Page 19 - Module2_B1_B2_i2_Portrait

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Temperature

Thermometers
A Thermometer is a device that measures temperature or a temperature gradient.

A thermometer has two important elements:

 a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer) in which some
physical change occurs with temperature, and

 some means of converting this physical change into a numerical value (e.g. the visible
scale that is marked on a mercury-in-glass thermometer).

Thermometers are widely used in industry to control and regulate processes, in the study of
weather, in medicine, and in scientific research.

There are various principles by which different thermometers operate. They include the thermal
expansion of solids or liquids with temperature, and the change in pressure of a gas on heating
or cooling. Radiation-type thermometers measure the infrared energy emitted by an object,
allowing measurement of temperature without contact.

Most metals are good conductors of heat and they are solids at room temperature. Mercury is
the only one in liquid state at room temperature. It has high coefficient of expansion. Hence, the
slightest change in temperature is notable when it is used in a thermometer. This is the reason
behind mercury being used in thermometer.

Mercury freezes at approximately -34°C. So for measuring the very low temperatures, alcohol
thermometers are used. The alcohol thermometer is an alternative to the mercury-in-glass
thermometer and has similar functions. Unlike the mercury-in-glass thermometer, the contents
of an alcohol thermometer are less toxic and will evaporate away fairly quickly. An organic liquid
is contained in a glass bulb which is connected to a capillary of the same glass and the end is
sealed with an expansion bulb. The space above the liquid is a mixture of nitrogen and the
vapour of the liquid.

If an alcohol thermometer utilizes a combination of ethyl alcohol, toluene, and pentane, its lower
temperature range may be extended to measure temperatures down to as low as -200°C (-
328°F). However, the measurement temperature range c. -200°C to 78°C, is highly dependent
upon the type of alcohol used.

How Its Made Thermometers
https://youtu.be/SZvAmvGQKEA

Measuring Temperature
https://youtu.be/JKuoQ5FV2c8

Total Training Support Ltd 3-9 Issue 2 – September 2016
© Copyright 2016 Module 2.3 Thermodynamics