A constant-volume gas thermometer depends only on the properties of an ideal gas. Since it doesn’t change over a wide variety of temperatures, it is used to calibrate thermometers based on other materials. A constant-volume gas thermometer uses a gas-filled bulb of unknown temperature along with a mercury manometer, as shown in Figure 16.

Constant-Volume Gas thermometer functions by the principle that the relationship between pressure and temperature in a system of an ideal gas is linear. Such a thermometer measures temperatures in the range from 0 K to 500 K.

Consider the P-V diagram in Figure 17.

• The pressure of water and ice is measured, that point is 0 °C.
• The pressure of water and steam are again measured while the volume is kept constant, that point is 100 °C.
• Then, connect the two points by a straight line.
• To find the temperature of the substance, first measure the pressure, then draw a perpendicular line from the pressure axis to meet the solid line, then drop a perpendicular line to the temperature axis, as shown in Figure 17. This is the temperature reading of the substance.