The Mark 53 maintains a constant differential between the inlet pressure and the pressure loaded on top of the diaphragm. It can be used for both differential and remotely adjusted applications.
The Mark 63 is designed to maintain a constant differential between the pressure on the discharge side of the regulator and the signal pressure loaded on the diaphragm. The Mark 64 provides the same flow capacity as the Mark 63, but with less offset in controlled pressure due to a larger diaphragm.