The change of temperature (T) with a decrease of pressure (P) at constant enthalpy (H) in a Joule-Thomson process is the Joule-Thomson coefficient denoted as and may be expressed as:

$$μ_{JT} = (\frac{δT}{δP})_H$$

The value of is typically expressed in K/Pa or °C/bar and depends on the specific gas, as well as the temperature and pressure of the gas before expansion.

Significance:

• It tells how close the gas is to ideal gas behaviour.
• It is an important consideration in the selection of refrigerants, since the best refrigerants have the greatest negative Joule-Thomson coefficient under the conditions selected.