CH with SWCG

rinaldok

Gold Supporter
Sep 16, 2016
113
Peoria, AZ
Why is the recommended CH for SWCG pools higher than bleach pools?

I have been getting around 310 or so with my tests. 2 pool store tests today (yes, I know...) gave me 241 and 250. So, either they're coincidentally consistently wrong, or I'm wrong. I admit that I have trouble with the colors on some of the tests.

My question is should I raise my CH and why? Doesn't that just drive up the CSI?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,765
Tucson, AZ
Ignore the pool stores, they never get the CH correct.

Pools in our part of the country typically need to have the TA a lot lower in order to avoid pH rising precipitously due to aeration and outgassing of CO2. Pools around here are also typically plaster and so the slightly higher CH values are there to ensure that the calcite saturation index (CSI) stays within reasonable values. You don't want to have aggressive water as that can lead to plaster damage over long periods of time.

So what are your other chemical levels? Where do you keep your TA and pH?

For reference, because my CH is currently very high (~950ppm), I typically keep my TA between 50-60ppm and my pH in the 7.6-7.8 range. If I keep my TA on the low side, I can usually get 10-14 days between acid additions.
 

rinaldok

Gold Supporter
Sep 16, 2016
113
Peoria, AZ
I know the "ignore the pool store" refrain, but as I mentioned I have trouble with the colors on the tests sometimes. Reds in particular and I can't always tell the difference between purple and blue, so I periodically get outside testing to ensure I'm staying within a margin of error.

In this case, 2 different stores with 2 different testing methods (one chemical, one digital) returned almost identical results, so I thought it worth considering the likelihood that I may have been off, and if I was off, I wanted to address it because I certainly don't want water that is potentially damaging to the new pebble surface I just had put in last summer.

I haven't been diligent with logging all my tests in the app, but I put the last couple in there and they're available:


I boosted my CYA because I just got my SWCG up and running. My TA is around 70 (by my tests), but I battle high pH. I have to add acid every 2 days or else it's well over 8. With us traveling a lot lately and I'm gone for a week+ at a time, I was looking for things I could do to get the pH rise under a bit more control. Today's pH of 7.5 is much lower than normal, I suspect because I added MA and CYA both yesterday. I'm sure that'll be back to 7.8-8 tomorrow.

*Purely* for amusement and reference, to compare against the test numbers that I logged, today's results from 2 pool stores:

Leslie's:
FC: 5
TC: 5
Salt: 3700
CH: 250
CYA: 60
TA: 80
pH: 7.5
Phosphates: 100

Poolwerx:
FC: 4.31
TC: 4.39
CC: 0.08
pH: 7.5
Alkalinity: 66
Adjusted Alkalinity: 37 (see my other thread on this)
CH: 241
CYA: 86
Salt: 3636
Phosphate: 86

Generally speaking these results agree with my tests, but the CH on both of these are consistent and 25%+ off of my number. As an engineer, that sort of data anomaly warrants investigation.

All that being said, I'm still unclear why a pool with a SWCG has a higher recommended CH than the same pool without a SWCG.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
21,967
Laughlin, NV
In other areas of the country, with very soft water, low TA, low CH, if you stay in the recommended ranges, you can ignore CSI.

We in the SW desert are not that lucky. We must manage our CSI, as Matt explained above. So the we do not follow the 'recommended ranges' explicitly for CH and TA. We keep the CH at or above 200 ppm and then manage CSI.
 

Saturn94

Well-known member
Mar 11, 2015
673
Virginia
Interesting. Since I have a vinyl pool, I haven’t paid much attention to recommended CH levels. I just saw this in Pool School:

“...A plaster pool without a SWG should have CH levels between 250 and 350 if possible. With a SWG, CH should be kept between 350 to 450...”

I’m also curious why the difference?

Like the OP, I have trouble with the CH test (hard to tell when it goes from purple to blue). I do have an alternate method of testing CH (suggested to me by Ben, aka Pool Doc from Pool Forum) that may help you. Just PM me if interested.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,765
Tucson, AZ
Generally speaking these results agree with my tests, but the CH on both of these are consistent and 25%+ off of my number. As an engineer, that sort of data anomaly warrants investigation.

