Hayward OmniLogic - huge pH sensor offset

[nickmick]

I have a new pool (built last June) with an OmniLogic system. It uses the "sense and dispense" to record pH and ORP, but it doesn't actually "dispense" anything (aside from the ORP controlling the SWG).

Last year the pH sensor reading I got from the OmniLogic system seemed to be dead on with the Pentair Rainbow test kit I was provided. This year it seemed to be drifting, and I woke up this morning and it read 7.7 after it was reading 7.5 yesterday. I just tested it now with the Rainbow kit and it indicated 7.2

I broke out an old Sensorex SAM-1 pH probe I used to use when I was doing a bit of homebrewing. It's an older probe, and the calibration solutions I have are out of date - but after cal-ing and testing I got 7.19.

So, is it possible for that Hayward pH probe to be off by a whole 0.5? That seems like a lot to me. At this point I probably trust the Pentair Rainbow kit more than anything.

 

[ajw22]

When did you last clean or calibrate the pH probe?

 

[nickmick]

I suppose I have to talk to the builder. I would have assumed he did this when they opened the pool.

 

[nickmick]

Thoroughly cleaned the probe and it dropped by 0.1. Still 0.4 off.

 

[ajw22]

Thoroughly cleaned the probe and it dropped by 0.1. Still 0.4 off.

Use the calibration function.

 

[nickmick]

I’m not sure what you mean by that. As far as I can tell, there is no way to “calibrate” (use reference buffers to generate a new slope and offset) this system. Best I seem to be able to do is to just manually offset it.

 

[ajw22]

I’m not sure what you mean by that. As far as I can tell, there is no way to “calibrate” (use reference buffers to generate a new slope and offset) this system. Best I seem to be able to do is to just manually offset it.

I mean the calibration procedure described in the manual and that I put in post#2

Yup manually offset it to get a closer value...

 

[nickmick]

I guess that’s the crux of my problem. I have two values, and when you have two values, you have none.

I have no idea what to “trust”. Are the reagent kits golden?

Hayward claims this system is “instrument grade”. Do I trust that over the reagent kits? Do I go buy some calibration buffers and test against that?

To be off by so much is just concerning. I can manually offset but offset against what? Also none of this affects the slope…?

Also I should mention that I’m a bit neurotic in that unless I can be certain of NIST-grade measurements I won’t be happy.

 

[ajw22]

Rainbow kit said 7.2. Your Sensorex SAM-1 pH probe said 7.19. Those two votes say you set the S&D probe to 7.2. Then see how the S&D reading tracks your other tests.

Or you can replace the S&D pH probe and see if it gives a better reading.

We think those systems are more trouble then they are worth, for the reasons you are seeing, and don't recommend them.

 

[nickmick]

I bought some fresh calibration solution and confirmed the pH sensor is off by 0.5. Why even have it if it’s going to be so unreliable?

 

[ajw22]

I bought some fresh calibration solution and confirmed the pH sensor is off by 0.5. Why even have it if it’s going to be so unreliable?

Now you understand why we don’t recommend them.