Why is my CH always dropping?

[screamingwalls]

My pool is four years old, and the CH is always dropping. I had scaling in year two, but got rid of it (sanded it off - the calcium nodules were removed via the filter and the Polaris). I've had no scaling in year 3 or 4. Regardless, every year I've had the pool, my CH has always dropped consistently.

This year I've done no backwash, and very little splashout. My pool has been uncovered for about 6 weeks, so I've only had water loss due to evaporation I can't figure out why evaporation and refill would cause the CH to *drop*. I've read many articles on here that say it will, but I don't understand why that would be the case. I'd like to figure out where this calcium has been going.

As I understand it when water evaporates it takes only H2O with it - all calcium is left behind in the pool. So for example, if I have a 10K pool with 0 CH and add 10lbs calcium chloride I get (for the purpose of round numbers) a CH of 100. Now, say evaporation comes along and takes away 1000 gals of water. I'm left now with a pool containing 9K gallons of water and still 10lbs of calcium (since the calcium didn't evaporate). Due to the new ratio, I would expect my CH to rise by about 10% to around 110.

To help keep it simple, let's assume my fill water source has a CH of zero. I know fill water is, in reality, non-zero CH, but it doesn't really help with figuring out why the CH is dropping - the CH in the fill water should actually help the CH rise, not drop.

So now I add back that 1000 gals of water with a hose or rain, but this adds no CH. I should now be back to my original CH level of 100. The refill put me back to 10K of water, my calcium remains 10lbs (since it never left the pool), and that should put me back to the starting value of 100 for the CH.

However, you can see from my records this year that my CH has been dropping, even though I'm only losing water to evaporation (which should not lose calcium). Can someone explain where the calcium has gone? Is it scaling somewhere I can't detect? Or is it turning into granules that are stuck in the filter (I haven't seen any in the polaris this year)? Where does the calcium go?

3/30 ch 150 (opening)
4/05 added 25lb CC
4/20 added 200oz (wt) CC
4/24 ch 275
5/13 ch 275
5/16 added 200oz (wt) CC
6/03 ch 300
6/27 ch 250

My other stats:
FC 4.5
PH 7.8 (7.4 to 7.8 is my range)
TA 85
CYA 40-ish

 

[Teald024]

Do you have a water softener in the house? Added any water from a hose? How much rain have you gotten? Have you had to drain water from the rain?

 

[screamingwalls]

Do you have a water softener in the house? Added any water from a hose? How much rain have you gotten? Have you had to drain water from the rain?

No, no water softener. Yes, I did add water from my hose, but that would tend to increase the calcium in the water, not lower it. I have not had to drain the water due to the rain.

I see other posts on TFP that say evaporation plus rain lowers CH. That may what's happening to me too, but if so, where does the calcium go? If calcium doesn't evaporate then I think it must still be in the pool somewhere.

 

[BadDogPSD]

What is the CH of your fill water?

 

[screamingwalls]

What is the CH of your fill water?

I don't know. But assuming it's non-zero it should raise my CH, not lower it.

 

[mgtfp]

I agree with you, evaporation can't lower CH, the calcium stays in the water when H2O evaporates. It can only rise when adding more calcium with the fill water.

Could there be a small leak somewhere, and you are misinterpreting some of your water losses as evaporation?

 

[screamingwalls]

I agree with you, evaporation can't lower CH, the calcium stays in the water when H2O evaporates. It can only rise when adding more calcium with the fill water.

Could there be a small leak somewhere, and you are misinterpreting some of your water losses as evaporation?

Yeah, I was hoping for some chemistry explanation, but leak seems to be the best guess so far. The pool is going to be very busy this weekend, so maybe once things quiet down I can do a leak test. I don't want someone to splash water in the bucket, etc, and give me a bad result.

 

[mgtfp]

Did you always compare measurements at the same water level? Could it be that you measured 300ppm while you had a low water level and the 250 measurement was after a good top up? Or taken too soon after rain or fill water had mixed with the pool water, resulting in a lower concentration in the top layers?

And remember that one drop of titration reagent is 25ppm, the actual difference between the two measurements might be closer to 25 than 50. Do you always add another drop once you think the colour has changed? Add until the colour really doesn't change anymore and don't count that last drop.

