Wildly different pH readings...help?

[convan23]

I have two ways to test pH:

1. Standard Taylor test kit from TFTestKits
2. Aptera ph20 digital (originally purchased b/c I was having trouble differentiating colors on the Taylor kit and to get more accurate high/low readings during startup as our startup "guy" was all over the place)

The digital tester has been working great since the pool was filled in October and readings seemed to be in line with the Taylor drop kit. It's a lot quicker and easier to use too. I have re-calibrated it once or twice since receiving it and has been working great.

Over the last couple of days (when our pool water started dropping temperature - now down to the low to mid 50s water temp) I've noticed that the digital tester has come up with wildly different readings then the Taylor kit. Originally it was testing very high - 8.0-8.2 when I knew the water was more like a 7.6-7.8. Then I re-callibrated it last night using the included solutions and it's consistently reading the water at a much lower pH then the Taylor Kit. I've let my sample pool water come up to room temperature to confirm that it's not just the temperature causing issues.

See below photos. All 4 photos were taken with the pool water and sample solutions at the same temperature. To me, it looks like the Taylor Kit is showing a fairly high pH (8.0-8.2), while the Aptera tester is showing 7.4. It is reading both the 7.0 and 4.0 solution correctly.

Any ideas on what could cause this?

 

[mknauss]

You just added borax which raises pH. Why did you do that?

 

[kul]

i use both the Tf100 and a digital meter I bought at the same time as my kit. My digital meter is always off by 0.3 and I am out of calibrating solution so I just subtract the .3 each time.
I’ve had similar inconsistency between the test kit and digital meter - and it turned out to be that I had not thoroughly shook out the digital meter. Once I did that, it helped resolve my issue. Maybe try a thorough drying or reset the meter (remove battery and put it back) and start over?

 

[convan23]

@mknauss I think that's unrelated, but for these reasons...please let me know if this is incorrect thinking:

1. I had my pH settled at around 7.6 over the last week or so when my pool water was heated. This was maintaining a neutral CSI at my 80-90 degree heated water
2. It got very cold here, my water temp dropped to the 50s (I stopped heating the pool). The pH was staying at 7.6 and my CSI had creeped very negative.
3. Raising my pH to 7.8-8.0 will neutralize my CSI down to almost zero. Pool Math calculated 7.6 to 7.9 pH raise needed 20oz of Borax
4. My pool is getting drained next week for a re-polish so not worth it (to me) to worry about adjusting other things like CH to help with CSI

 

[mknauss]

The chemical adjustment of pH is not a precise process. So you adding borax raised your pH.

I trust the drop test before any probe. As long as your FC is 10 ppm or less, and you are using a proper pH test kit that is designed for up to 10 ppm FC, then it is correct.

 

[convan23]

The chemical adjustment of pH is not a precise process. So you adding borax raised your pH.

Correct. My pH was reading 7.6 (drop test was showing it might've been a little lower then that, hard to tell) and I added realistically only like 16oz of borax (pool math wanted 20, which is what was logged, but I didn't add quite that much). I wanted to raise pH a little bit to help with CSI. I had a little borax on hand so I dumped it in. Is that wrong to do?

I trust the drop test before any probe.

@mknauss, I agree. I'm not doubting the drop test, I'm frustrated that a relatively new tool is seemingly inaccurate with only my pool water - my tap water, the tester solution, etc are all testing right on par. It's reading my tap water at 8.0 pH which is what the drop test reads it at. They are ONLY varying with my pool water... and I'd love to know why

 

[Newdude]

Hey Convan !! If it helps any going forward, I hold the block so 2 fingers block out the values above and below what I want to compare. It’s still subjective but my feeble brain has an easier deciding if one color matches at a time instead of looking at 3.

 

[mknauss]

Is that wrong to do?

No. But read the fine print on adjusting pH --

Note: pH calculations depend on TA and Borate. Results are approximate and can be off significantly for large pH changes. Changing your pH will also change your TA. Users should be very mindful of their CSI when their pH is at the top of bottom of the suggested ranges.

 

[mknauss]

They are ONLY varying with my pool water... and I'd love to know why

No idea. The drop test in only really effected by high FC levels. The probe - not sure what effects it.

 

[ajw22]

I wanted to raise pH a little bit to help with CSI.

Don't go chasing short term fluctuations in your CSI.

And short term for CSI is anything less then a month.

In cold weather the CSI will go more negative. Just let the pH rise naturally over a few weeks.

Trying to manage a specific CSI in fluctuating temperatures is an exercise in frustration.

Any CSI between -0.6 and +0,6 is OK. And CSI outside of that range is ok for a few weeks as you let nature bring it back in.

 

[kul]

Don't go chasing short term fluctuations in your CSI.

And short term for CSI is anything less then a month.

I was totally doing this too. Thanks for this info. When new and trying to get everything in range it’s tough to know when to chill out!

 

[Newdude]

Don’t worry Kul, we’ve all over managed new skills/ toys. Over time you’ll just see the PH is 7.8 (etc) and say ‘look at that. The world didn’t end, I’ll just keep an eye on it in case it continues rising.’

Then you’ll be bored and come here needing something to do and help a newb all ‘HALLLLP my TA is 70 and I wanted it 90’.

 

[convan23]

Reading the above helps me understand the types of responses I've gotten to this thread

I was hoping to nerd out on how these pH probes work, and instead the focus has been: over management of pH, accuracy of different pH tests, etc...which I still totally appreciate and makes sense! But as all us newbs come on here with things like:

‘HALLLLP my TA is 70 and I wanted it 90’.

Or in my case, trying to move pH from 7.6 to 7.9 chasing a CSI number....when you can't hardly tell the difference between 7.6 and 7.9 on our pH drop test....

I can understand why the first response is "why the heck you adding borax bro?!" (@mknauss I mean no disrespect, really appreciate you jumping in ).

Alas...now I will turn to the old googs and see if I can find out how these Dang probes work

 

[kul]

Reading the above helps me understand the types of responses I've gotten to this thread

Haha sorry about that! I didn’t know what you already knew!

 

[ajw22]

Alas...now I will turn to the old googs and see if I can find out how these Dang probes work

Back to the regularly scheduled topic...

You are trying to measure a moving target using two very different measurement techniques. If they happen to be close all is fine. If not trust the chemical test.

For pool water chemistry any pH in the 7's is equally good. A pH of 7.1 is as good as a pH of 7.8. Accuracy does not matter much. The chemcial test is more then adequate for pool water chemistry. The precision of a digital meter is not needed. There are other activities you measure pH for where closer tolerances are required.

The Apera pH meter has temperature compensation. pH electrodes and measurements are temperature sensitive and so the meter adjusts its reading. The chemical test is not temperature sensitive. We don't know how well the meter is measuring the pH and making its temperature compensation. When in doubt trust the chemical test.