Using manometers with fluctuating pressures.

Don’t is the best advice. Sometimes the pressure in a pipe will fluctuate as a result of pumping. Then a manometer connected to a measuring device will produce a reading that might appear to indicate flow but, in fact, because of the poor time-response of the manometer, that reading could be wildly inaccurate. As the connecting pipes cannot be  a part of any standard there can be no panacea for this. However an electronically operated gauge could be attached in a standard way and read remotely and it is conceivable that it could have some averaging circuitry that could produce an accurate measurement of flow.