Flush rings are used for diaphragm seals in order to prevent clogging of the instrument connection. They are mounted between the process flange and the diaphragm seal and an added benefit is that they are useful for field calibration of a transmitter.
Flushing ring allows in line rinsing of sediments (solids, oils etc.) from the diaphragm of a flanged pressure (or d/p) transmitter.
Sometimes flushing connections are used to apply known pressure and calibrate transmitter against known pressure hence may be called calibration ring also