Math Formula in CompactLogix

smohamed · Mar 24, 2011

Morning everyone,

We've been given a formula for a relationship between flow and level for a treatment plant. The formula is complex in a math sense but can be solved.

The real question is: Can we put it into a PLC to solve? I am only familiar with a SQT function but this one seems a bit more odd.

Here is the formula:

L = (F/2.5)^2/5 * 12

We are attempting to solve for F (flow) where L = level.

Any ideas? I'm really scratching my head at this point.

Thanks!

rdrast · Mar 24, 2011

Someone check my Algebra...

L = (F/2.5)^2 / 5 * 12

L/12 = (F/2.5)^2 /5

5L/12 = (F/2.5)^2

(5L/12)^2 = F/2.5

(5L/12)^2 * 2.5 = F

bernie_carlton · Mar 24, 2011

To me the formula is unclear. Is

L = (F/2.5)^2/5 * 12

actually

L = ((F/2.5)^2)/5*12

or

L = (F/2.5)^(2/5)*12

rdrast · Mar 24, 2011

/shrug. I just assumed common order of precedence.

smohamed · Mar 24, 2011

OK seems that we just solved for F and used a few compute instructions and a square root function and the result was what we expected.

Thanks for everyone's help.

TConnolly · Mar 24, 2011

First step, lets combine all those constants to a single term.

Like Bernie, I saw some ambiguity about the equation.

Is it L = ((F/2.5)²)/5 * 12

Which can be dramatically simplified L = F² * .384

or is it L = (F/2.5)^(2/5) * 12,

Which can be simplified to L = F^.4 * 8.31773812

From there its pretty simple to write a single CPT statement.

smohamed · Mar 24, 2011

Sorry about the confusion. You are all correct, it can be simplified quite easily. I guess I looked at it initially and wondered if I could actually calculate it.

We ended up getting the results we were looking for.

Thanks again.

Peter Nachtwey · Mar 24, 2011

Use wxMaxima to solve symbolic math problems.

smohamed · Mar 24, 2011

bernie_carlton said:
To me the formula is unclear. Is

L = (F/2.5)^2/5 * 12

actually

L = ((F/2.5)^2)/5*12

or

L = (F/2.5)^(2/5)*12

To close the loop, it was the 3rd equation, I had put it down in error the first time.

L = ((F/2.5)exp(2/5))* 12

Thanks.

TConnolly · Mar 24, 2011

Then L = F^.4 * 8.31773812

and

F = (L/8.31773812)^2.5

Math Formula in CompactLogix

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