The high and low values of pressure applied to the diaphragm to produce rated travel with stated conditions in the valve body. The “stated conditions” referred to here means the actual pressure drops at operating conditions.
Example: A control valve may have an inherent diaphragm pressure or bench set of 8 to 15 psig. But when subjected to a 600 psig. Inlet pressure, it may start to open at 3 psig and be full open at 15 psig.
It is because of the forces acting on the valve plug and the direction of flow through the valve (flow-to-open or flow-to-close) that the installed diaphragm pressure will differ from the inherent diaphragm pressure.