pH measurement application

Wasn’t sure what category to select as the root of my issue is currently undetermined.

I have a groov Epic that is getting pH readings from 5 dual channel pH/ORP modules via and EB2.

The groov system has been installed to replace a bunch of individual Jenco 3671 analog units.

I’ve been troubleshooting a long list of independent issues and now have the groov system running fairly well with he exception of the pH readings. I am consistently getting readings from the groov unit that differs by 0.1 to 0.3 from the analog system. We are quite confident that the analog system is giving us the more accurate reading as our process fluid is seawater that has a nominal pH of 8.0 to 8.2 in our locale. The pH of the process fluid as measured by the groov system ranges from 7.7 to 7.9.

On the program side of things I use a simple interpolation-like equation that compares the raw analog output from the pH modules at buffers of 7.0 and 10.0 and the raw process actual reading and compare that with the ideal range to spit out an adjusted pH reading. It’s a standard 2-point calibration technique.

Without going into all the details of what I’ve done to attempt a solution to the slightly off readings I’m looking for some input from anyone with pH measuring experience on what they’ve done, things I should look at to resolve the accuracy issue I appear to have.


I don’t have a lot of experience with pH readings and only have one SNAP pH module which is offline at the moment… but, I just want to double check something here…

You mention groov EPIC a lot and that the error is showing up in the EPIC… I just want to double check that you have double checked the SNAP-PAC-EB2 side of things…

Using PAC Manager, have you looked at the analog module and seen the error in there as well?

Do an inspect of the EB2 IP address, then select Analog Point from the side menu, then select your module number and point number and see what it shows.

I am just wondering if the error is directly from the module, or is sneaking in somewhere else.

EDIT, clearly, this is not a pH module, but an AIMA, but it shows where to look for the raw value.

Thanks for the quick reply.

Although I am able to ping the device I can’t get on with PAC Manager. I am working remote at the moment, I will be on-site tomorrow so will connect directly and check as you’ve suggested - will report findings then.

In PAC Terminal the raw signal coming in is substantially in error, so on that one channel I’m suspecting a faulty probe or an issue with the signal cable.

On all the other channels I am still dealing with what appears to be an offset from the actual. The readings from the analog system are assumed to be correct. I think I will need to get a calibrated hand-held to use as a third independent comparison.

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Great, that’s some good helpful information there.
The pH/ORP modules are calibrated at the factory with a precision voltage source to remove any issues with probe calibration. In other words, they are set to be spot on for the voltage they expect to see.

Getting a hand-held calibrator sounds like a good idea to narrow down were the offset is creeping in.

I was able to get on the modules once directly connected to the PLC. On the default screen for Analog Point there is a default calibration option in the lower right of the screen. A small button is there titled “2 Point Calibration” (where you see the “Default Method” button). This gives the option that is outlined in red which is exactly what we’re doing with our program.

The procedure is to enter theoretical minimum calibration set-point and take the module reading, then do the same for the maximum and take the reading. For us that’s 7.00 and 10.00. Step three is to “calculate offset and gain” and send the settings to the module.

When I initially calibrated the probes the difference in the pH reading on the display was ~0.4, which was on the high end of what we were seeing using a program based calibration method. Then I recalled that the pH modules are set up kind of backwards (see blue-outlined inset taken from the Point Config screen), I think this is because in the module -500mV equates to a pH of 14 and +500mV equates to a pH of zero, so the scale is “min for max”. When I used 10 as the min and 7 for the max the reading being sent to the groov display was ~0.2 off from the analog system. The OPTO system is measuring 7.8 in seawater when calibrated this way, a bit of a conundrum yet as it is known that the pH of seawater varies 8.0 to 8.1.

We’re going to monitor the difference between the analog and OPTO system for a week to see if the offset remains constant.