Fieldbus transmitter ranging application, this time a diﬀerential pressure transmitter sensing pressure dropped across an oriﬁce plate in order to infer the rate of ﬂow for ﬂuid inside the pipe. The transmitter senses small amounts of pressure diﬀerence (expressed in a unit of pressure called inches water column), but what we want it to report to the Fieldbus network is an actual ﬂow rate in gallons per minute.
If we happen to know that this oriﬁce plate produces a pressure drop of 125 inches water column (125 ”WC) at a ﬂow rate of 350 gallons per minute (350 GPM), we could set up the scaling parameters as shown:
Note the use of the “indirect square root ” L_Type parameter value instead of just “indirect” as we used in the ethanol tank example. The square root function is necessary in this application because the relationship between diﬀerential pressure (∆P) and ﬂow rate (Q) through an oriﬁce is nonlinear, as described by the following formula:
This particular non-linearity is unique to pressure-based measurements of ﬂuid ﬂow, and does not ﬁnd application in any other form of process measurement.
As before, though, we see a common theme with the XD_Scale and OUT_Scale parameter ranges: we set the XD_Scale parameter to the physical range of measurement directly sensed by the transducer, we set the OUT_Scale parameter to the corresponding range of measurement we wish the transmitter to report to the rest of the control system, and we set L_Type to “indirect” to enable this translation from one range to another.
Also Read : SMART Transmitter Calibration