Calibration Offset

I have a slc 5/04 plc and I am using the SCP instruction to scale a 4-20 ma analog value for my machine.

The sensor is about 10 LBS off and i would like to adda calibration offset to the output.

My output is N7:50 so on a rung below can I just add a number to N7:50 then move the result with the move instruction to my control word N7:60?

I am would think I could add a positive number if the PSI was low and add a negative number if the PSI was high? Will this work?

I there a better way I should try?

Have you considered calibrating the instrument?

Yes but there is no way to adjust the zero and span. Closed unit psi tranducer. Sealed.

Why not just change the scaling right at the SCP instruction?

FWIW, I usually use word addresses instead of constants for the EU min and max values on an SCP - word address values can be changed online without doing an online programming change, by an HMI, or by the PLC program. Changing the EU Min/Max of an SCP helps you avoid any non-linearity introduced by using an offset only. For example, what if your zero is ok but the span drifts? Adding an offset helps accuracy at the high end but then low end accuracy suffers. If the zero drifts then you reverse the problem. If you have the option to change the EU Min/Max of the SCP then you can compensate not only for changes in calibration end points, but also changes in the line slope that are inevitable as the end points change.

I am would think I could add a positive number if the PSI was low and add a negative number if the PSI was high? Will this work?

Click to expand...

Sure, should work.

What are you doing with the results? Some instructions, math operations don't like negative numbers.

Tim Ganz,

in this case, i would input 4ma to the input to get the raw data number and then apply 20 ma for the raw data number.
then input these numbers into the SCP instruction.

while you can enter the data in registers for the scp inctruction, we have made it a policy to never do that. it's
too easy for someone to change the calibration numbers and foul up everything. hard coding requires a pc, programming software, and the ability to make those changes.

i don't know your application but i have found it way to tempting for supervisors to change the parameters just so production can run, in the mean time, your customer suffers due to bad product and your company risks loosing the contract.

regards,
james