Thermometers for Every Use

Why do you need to measure temperature? Think of all the ways temperature measurement is important in your life. If you know the temperature outside, you know how to dress to go out. If you know the temperature inside your oven, you know if it is hot enough to cook a pizza. You can probably think of many more examples. Different types of thermometers can be used to measure temperatures. Since solids, liquids, and gases usually expand when heated and contract when cooled, almost any substance could be used in a thermometer. The choice usually depends on the way the thermometer will be used.   

Outdoor Thermometer

One thermometer that you are probably familiar with is the outdoor thermometer. It uses the expansion and contraction of a liquid to measure temperature. The liquid in the bulb (usually ethyl alcohol with colouring added) expands when it is warmed and is forced up the narrow bore on its temperature. The more the liquid cools. It contracts, dropping lower in the bore.
 

Clinical Thermometer

Another thermometer that you are probably familiar with is the clinical thermometer. It also uses expansion and contraction of a liquid to measure temperature. But it must be sensitive to a very small changes in temperature, and able to measure temperatures only within a few degrees of normal body temperatures (37ºC). 
As the liquid is warmed by the patient's body heat, it expands past the constriction to show the patient's temperature. After the thermometer has been removed from the patient's mouth, the liquid cools and contracts. But the liquid cannot move past the constriction; it must be forced back by shaking the thermometer downward.

Thermostat

A thermostat can be used to measure temperature in an appliance, such as a furnace, or in a room. It can also switch appliances on or off at a preset temperature. Thermostats use the expansion and contraction of solids to measure temperature. They contain a strip of two metals (called a bimetallic strip). When heated or cooled, the two metals expand or contract by different amounts, causing the strip to bend. The amount of bending depends on the temperature. It provides a measure of the temperature.

Thermocouple

A thermocouple is a thermometer that doesn't use the expansion or contraction of a solid, a liquid, or a gas to measure temperature. Instead, a thermocouple uses electricity. A thermocouple contains two wires, each of a different metal, that are joined (coupled) together at one end. When two different metals touch each other, a tiny electrical depends on the temperature. The other ends of the wires are connected to a meter that measures electricity. By measuring the amount of electricity that flows through the meter, you can measure the temperature of the metal wires. 

 
The thermocouple is useful as a thermometer for measuring very high temperatures in places where people cannot go, such as inside a kiln or a blast furnace. Also, since the thermocouple creates electricity, it can be connected to a computer to record the temperature. 

Self Check
  1. Describe the different devices that measures temperature. Explain where each device is used. 
  2. a) What kind of temperature-measuring device is best to measure body temperatures? 
  3. b)What kind of temperature measuring device is best to measure very hot temperatures? 

  4. Describe how a thermostat can be used to keep your home at a constant temperature. 
  5. Explain how a thermocouple works. Why can it be connected to a computer? 
 
Journal Entry
Make a list of all different devices that you see in a day that measure temperature. Next to each device, explain how the device works. 
 
 
' From Heat and Temperature, ITP Nelson. © Copyright 1997 
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