7.1.2

Measuring Temperature

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Thermocouples

A thermocouple can measure high and fast-changing temperatures.

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Structure

  • Two different metals are joined at different points into a strip.
  • The temperature is fixed at one point and if the temperature at the other point is different, a voltage is induced.
  • The size of the voltage depends on temperature difference.
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Advantages

  • Thermocouples are made of metal, so they can measure very high temperatures. Unlike thermometers which use ethanol that boils at 78°C.
  • Thermocouples are quick to respond to temperature changes.
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Disadvantages

  • Thermocouples are non-linear.

Liquid-in-glass Thermometers

We usually measure room temperatures using a liquid-in-glass thermometer.

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Structure

  • Thermometers today usually contain alcohol, but they used to contain mercury.
  • A thinly-walled bulb of alcohol (called the reservoir) is attached to a capillary tube with a narrow diameter.
  • When the alcohol expands it is pushed up the capillary tube.
  • The distance the liquid moves is a way of measuring temperature.
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Disadvantages

  • Because glass insulates the bulb of alcohol (reservoir), thermometers can be slow to respond.

Thermistors

Another way of measuring the temperature of an object is to use a thermistor.

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Thermistor

  • A thermistor is a semiconductor with a resistance that changes with its temperature.
  • Usually, increasing temperature decreases the resistance.
  • We can use a thermistor to turn a heater off when a house reaches certain temperatures.
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Advantages

  • Small size
  • Quickly comes into thermal equilibrium with the object being measured.
  • Thermistors are very efficient.
  • Can produce a temperature reading at a distance from the object.
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Disadvantages

  • Thermistors require recalibrating often.
  • Thermistors are non-linear.

Jump to other topics

1Physical Quantities & Units

2Measurement Techniques

3Kinematics

4Dynamics

5Gravitational Fields

6Deformation of Solids

7Thermal Physics

8Oscillations

9Communication

10Electric Fields

11Current Electricity

12Magnetic Fields

13Modern Physics

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