Ph accuracy with low water temp

jimim

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Jun 20, 2016
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NE/Pa
Hey all. I know I have asked this before but. When I open my water will be mid to high 30’s maybe low 40’s. I know to test my cya I have to do a warm bath to get the pool water temp up to about 70.

But ph. It’s not accurate with low water temp. Should I be testing that at 70 degrees also? I know the other tests the pool water can be low.

Thanks! Almost time. I added in chlorine last night. My pump that I use to stir it up broke so I had to drive to look for a new one. I ended up at Leslie’s. Hopefully this one lasts longer. I only got 4 seasons out of my other.
 

mgtfp

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I've noticed different pH readings with cold water vs when I bring the water inside and get it up to room temperature.
And this chemistry forum indicates that pH declines at higher temperatures: Temperature Dependence of the pH of pure Water

Note that the pH of pure water at 10C (50F) is 7.27
and the pH of pure water at 30C (86F) is 6.92

That's correct. That's why it is important to make sure to measure pH at the actual pool water temperature. E.g. don't let a pool water sample in winter warm up to room temperature before running the pH test, that would make you underestimate the pH.

When using a pH-meter, it is important that the meter has sufficient time to reach the water's temperature, otherwise the automated temperature compensation will work based on a wrong temperature reading. As long as meter and water temperature are not too different, this effect will be negligible (smaller than 0.1), but when dipping a meter that's been stored at room temperature into near freezing water, then the initial reading might be off more significantly.
 
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jimim

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Jun 20, 2016
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i thought that also. so then why don't we bring it up to rom temperature first to know exactly what it is before adding acid. cause i'm open right now and maintaining but my pool water is 51. so i would be adding different amounts of acid, so which is correct?
 

jseyfert3

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i thought that also. so then why don't we bring it up to rom temperature first to know exactly what it is before adding acid. cause i'm open right now and maintaining but my pool water is 51. so i would be adding different amounts of acid, so which is correct?
The pH of water changes with temp. So the pH test needs to be done at the temp of the pool water, because that is the pH that’s important.

If you use the pH at room temp then you would be adding the wrong amount of acid. Always do the test at the temp of the pool/spa water and adjust as needed from that test.
 
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mguzzy

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The pH of water changes with temp. So the pH test needs to be done at the temp of the pool water, because that is the pH that’s important.
That is why I do all tests at poolside...(and I am too lazy to walk back and forth) The only test that is negatively affected by temp is the CYA test to my knowledge. Right? That one is more accurate if you let the water warm up. I am sure the Pool Chem Gurus will correct me if I am wrong, but that is what I have collected from hanging out here. Calling @mknauss too.
 
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CrystalRiver

Well-known member
Jun 19, 2020
648
Massachusetts
It's not just the test result that changes when you warm up the water. It's the actual pH of the water that changes. The test is accurate for the temperature at which it's performed.

You don't need to know the pH your pool water would be if it magically warmed up to 75, you need to know the pH of the water you actually have right now, and that water is cold. Test a cold sample so you know the pH actually in your pool.
 
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mgtfp

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The only test that is negatively affected by temp is the CYA test to my knowledge. Right? That one is more accurate if you let the water warm up. I am sure the Pool Chem Gurus will correct me if I am wrong, but that is what I have collected from hanging out here

That's also my understanding. I bring my water sample in, fill my CYA mixing tube to the fill line and out it aside. Then I test pH and FC as quickly as possible. Then TA and CH, and then I get back to CYA once the sample has warmed up. Usually, I just run pH and FC, and from time to time I add one of the other tests.

For me, the water temperature is only an issue in winter (pool remains open, but only used by ducks from time to time). Water temperature goes down to about 7-8°C (44-46°F), freezing is never an issue. In summer, pool water temperature and room temperature are quite similar, I don't have a pool heater, and the timing of the tests is not critical.
 

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mgtfp

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Mar 5, 2020
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i thought that also. so then why don't we bring it up to rom temperature first to know exactly what it is before adding acid

To add to what has already been said: Let's say your water temperature is close to 40°F. You bring the water sample in, let it warm up to room temperature and test your pH at 7.9, and think that's pretty OK for winter, keeping pH a bit higher is good to compensate for lower temperatures to keep CSI from getting too low. But in fact, pH at the actual pool temperature is probably well above 8, creating a risk for metal staining.
 
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