Author: John M. Cimbala, Penn State University
Latest revision: 14 November 2007 [minor modification 09 April 2008]
There are four basic types of temperature measuring devices, each of which uses a different principle:
(liquid-in-glass thermometers, bimetallic strips, etc.).
(RTDs and thermistors).
(infrared and optical pyrometers).
Each of these is defined and discussed in this learning module, with most of the emphasis placed on
thermojunctive temperature measuring devices – thermocouples.
Mechanical temperature measuring devices
Principle of operation
A change in temperature causes some kind of mechanical motion, typically due to the fact that
materials expand with a rise in temperature
. Mechanical thermometers can be constructed that use
liquids, solids, or even gases as the temperature-sensitive material.
The mechanical motion is read on a physical scale to infer the temperature.
The most common and well-known thermometer is the
As the temperature rises, the liquid expands, moving up the tube. The scale is calibrated to
read temperature directly.
Usually, mercury or some kind of alcohol is used for the liquid.
Bimetallic strip thermometer
Two dissimilar metals are bonded together into what is called a
, as sketched to the right.
Suppose metal A has a smaller coefficient of thermal expansion than does metal B.
As temperature increases, metal B expands more than does metal
A, causing the bimetallic strip to curl upwards as sketched.
One common application of bimetallic strips is in home
thermostats, where a bimetallic strip is used as the arm of a
switch between electrical contacts. As the room temperature changes, the bimetallic strip bends as
discussed above. When the bimetallic strip bends far enough, it makes contact with electrical leads that
turn the heat or air conditioning on or off.
Another common application is for use as oven thermometers or wood burner thermometers. These
thermometers consist of a bimetallic strip wound up in a
, attached to a dial that is calibrated into a
, while still considered mechanical,
operates by the expansion of a gas instead of a liquid or s
: There are also pressure thermometers that use a
instead of a gas.)
Suppose the gas inside the bulb and tube can be considered
an ideal gas. The ideal gas law is
is the pressure,
is the volume of the gas,
is the mass of the gas,
is the gas
constant for the specific gas (not the universal gas constant), and
is the absolute temperature of the gas.