Calibration instrument in program

I was given a pH sensor to put in our process tank. There isn’t a transmitter for it but it has a 4-2mA out.

I’m assuming I will just have to put some logic in the program and make a calibration screen on the HMI.

I’m pretty sure I thought of a way to do it. It should be pretty easy to implement a 2 step calibration process. Anyone do this before?

Several questions for you.

What model of plc are you using?

What is your process?

if the ph level has to be within +/- x.x ph, you will need to program the unit using a calibration meter or use the sensor with known ph levels.
if you use calibration screens, You will need to password protect the screens.

A word of caution, using programming screens is ok, but you MUST protect the screens or only have different users log into the system.

you will have to defend your programming and prove your programming is accurate, after all, production never makes a mistake, it's always the plc that's in error.

james

James Mcquade said:

Several questions for you.

What model of plc are you using?

What is your process?

if the ph level has to be within +/- x.x ph, you will need to program the unit using a calibration meter or use the sensor with known ph levels.
if you use calibration screens, You will need to password protect the screens.

A word of caution, using programming screens is ok, but you MUST protect the screens or only have different users log into the system.

you will have to defend your programming and prove your programming is accurate, after all, production never makes a mistake, it's always the plc that's in error.

james

Click to expand...

James, thanks for the input.

This screen will detrimentally be password protected. The service guys will have two known solutions for calibration purposes. Instead of doing it on the transmitter I’m going to attempt to duplicate he process in the HMI/PLC.


Compactlogix & wonderware.

I’m going to find some time and write ST this evening and I’ll post it and get your thoughts.

You might want to read the calibration procedure for the device to see if pH probe calibration can be done 'remotely' through an HMI.

Usually pH calibration involves changing constants inside the device so that the probe properly reports the correct pH value when reading a 'standard' solution. That pH value then is converted to 4-20mA. Internally, this involves 'shifting' slope of the response curve.

The issue with pH is not usually the 4-20mA calibration, it is degradation of the pH probe, which re-calibration brings back into tolerance. So in essence, pH calibration calibrating the probe response to standard solutions, which is done through some keyboard procedure on the pH transmitter/analyzer.

pH calibration might not lend itself to HMI calibration due to the inaccessibility of calibration prompts and feedback.