibrating using a known lower and upper fixed points, a scale relating pressure to temperature may be obtained. Hence, by reading off the pressure of the gas, the temperature may be obtained. Other thermometric properties may also be utilised using the same method as described above.

CALIBRATING A TEMPERATURE SCALE

A temperature scale is calibrated by choosing two fixed points, usually the lower fixed point of melting ice (0 °C) and the upper fixed point of boiling water(100 °C). Firstly, the thermometer is placed in a beaker of melting ice. Mark the reading on the thermometer as 0 °C. Next, place the thermometer in boiling water and mark this reading as 100 °C. Divide the two readings into 100 equally spaced units, each corresponding to 1 °C change. After getting the scale, we can put the thermometer into the system which the temperature is to be measured. By reading the thermometric property at that temperature, we can interpolate the temperature according to the scale obtained earlier.

CHARACTERISTICS OF A THERMOMETER

All thermometers have limitations. In general, there are three characteristics concerning a thermometer. Sensitivity Sensitivity refers to the ability to give a large response to a small change in temperature. A sensitive thermometer is able to detect small changes in temperature. It can also give a rapid response to temperature change. To make a liquid-in-glass thermometer  sensitive, a large bulb is used. Large bulb will cause a big change in volume of the mercury, which will appear as a change in the length of mercury up the capillary tube. Making the capillary tube small also increase the sensitivity of the thermometer because volume change results in a big change in the length of liquid up the tube. Lastly, a liquid-in-glass thermometer may increase its sensitivity by choosing a liquid that expand more. Alcohol expands more than mercury, and would make a thermometer more sensitive than a mercury-in-glass thermometer. Linearity A temperature scale is calibrated using two fixed points. Between these two fixed points, 100 equal divisions are marked to represent temperature change of 100 °C. To obtain the temperature, one read off the scale using the interpolated divisions. This assumes that the thermometric property changes linearly between the two fixed points. In actual case, this is not true. Different materials change their thermometric properties differently at different temperatures. Hence, a good thermometer should have thermometric property that changes linearly in between the two fixed points such that the thermometric property at a particular temperature corresponding to the reading on the interpolated scale. Range Range refers to the operating temperature which the thermometer can be used. A laboratory thermometer can measure from -10 °C to 110 °C, beyond which the scale will not be able to register any readings. The expanding liquid column may even break the thermometer if the expansion is restrained beyond the maximum 110 °C. A thermocouple which uses two metal junctions can have a large range from -200 °C to over 1000 °C. This make thermocouple a very versatile thermometer and is suitable in many situations.

 Thermocouple

Thermocouple is a very common electrical thermometer used widely in industry. Often, two different metals, copper and constantan are used. Advantages of a thermocouple are

  • since it is electrical, it can be read from a remote site.
  • a thermocouple can be made very small. Hence, it can respond to changes in temperature very fast.

A thermocouple works because when two different metals are in contact, they will generate a tiny electromotive force, which can be read by a sensitive voltmeter. In order to measure this voltage, the wire need to form a closed circuit, forming two junctions. No voltage is produced when the two junctions are placed at the same temperature. However, if one junction is placed at the lower fixed point and the other junction is placed at the temperature to be measured, then the generated e.m.f.can be read as the temperature.

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