DP-Transmitter is the secondary measuring element which its sensor converts the sampled representative process variable by taken from a primary element said to be Orifice. Here,
1.)The Transmitters has two pressure ports(HP and LP) across which the DP gets created and then the Transmitter Converts this DP to Electrical signals which is used in Automation.
2.)Orifice creates the DP by Blocking the flow across it. This DP plays the measure role for the measurement. DP varies in square function of Flow. So small change in Flow make the DP big by squaring it.
3.)In your case it is gas (Compressible/ expansive )service, so every surrounding & other process parameters(mainly temp. & Pressure) plays a role in variation of the Flow rate. So calculate the DP at lowest flow rate & lowest temp for LRV and at highest flow rate & highest temp. for URV along with other factors as per the specified formula. Balance all combinations of Flow/temperature will fall between your LRV and URV.
4.) Since the linear range of sensor fall between appx. 35% to 65% of the measuring range , chose LRL and URL such that LRV= LRL+35% of span and URV=URL-35% of Span. Manufacture has to select the transmitter with sensor of measurement range accordingly which will be higher than the LRL/URL. But they will calibrate the instruments as per the LRL and URL in the software of the transmitter which will automatically take care your LRV and URV.