DP to Flow Calculation

B

Thread Starter

Bob

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.

Thanks.
 
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.
 
M

Matthew Hyatt

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.
 
G

German Lopez

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
 
S

S.Elavazhagan.

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,
Elavazhagan.S
Engineer-Instrumentation.
 
<p>Use this website www.clabberhead.com 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).

<p>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.
<pre>
%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
</pre>
 
D

Donald J. Jenkinson

This chart says a lot. For example that these dP devices should not be used if you desire to measure at the low end. Above at 20% flow you only have 4% percentage of DP rage applied, and almost no mA output (0.64 mA), almost the noise level. I would not consider that this can measure flow below 30% F.S. This does not count the fact that you may be working with compressible gases which means that pressure/density/Temp variations can change the accuracy of measurement. I use an optical scintillation flow device where none of these variations affect the reading since we are measuring velocity, instead of flow.

>Use this website www.clabberhead.com you will need to
>know the bore of the orifice plate you are measuring the DP
>across. --- Snip ---
<pre>
>%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</pre>
 
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