All that being said, I'm still unclear why a pool with a SWCG has a higher recommended CH than the same pool without a SWCG.
The tests are all independent of one another so having agreement on one variable does not imply that all variables should agree. Does it warrant investigation? Well, sure, and it has been....thousands and thousands of times over with posts on TFP detailing just how wrong pool stores get it. So, if you look at the data in aggregate, not just specific instances, it is much more likely that the pool store is wrong than you are. For example, not but 2 weeks ago there was a post from a user about CH testing where they we're worried about the exact same issue - pool store testing of CH did not match their testing. Well, the OP described in detail the test method the store was using and the person behind the counter was adding, via pipette, well over 1 mL of what was likely the buffer reagent (similar to R-0010) rather than the 10 drops prescribed by the test protocol. If that is the case, then the pool store was adding about 4-5 times more buffer than the test calls for. If one understands the detailed chemistry behind the CH test, you can not add the buffer indiscriminately - its purpose is to raise the pH of the test sample to a well defined level so that the interference from magnesium is removed. Go too far, and too much buffer will also arbitrarily reduce the calcium levels giving a false-low test result. So the OP's tests were higher than the pool store and the pool store was not following the proper protocol....I think that's an obvious case of who's data one should trust. As for digital testers, that falls squarely into the mistaken belief that just because a meter gives more precise values (+/- 0.1 ppm readings) that it is somehow more accurate. Simply not true and, as one engineer to another, I think you well know that digital meters need consistent zeroing out and calibration against standards to ensure accuracy. In my days in the chemistry lab, calibrating meters was a daily event and, for highly sensitive tests, meters needed to be calibrated both before and after a test to ensure accuracy. Find me a pool store that has a digital testing setup that is calibrated on a daily basis...I won't hold my breath.

As for the question at hand, I already answered that. Plaster pools need to have their water saturation (tracked using CSI calculations or, if your prefer the industry standard, LSI) to ensure that the water does not get too corrosive. With an SWG it is doubly important to track CSI as the chemistry inside the cell tends to be different than bulk pool water. For TFP recommended levels, plaster pools and pools with SWG's tend to run at lower TA to avoid the pH rise that you seem to be experiencing. Since TA has a big effect on CSI, one needs to balance the lower TA against another variable to keep the CSI from dropping into a corrosive zone. Adding a little extra CH to the water helps to allow TFP users to maintain a lower TA and still keep a balance water saturation. If you prefer to not deal with your low CH, you can leave it where it is and just run at a higher TA BUT, as you have experienced, that means you will be adding acid more frequently to keep you pH in check. Borates can help to slow pH rise but you'll still be adding the same amount of acid to the pool - it just takes longer for the pH rise to happen and, when it does, you need to add more acid.
 

rinaldok

Gold Supporter
Sep 16, 2016
113
Peoria, AZ
Does it warrant investigation? Well, sure, and it has been....thousands and thousands of times over with posts on TFP detailing just how wrong pool stores get it. So, if you look at the data in aggregate, not just specific instances, it is much more likely that the pool store is wrong than you are.
Understood and agreed. I never rely on pool store tests, only use them on occasion to bounds-check my own results due my color perception issues. If something of concern comes of it, where else to go to clarify than here?

I usually ignore the results, but this *particular* case was odd enough to make me raise an eyebrow and ask the question, just in case I was off on my tests. While certainly within the realm of coincidence, it would have been much easier to ignore if they were wildly off in different directions. 🤷‍♂️

To be clear that I'm being clear, I'm not saying I trust their tests. I'm simply saying I question my own at times and this is the only way I have to get additional data points to check mine against. No good can come of pool stores, I know.

With an SWG it is doubly important to track CSI as the chemistry inside the cell tends to be different than bulk pool water. For TFP recommended levels, plaster pools and pools with SWG's tend to run at lower TA to avoid the pH rise that you seem to be experiencing. Since TA has a big effect on CSI, one needs to balance the lower TA against another variable to keep the CSI from dropping into a corrosive zone. Adding a little extra CH to the water helps to allow TFP users to maintain a lower TA and still keep a balance water saturation.
Ah, THAT answers my question. It hadn't occurred to me that we run a lower TA range with SWG, so balancing it with higher CH now makes sense.

With my TA around 70, as long as my CSI is staying within range there's no need to increase my CH, correct?

For the pH issue, I have considered borates, and I may revisit. I don't mind adding a bit more acid if I have to add it less often. Last time I brought it up the response was not overwhelmingly positive.

