pH balancing in winter (low temperature)

iflyjetzzz

Well-known member
Jun 16, 2011
60
Las Vegas, NV
#1
I have a new pool, completed in August. The current water temperature is now starting to warm a bit but is still in the (high) 40s. It has gotten as low as the high 30s this winter here in Las Vegas.
The pool is a salt water pool and I occasionally use the spa portion of the pool. This allows me to run the salt cell when the spa water heats up. This has easily taken care of chlorination in the entire pool, as the spa water mixes with the rest of the pool water daily and chlorine demand is low with the low temperatures.

Here is my question/comment. When testing the pH, I've found that it reads low when the temperature is in the 40s (say 7.0 for example) and rises considerably (say 7.8) when the sample is warmed to room temperature. I've found that this has occurred every single time I've let the sample rise to room temperature. So, should I try to make my pH correct for the pool temperature or for room temperature? So far, I've been trying to keep the room temperature pH closer to 7.2, as I have a new plaster pool that is still using a considerable amount of muriatic acid.

Interestingly, I looked online for how pH is effected by temperature. I was shocked to see that more than a few academic chemists state that pH falls with higher temperatures, while others state that pH rises with higher temperatures.

TFP community, is this something that you check for? How do you handle pH during winter?
 

AUSpool

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Sep 23, 2015
503
Sunshine Coast, Australia.
#2
What is your TA at? I don’t think its your temp warming up but out gassing of CO2. With your auto cover on your water will not off gas as much as it could and your accumulating CO2 which will lower the pH. When the water sample is allowed to warm and out gas the pH will rise accordingly.
 
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iflyjetzzz

Well-known member
Jun 16, 2011
60
Las Vegas, NV
#3
Like I stated, around 7.0 when I test it cold. About 7.8 when I test it at room temp.
Perhaps just outgassing - I'll check it with one sample sitting outside for a while and one sample warmed to room temp.

I hadn't considered that; good point.
 

AUSpool

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Sep 23, 2015
503
Sunshine Coast, Australia.
#4
I was asking about your total alkalinity, TA, but a list of your complete current test results would be very helpful. Some SWG shut down when the temps drop, check on this too. When the temps drop in winter everything slows down so there’s normall not to much to worry about, just keep an eye on your CYA and in particular your FC. You don’t want to end up in slam territory due to running low on chlorine.
 
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iflyjetzzz

Well-known member
Jun 16, 2011
60
Las Vegas, NV
#5
AUSpool, sorry for the very slow response. We moved into our newly built house in March 2018 and I've got too much work going on. I had a bunch of soil hauled in and rented a bobcat to move all of the soil - between that and work (and very cold temps), I haven't had a chance to devote time to my pool.

As far as chlorine level, I've got a spa and I was putting the chlorine generator to work when I heated up the spa but my chlorine level has not dropped much at all so I shut off the chlorine generator for the spa about 3 weeks ago. I just took chlorine and pH readings (been a couple of weeks). Chlorine's still a bit high (reading in excess of 10) and my pH read very high with cold (42F) water and pool covered for the last few weeks at greater than 8.2.

My TA's been reading a bit on the low side, although I haven't measured it in a while. We're supposed to get some more cold weather here in Las Vegas in the next few days so other than a high pH, I haven't been too worried about pool chemistry.

CYA's been steady at ~40 for a long time. It should remain stable there, as I'm not using tabs.

I just added 1 gallon of 20 baume (double pool strength; branded as masonry cleaner) muriatic acid. That should get the pH back to normal levels. I'm not surprised that the pH level is high, as it's a new pool (completed August 2018).
 

Texas Splash

Mod Squad
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#6
I just added 1 gallon of 20 baume (double pool strength; branded as masonry cleaner) muriatic acid.
As a note, just be careful about adding too much MA at once. It's usually best to use something like 7.8 as a target (from 8.2) then re-check in about 30 minutes. One full gallon is quite a bit for your 15K pool. Since your water is so cold, it's already trying to knock down the CSI which is important for your plaster. Ideally you want your CSI between -0.3 to -0.1. A pH around 7.8-8.0 may be just fine this time of year to counter that cold water. Once Vegas decides to stop playing with snow and getting back to typical desert temps, then you'll probably need the pH lower. Always use the Poolmath tool as well to confirm.
 
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iflyjetzzz

Well-known member
Jun 16, 2011
60
Las Vegas, NV
#7
As a note, just be careful about adding too much MA at once. It's usually best to use something like 7.8 as a target (from 8.2) then re-check in about 30 minutes. One full gallon is quite a bit for your 15K pool. Since your water is so cold, it's already trying to knock down the CSI which is important for your plaster. Ideally you want your CSI between -0.3 to -0.1. A pH around 7.8-8.0 may be just fine this time of year to counter that cold water. Once Vegas decides to stop playing with snow and getting back to typical desert temps, then you'll probably need the pH lower. Always use the Poolmath tool as well to confirm.
Thanks; I hadn't factored in the effect of the temp on CSI. I just checked the pH a few minutes ago and it's ~7.4. I'll have to use the tool to determine CSI.

Edit: looks like I got my CSI too low; it's right around -.6 right now.
 
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Texas Splash

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#8
Edit: looks like I got my CSI too low; it's right around -.6 right now.
Perhaps not the worse thing for a few days since the pH appeared to be high for a while. Depending on the current TA and water movement, I suspect the pH will try to rise again on its own soon. Then you can watch it from there. This is one of those areas where Poolmath shines by do the hard math for us. :hammer:
 
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iflyjetzzz

Well-known member
Jun 16, 2011
60
Las Vegas, NV
#9
I took a few water samples this morning.
Initial pH read was 7.2. After sitting inside for a few hours, it rose to 7.6

I also took two water samples, one sat outside for a few hours and the other was inside and warmed to room temp for a few hours.
I checked the pH on both of those samples and they were both 7.6.

So I would conclude that AUSpool is correct in that my pH initially reads low due to my pool being covered and requires some time to outgas CO2 before getting an accurate reading.

Texas Splash, the pH tends to rise fairly quickly in the pool due to it being new. I'll be out of town for a few days but will likely need to add some muriatic acid when I return to get the pH back below 8.0. Up until now, I'd been shooting for ~7.2 after adding muriatic acid because the pH climbs so quickly. I will adjust to a target of ~7.6 after adding muriatic acid to keep the CSI above -.6 until the water warms up. I'm hoping by then the pool will be more fully cured so it won't be using as much muriatic acid.
 

Texas Splash

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#10
Texas Splash, the pH tends to rise fairly quickly in the pool due to it being new. I'll be out of town for a few days but will likely need to add some muriatic acid when I return to get the pH back below 8.0. Up until now, I'd been shooting for ~7.2 after adding muriatic acid because the pH climbs so quickly. I will adjust to a target of ~7.6 after adding muriatic acid to keep the CSI above -.6 until the water warms up. I'm hoping by then the pool will be more fully cured so it won't be using as much muriatic acid.
Sounds good. :goodjob: