Answer / carl_ellis

The situation must be an installed, running system, with a receiver that displays a flow rate from the DP pressure signal, because no one just has a DP transmitter across an orifice plate for fun.

Every differential pressure primary flow element is assumed to be zero flow at zero DP. So the DP low side pressure 'zero' will always be zero DP, or when the 3 valve manifold has its process connection valves closed and the equalizing valve open.

The measured DP represents some flow rate on the square rooted curve of the normalized percentage of span.

In order to determine the DP span value, you measure the actual DP and note what the flow rate reads on the receiver (PLC/DCS/recorder/indicator/controller) at that DP.
Then you can calculate what the DP span value is for the max flow rate that receiver is configured for.

The receiver has some configured flow range: 0-500gpm? 0-1,000 bpd? 0-650pm? Find out what it is.

For example, say you measure a DP value of 700mm wc and that 700 mm wc shows a flow rate reading of 264.5 lpm on the receiver.

You have discovered that the receiver is scaled 0-500 lpm for that DP transmitter and the receiver does the square root extraction.

What is the flow rate as percentage of flow span?
264.5 lpm/500 lpm = 52.9% of flow rate span

52.9% normalized is 0.529

Since the flow rate is the square root of the normalized DP span, you need to square the normalized percentage of flow rate to find the equivalent percentage of DP span.

0.529^2 = 0.2798, or 27.98% (~ 28%) of the DP span.
If 700mm is 28% of span, 100% of span is 2500mm wc.

So the DP span for the transmitter is 0 (no flow) to 2500mm wc (max flow).

Carl Ellis
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