Help with flow curve controllogix

[Peter Nachtwey]

The equation is a square root function

The obvious thing is to try a polynomial. If there are enough terms the polynomial will be accurate enough. However there is a give away, asterof said the pressure drop was across an orifice and the equation for fluid flow through an orifice is:
Flow=K*sqrt(DeltaPressure)
This makes find K easy as the data is almost too perfect. It looks like
Flow=2600*sqrt(DeltaPressure)
Simple, no AOI or look up function is necessary.

 

[Bob O]

Simple and clean.
Thank you Peter.

 

[asterof]

How did you figure the K factor ?

 

[Bob O]

Flow/sqrt(Delta Pressure) = k
26000/sqrt(100) = k
26000/10 = 2600
k = 2600

I have never used this type of meter but the math appears to work and I trust Peter.

 

[asterof]

Cool most times the meter

Comes with K factor
This time it came with calibrated flow values
no K
Thanks

 

[Peter Nachtwey]

I did what Bob O did an then tried for different pressure drops

I got the same K each time so the equation is a perfect fit which is suspicious. This means the data is generated not measured. Valve vary from the nominal. I suspect this data is for a typical valve but not your valve.

Comes with K factor
This time it came with calibrated flow values
no K
Thanks

You should have asked the manufacturer for the K value since they must obviously have it to generate the table of data you showed.

Now that the exact formula is known it would be interesting to compare the function generators. I wonder what method they use to interpret between the points.

 

[asterof]

data was on the spec sheet from the OEM

The data was supplied on the meters spec sheet that came
with the meter.

 

[Tom Jenkins]

This is a good example of why you should understand the basic physics of the system you are programing. The PLC end is simple - the process is what counts.

In this case if helps to know that the pressure drop through an orifice is proportional to the square of the flow - the data tracks quite well.

 

[Sheldn]

It has been quite a while since I posted a comment, but...
Does the DP transmitter have square root extraction as an option so the 4-20 mA output is proportional to flow rate? If it is an instrument greater than say 10 years old you had to buy a transmitter with this option. Today, it usually is a parameter that can be set...