To get flow rate from a DP transmitter assumes that you have a primary flow element mounted in a pipe or duct, something like an orifice plate, an averaging pitot tube (Annubar), or one of several other specialized primary flow elements.
Primary flow elements are designed to produce a certain differential pressure at a certain flow rate as calculated from design information including medium viscosity, density, temperature, upstream static working pressure, and a dozen others.
The manufacturer of the primary flow element provides a "sizing sheet" with the primary flow element that includes all the data used to calculate the flow rate at a given DP. You need that sizing sheet, because that sizing sheet tells you what flow rate 100"w.c. equals for your specific primary flow element.
And you need to know whether the DP transmitter is configured to output the 'raw' DP or whether it extracts the square root of the DP.
And you should be aware that you cannot get SCFM (standard cubic feet per minute, a mass flow measurement) from just a primary flow element and a DP transmitter, unless your static working pressure and temperature never vary from the design standard, which never happens in the real world. 'Standard' cubic per minute is referenced back to STP. Your DP transmitter does not measure gas temperature and the static pressure and compensate for those changes, so your engineering units are CFM (a volumetric measurement), not SCFM (a mass flow measurement).
Dan
