The pressure instrument is direct mounted or through a capillary line to the diaphragm seal. The complete volume of the diaphragm seal, the capillary and the measuring system, is filled under high vacuum with the appropriate filling fluid.
The process pressure applied will be transferred through the diaphragm to the measuring system. The volume displacement has to be sufficient to steer the sensor of the transmitter or the bourdon tube of a pressure gauge.
For an accurate measurement the relation between diaphragm diameter and volume displacement is important.