from the Automation List department...
DP to Flow Calculation
Sensor technologies. topic
Posted by Bob on 9 September, 2004 - 12:31 pm
Does anyone have the calculation to convert a differential pressure transmitter to flow? I am reading in some 4-20 mA signals to a PLC from Rosemount DP transmitters and need to convert them to flow rate in GPM. The fluid is plain water at approx 55 deg F.


Posted by Dan on 10 September, 2004 - 1:46 am
Most PLCS have a scaling function built in. It depends on your brand of PLC, but most have the 4-20 mA range as number of counts. The formula would be something like

EU = Input Counts x (( Max counts - Min Counts)/Range)

You will need to check to see if the DP uses an internal Square Root Extraction, or if you need to do this prior to the scaling calculation.

Posted by Matthew Hyatt on 10 September, 2004 - 5:19 pm
The Rosemont should be able to be scaled for a process value of gpm based upon the pressure.

Otherwise, need to know orifice size to perform the calculations, and why use a DP meter when a mag meter would be far better and requires less effort ot install, setup and calibrate.

Posted by matthew hyatt on 11 September, 2004 - 1:11 am
drop me a note off line.

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Posted by S.Elavazhagan. on 5 October, 2004 - 8:01 pm
First tell us what type of flow element (primary) is used. The DP to Flow calcualtion varies on flow element also indicate operating parameters.

with best regards,

Posted by German Lopez on 12 September, 2004 - 10:18 am
As your message say aprox 55 degf it looks that you are just wanting an "estimate" of the flow
what is the source of the differential pressure?
oriffice plate? or column weight?
you might laugh but probably the most "accurate" factor in your basic conditions is just measure 55 gallons and get a factor and that's it

remember that you need to get sure that the dp and the transmitter are in the right range because you could be on 20 Ma or close to 4 all the time,just because of the right selection of the dp source

if you plan something more sophy call me 661 665 0999
German Lopez

Posted by Roley on 17 September, 2004 - 12:18 am
Rosemounts new model 3051S has a scaled variable option in the xmtr. Hope this helps!

follow this link:


1 out of 1 members thought this post was helpful...
Posted by Tom on 7 December, 2004 - 4:49 pm

Use this website you will need to know the bore of the orifice plate you are measuring the DP across. The internal diameter of the pipe, you will find this from ppipe table by comparing it to the pipe specification, normally marked on the PID's. Basically the DP is equal to the (square of the mass flow)/((square of (pipe CSA X square of beta ratio))x density).

now we assume density, internal area and beta ratio are all constants then flow squared is proportional to DP.
so to get linear flow reading we must sqare root DP either at the transmitter or ar the PLC but not at both.
use table below for quick calibration of a transmitter with square root extraction.

%flow percentage 0f DP rage applied mA signal
0 0 4
10 1 4.16
20 4 4.64
30 9 5.44
40 16 6.56
50 25 8
60 36 9.76
70 49 11.84
80 64 14.24
90 81 16.96
100 100 20

Posted by James Havenga on 20 May, 2010 - 2:44 pm
There is a good Excel Add-In available from Vengatech at

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