Our PN 3565, 0-100 PSI pressure gauge assembly is used to measure static pressure and/or differential pressure (delta-P) in a system via a Pressure-Temperature (PT) plugs. For static pressure measurement, insert the needle into a PT plug when the system pump is not running. To measure delta-P, ensure that the system pump is running then insert the needle into one PT plug, record the value, and repeat for the second PT plug. The differential pressure between the two measurement locations is the difference between the two values recorded. Geothermal and water source heat pump systems should have heat pump elbows with PT plugs on both the entering and leaving water/fluid connections. By determining the delta-P, which in this case is also known as the pressure drop across the unit’s heat exchanger, you can determine the fluid flow rate. Geothermal heat pump manufacturers publish a table of pressure versus flow rate for various entering water temperatures for each heat pump that they manufacture, so you simply need to look up your pressure drop to determine the flow rate.