A diaphragm gas bag, bladder or sleeve, "a membrane", will move up to 15 times per second. 15Hz. When it is hidden from system fluctuations by being "Td" off the line. All the above applications require an in the line, flow through, multiconnection, and in and out port, damper. A flow through damper also intercepts pressure waves, which typically travel at 3500mph. For all those applications below a 15 Hz. frequency, flow through enable: Flush in place before system service. Constant temperature by first in first out. No sedimentation and intercepts the pressure disturbances with make meters and instruments inaccurate.
FOR THESE REASONS, INVENTORY - LOWEST COST DAMPERS - HAVE 2 CONNECTIONS.
For those who do not want the advantages of FLOW THROUGH, save buying a T for drain, RV, or gauge. Use the extra connection for that, and still buy the lease expensive - the flow through damper.