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Flow calculation
Formula for Square root extraction for doing in PLC for flow measurement.

Please let me know the formula for Square root extraction for doing in PLC for flow measurement.

Flow range: 0 -928 m3/hr
Calibration range: 0 - 2500 mmwc


There is a simple formula that could work for the calculation of flow in your situation.


K is a proportionality constant that will take into (during the original design) the effects of units, temperature, expansion factors, density, etc.

If the original data has changed significantly the formula may not work.

You use the original design capacity of what (Q) and (Hw) is at maximum flow then calculate K.


Once you have K the process is quite easy to calculate (SQRT(Hw)) -- providing your PLC can accomidate the function ((X/Y)^(Z)).

Set Y to 1

Set X to the value of Hw

Set Z to 0.5

Then calculate the flow using Q=K(SQRT(Hw)).


By Gopal Krishan on 26 June, 2007 - 5:19 pm

Hello Sumit,

The simple formula for flow calculation is
= Transmitter range in eng unit x Sqrt (Diffrential pressure Field I/P in mmWC/DP range in mmWC).

In your case:
Transmiter range = 928 m3/hr
DP range = 2500 mmWC

Thus, the formula will be in your case:
Calculated Flow in m3/hr= 928 X Sqrt (Diffrential pressure Input/2500)

So you can calculate using apprprite multiplier, divider block, etc. easily.



This reply was very helpful.I have one more question. Is it possible to find out that how much DP should i apply to getout put equivalent to 25%,50% & 75% flow for the same transmitter having a flow range of 928 m3/hr & DP range of 2500 mmH2O? it will be very helpful while calibrating the transmitter.