4-20 mA analogue signal conversion

[vlc]

Hi, I have the following control setup:

For that, I have data recorded for every I/O block, and I am asked to determine the function used by the AIN block to convert from an analogue 4-20 mA signal to a flow value in the range of 0/32 L/m. The plotted data looks like this:

How would you obtain that function?
Thanks for your time

 

[btrueblood]

Start by plotting on logarithmic scales and find the slope of the best fit line, then pursue manual fitting, looking to minimize the errors between data points and your function.
Google "least squares curve fitting"
Or try various functions using the curve fitting routines in Excel.

 

[IRstuff]

Looks like a 5th order polynomial fit could work:

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[waross]

If you offset your 4-20 mA signal by -4mA, and work with a 0-16 mA signal the curve should be a square function of the flow.
I haven't worked much on process controls for many years but back in the day of pneumatic instrumentation and 3-15 PSI air signals, we used a component called a square root extractor ahead the DP transmitter.

Have you check the instrument calibrations?
A properly calibrated instrument should output 4 mA with zero air flow.

If you are working in a lab, that DP cell may last been used for an application with an elevated or depressed zero.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Here is one process:

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!