Based on my test results, can you see any other factors for my pH rise? I don't run my aerator, I have no waterfalls or features, the pool runs at night, and the return jets are positioned to just move the surface of the water but never break it. I don't have a large bio-load. The pool is honestly barely used. No kids, only occasional use by the 2 of us, so extremely low bather load. Leaves and other debris are cleaned out as often/quickly as possible and never left to sit in skimmers, etc., to break down. Fill water is softened and the pH is around 7.7. No sprinklers or other runoff. I think I've done as much preventative steps as I can to keep pH in check, yet it still climbs fast.
 

mcdickey

Well-known member
Apr 19, 2012
59
Tucson, AZ
As for the question at hand, I already answered that. Plaster pools need to have their water saturation (tracked using CSI calculations or, if your prefer the industry standard, LSI) to ensure that the water does not get too corrosive. With an SWG it is doubly important to track CSI as the chemistry inside the cell tends to be different than bulk pool water. For TFP recommended levels, plaster pools and pools with SWG's tend to run at lower TA to avoid the pH rise that you seem to be experiencing. Since TA has a big effect on CSI, one needs to balance the lower TA against another variable to keep the CSI from dropping into a corrosive zone. Adding a little extra CH to the water helps to allow TFP users to maintain a lower TA and still keep a balance water saturation. If you prefer to not deal with your low CH, you can leave it where it is and just run at a higher TA BUT, as you have experienced, that means you will be adding acid more frequently to keep you pH in check. Borates can help to slow pH rise but you'll still be adding the same amount of acid to the pool - it just takes longer for the pH rise to happen and, when it does, you need to add more acid.
JoyfulNoise - Let me know if I should post this in a new thread instead of here...I would like your advice please on my pool based on what you said above. Here are my numbers as of this morning in Tucson:

FC 5.5
pH 7.5
TA 70
CH 200
CYA 70
Salt 3200
Temp 84
CSI -.58

I do get consistent pH rise up to 8+ within three days of lowering to 7.6 or so. Do you recommend I raise CH and lower TA? What levels would you recommend, keeping CSI and potential corrosion in mind on my plaster (pebble) pool with SWG? Thank you.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,765
Tucson, AZ
JoyfulNoise - Let me know if I should post this in a new thread instead of here...I would like your advice please on my pool based on what you said above. Here are my numbers as of this morning in Tucson:

FC 5.5
pH 7.5
TA 70
CH 200
CYA 70
Salt 3200
Temp 84
CSI -.58

I do get consistent pH rise up to 8+ within three days of lowering to 7.6 or so. Do you recommend I raise CH and lower TA? What levels would you recommend, keeping CSI and potential corrosion in mind on my plaster (pebble) pool with SWG? Thank you.
Your CH is quite low for a Tucson area pool. I’m guessing you either have a low CH well (they do exist around here) or you keep your pool filled via a water softener. Either way, 200ppm CH is a bit low. With your TA down at 70ppm your CSI is very aggressive all throughout your pH range (7.6 to 8.0). A more optimal point for your pool would be to let your CH rise up to 300ppm or even 350ppm and then you can keep your TA lower at 60ppm and not be as aggressive. You could also add 50ppm borates and that would help to keep the pH rise under control. Don’t try to force your pH any lower than 7.6 and don’t let your TA go any lower than 50ppm for your plaster’s safety.
 

mcdickey

Well-known member
Apr 19, 2012
59
Tucson, AZ
Your CH is quite low for a Tucson area pool. I’m guessing you either have a low CH well (they do exist around here) or you keep your pool filled via a water softener. Either way, 200ppm CH is a bit low. With your TA down at 70ppm your CSI is very aggressive all throughout your pH range (7.6 to 8.0). A more optimal point for your pool would be to let your CH rise up to 300ppm or even 350ppm and then you can keep your TA lower at 60ppm and not be as aggressive. You could also add 50ppm borates and that would help to keep the pH rise under control. Don’t try to force your pH any lower than 7.6 and don’t let your TA go any lower than 50ppm for your plaster’s safety.
Thank you. I do keep my pool filled via a water softener as you guessed above. This is the 3rd swimming pool I have owned in Tucson and the other two were filled with high CH city water and not softened water. Its hard to imagine having to raise CH here right? My other two pools always had sky high CH.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
15,765
Tucson, AZ
You can just fill it with city water for a while and the CH will drift up naturally. You just need about 100ppm more and that should take about 4-6 months on city water. It’ll save you some nickels by regenerating your softener less 😂