Water Chemistry Balance - Can I do better?

[YesRushGen]

Greetings:

For the past 7 years, I've been quite happily using the BBB techniques I learned about here on TFP. I never have an issue with algae, my water sparkles and my guests are always happy with the subjective feel of the water. In that time, I've not had reason to change anything I've been doing.

Until possibly now! I am planning for the addition of a heater for the 2017 pool season. (I'm starting another thread about that - Preparing for New Heater) I've read that "improperly balanced water" is one of the leading causes of early failure in heaters. (ie, scaling/corrosion) To that end, I'm trying to pay more attention to my CSI this year.

My current TF-100 test results:

TC = 6.0
CC = 0.0
FC = 6.0
pH = 7.2
TA = 90
CH = 210
CYA = 80
Salt = 3200
Temp = 76F

Pool Calculator shows these values result in a CSI of -0.77, which is way out of the suggested range! After fiddling with some numbers I find that:

1. raising pH to 8.0 would bring CSI to -0.02. My concern with that is two-fold. First, it is outside the acceptable pH range. Second, I've learned from experience that my SWG will raise my pH from 7.2 at pool opening to 7.8 at pool closing. Because of that, I never bother adjusting pH.

2. Some combination of raising pH, TA and CH. For example: raising pH to 7.5, TA to 120 and CH to 280 gives a CSI of -0.21.

Please share some of your wisdom! I've never had any visible corrosion or scaling issues with any of my equipment. However, I've never owned a heater and that is my primary concern.


Thanks!

Kelly

 

[woodyp]

If anything, I'd try lowering my TA to 60.

 

[RobbieH]

OK, here is what I see:

Your FC does not need to be that high. Shooting around 4 is just fine. 3 is minimum, 4 is great, 5 is failsafe. You can lower your SWG output a bit and save some wear on your cell.
You don't have to worry about CSI, you don't have a vinyl pool.
And, as stated above, if you have pH swing, you can try lowering your TA. If it is stable, don't worry about it.

 

[BuckeyeChris]

"lowering the TA from 90 to 60"

um that will push the CSI even farther negative....

Look, move your pH to 7.4, your CH to 300 and your temp up to 86 and you are sitting pretty at -0.26. If your heater is going to get the water even warmer, that will push the CSI even closer to the + range, so it will be helping itself.

Just minor tweaks on the margins.

 

[JoyfulNoise]

CSI is meaningless in terms of metal corrosion. It is simply a predictor of calcium scaling and it is really nothing more than a measure of the degree to which your water is saturated with calcium carbonate. Let me say it again - it has no value in predicting metal corrosion whatsoever.

The biggest driver of metal corrosion is pH. If you keep your pH within the recommended range (7.2-7.8), the heat exchanger will be fine. If you are getting a heater, then consider getting one with a cupronickel (gas) or titanium (heat pump) heat exchanger.

 

[RobbieH]

Note item 2 in my list.

 

[BuckeyeChris]

Wouldn't CSI matter because the heater could get scaling?

 

[Richard320]

Let the pH ride in the upper half of the comfort zone -- 7.5>7.8. It's acidity that dissolves metal. So keep to the basic side a little.

Plug your numbers in and see what the CSI is inside the heat exchanger where the surface is 120 degrees or higher. No danger of scale is there?

 

[RobbieH]

Good catch, Richard. I meant to mention pH was a bit low.

- - - Updated - - -

Wouldn't CSI matter because the heater could get scaling?

The best way I have seen to control scaling is to keep pH in range, keep TA in range, and the addition of borates if the first two don't work. And with that low CH, he has very little to worry about. Richard can talk about CH scaling all day long.

 

[JoyfulNoise]

Wouldn't CSI matter because the heater could get scaling?

Pool water CSI would have to get above +0.7 for scaling to be an issue. The OP's water currently sits at -0.8. Something would have to go wildly out of whack for there to be even the hint of an issue.

Richard320 did the math - your heater will be fine.

 

[YesRushGen]

Ok thanks everyone. In summary it sounds like I don't have anything to worry about. Raising pH to 7.5 is advised but not absolutely necessary.

If I take that route, I will have to monitor pH closely as experience has taught me that my pool naturally goes from 7.2 to 7.8 in the course of a season. If I adjust and start at 7.5, I'm not sure how that will impact the pH at closing time.

 

[JoyfulNoise]

Ok thanks everyone. In summary it sounds like I don't have anything to worry about. Raising pH to 7.5 is advised but not absolutely necessary.

If I take that route, I will have to monitor pH closely as experience has taught me that my pool naturally goes from 7.2 to 7.8 in the course of a season. If I adjust and start at 7.5, I'm not sure how that will impact the pH at closing time.

pH rise is NOT linear with time. When you lower your pH to 7.2, the outgassing rate of CO2 is much faster from 7.2 to 7.5 and then it slows down a lot from 7.5 to 7.8. This is because the concentration of aqueous CO2 is a lot higher at lower pH and the outgassing rate is proportional to concentration.

In my own pool, if I lower my pH to 7.2 it will rise back up to 7.5 in about 24hours. It will then take ~10 days to go from 7.5 to 7.8.

So, you can probably adjust your pH to 7.5 and notice very little difference in the time it takes to get near 7.8. Adding 50ppm borates to pool water also dramatically improves pH stability as it strongly buffers against pH rise.

 

[YesRushGen]

Update: After 2 weeks of aerating every-other day, I'm now at a pH of 7.5. I shall now closely monitor how this rises through the remainder of the season. If I've understood the information at TFP correctly, if my pH hits or exceeds 7.8, I should use muriatic acid to snap it back to 7.5, correct?

Now with pH 7.5, my water temp around 82F, and all others the same, PoolMath calculates CSI at -0.33. Also, as Richard indicated, a temperature of 120 on the heat exchanger gives an even more ideal -0.02. I feel much better now, just have to monitor the change, per above.

One last item: It was pointed out my FC of 6.0 is too high. With my setup, PoolMath's goal is 4.0 - 11.0. I guess I've made a habit of running it a bit higher than the lowest goal number as a form of "insurance." LOL. As Robbie suggests: I suppose I could play with the purifier, and target 4 to 5, save some SWG wear.

 

[BuckeyeChris]

at a CYA of 80 with a SWG, the minimum is 4.0 FC. 6.0 is right where you should be for a target. If your pump runs two times a day, you can maybe be at 5, since fresh chlorine is never that far away. If you are just running once a day, 6 is just fine.