Analogue input % error check

[Conor 4]

Hi all, I am measuring pH and reading via an Analogue input. I have a setpoint that the pH should be, and I want to generate an error when the actual value is 10% below/above the SP. Can anyone point me in the right direction as to how to code this? I am using a SLC500. Thanks, Conor

[Bob O 8]

Error = SP*1.1 or SP*.9. You could place these two values in a LIM instruction or put the SP*1.1 in a GRT and the SP*.9 in a LES.

[Ken Moore 23]

See attached, in this example F8:0 contains your PH setpoint, F8:10 is the scaled value from your analog input. I used two compares, because you may want to know if its high or low also. Ken PH_CHK.pdf