Explanation of Temperature Measurement: Thermometers - Succeed in Physical Science. Also refer to Physics, temperature, heat transfer, thermal energy, expansion, electrical resistance, color changes, physical properties, calibration, Ron Kurtus, School for Champions. Copyright © Restrictions
Temperature Measurement: Thermometers
by Ron Kurtus (13 December 2006)
Temperature is the average kinetic energy of a material's molecules or its average thermal energy. It is measured with a thermometer. Heat is transferred to the thermometer, causing changes in its physical properties. Thermometers can be classified according to the physical principles they employ to measure temperature, such as expansion, electrical resistance and color changes. Thermometers must be calibrated to be able to indicate accurate temperatures.
Questions you may have include:
- How do expansion thermometers work?
- What are other types of thermometers?
- How do you calibrate a thermometer?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
Expansion thermometers
Most thermometers work on the principle that materials expand and contract with temperature.
Liquid
By putting a liquid in a thin tube, you can readily see its expansion with increasing temperature.
You can put some colored water in a glass or plastic tube with one end closed. The water will move up or down the tube as the temperature changes. The thinner the tube, the greater the movement.
The problem with a water thermometer is measuring temperatures below the freezing point of water.
At one time silver-colored Mercury was used. It is the most accurate, but since it is dangerous to the environment it is only used today by physicians.
The red liquid in many thermometers is colored alcohol, which does not freeze until around -90 degrees F.
Bimetal thermometer
Another popular device is the bimetal thermometer that is used in many home thermostats. It consists of a copper and a steel strip of metal attached to each other. Since copper expands faster than steel, it causes the strip to bend. When it bends, it can make an electrical contact to start up your furnace.
Usually, these bimetal strips are in a coil. If you have an opportunity, take off the cover of your thermostat and observe the curved metal strip.
Other types
The newest type of thermometers are electronic. They often use electrical resistance to measure temperature and show the degrees in digital numbers.
There are also thermometers that have a chemical that changes colors with temperature.
Calibration
Picking your two end points, such as the freezing and boiling points of water, you can calibrate the thermometer and set the degree units.
Expansion type
The assumption is that the expansion of the liquid or metal is relatively linear. In other words, it doesn't expand real much for one degree change at a high temperature as compared to a low temperature. If it is linear, you can have all the units on your scale the same size.
Note that the glass or plastic also expands or contracts with the change in temperature. But the expansion is relatively small, so it does not affect the readings.
Other types
Calibrating electrical or electronic thermometers is relatively easy since changes are linear.
Thermometers that use color changes do not measure temperatures accurately, but they must be calibrated for temperature ranges.
Summary
The temperature of an object is the average energy of its molecules. The fact that materials expand when heated resulted in the invention of the thermometer. There are three commonly used systems for measuring temperature: Fahrenheit, Celsius, and Kelvin.
Measure your capabilities
Resources
The following resources provide information on this subject:
Websites
Books
Top-rated books on Thermometers
Mini-quiz to check your understanding
1. How can the expansion of a liquid be amplified?
2. Why does a bimetal thermometer element bend?
3. How is a thermometer calibrated?
If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.
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Temperature Measurement: Thermometers