A CH drop shouldn't be larger than CYA dropping (relatively speaking of course). There is no CYA in rain or fill water, and CYA is (very) slowly degrading over time, whereas CH is not. But CYA is more difficult to test. 50ppm CH drop from 300 due to water dilution (with no Ca in the fill water) should also decrease CYA by about 6ppm from 40ppm - but that certainly is still within the tolerance of the test.

Are you using a speedstir? For me that was a breakthrough, makes the test so much easier, not having to swirl and drop at the same time.

 

[screamingwalls]

Did you always compare measurements at the same water level? Could it be that you measured 300ppm while you had a low water level and the 250 measurement was after a good top up? Or taken too soon after rain or fill water had mixed with the pool water, resulting in a lower concentration in the top layers?

And remember that one drop of titration reagent is 25ppm, the actual difference between the two measurements might be closer to 25 than 50. Do you always add another drop once you think the colour has changed? Add until the colour really doesn't change anymore and don't count that last drop.

A CH drop shouldn't be larger than CYA dropping (relatively speaking of course). There is no CYA in rain or fill water, and CYA is (very) slowly degrading over time, whereas CH is not. But CYA is more difficult to test. 50ppm CH drop from 300 due to water dilution (with no Ca in the fill water) should also decrease CYA by about 6ppm from 40ppm - but that certainly is still within the tolerance of the test.

Are you using a speedstir? For me that was a breakthrough, makes the test so much easier, not having to swirl and drop at the same time.

I have not always measured at the same water level, I'm sure. But it should even out in the long run I would think. In the future I'll try to make a note each time on the water level - maybe I'll spot a trend there.

You're right, I should probably be more conscientious about the color change especially on a test that is not very precise to begin with. Adding another drop would be easy to confirm the color change (Taylor mentions this too).

I'm not really too confident about my CYA reading this year. I practice each time with a CYA Standard, but I can't read it consistently. I used to take it to one pool store near me where the CYA readings were pretty consistent on their machine, but this year I'm on my own. I'm not using speedstir, no.

Someone asked about my fill water CH - I measured it today and it is 40. I did the extra drop at the end to confirm the change.

Your questions about precision, etc made me think I just don't have enough data, and maybe I'm assuming a trend from "noise". So I went back and got my measurements from 2018 and they are listed below along with 2019. I added to the data the imputed CH values in []'s after each CC addition (what the CH *should* be after adding each amount of CC). Then after each measurement I added a relative value at the end noting how far up or down this measurement was compared to what I would expect from complete CH stability. As you can see my numbers were almost all in a downward trend last year as well.

I know there was more splashout last year (had many kids over last year vs this year when we had none), and I backwashed 2-3 times last year and I probably drained a few times due to rain. So, the 2018 data includes CH removal via those methods. Maybe that invalidates the 2018 data, not sure - if those normal water removal activities usually result in a CH drop then that explains what was going on in 2018.

2018
04/18 ch 150?
04/18 add 375oz CC [283]
04/27 CH 250 -33
06/01 CH 175 -75
06/05 added 25lb CC [317]
06/06 ch 375 +58
06/15 ch 375 +0
07/19 ch 325 -50
08/16 ch 300 -25
10/12 ch 275 -25
11/09 ch 250 -25

2019
03/30 ch 150
04/05 added 25lb CC [293]
04/20 added 200oz CC [365]
04/24 ch 275 -90
05/13 ch 275 +0
05/16 added 200oz CC [347]
06/03 ch 300 -47
06/27 ch 250 -50

 

[mgtfp]

Nice data collection. Looks like in early summer the losses are a bit higher than later in the year?

In my pool, the water volume can vary about 5000l in the range where the skimmer can work, that can lead to differences of about 25ppm in CH. Draining water after heavy rain from too much to workable skimmer levels can wash 10-20ppm of CH out the drain, depending on how much rain there was, and if I expected more rain to come or I wanted to leave room for evaporation.

When calculating the CH rise by a CC addition with PoolMath, you need to consider that these products are not always 100% CC, they often contain fillers. I have seen pool store products that specify the CC content as 60%-100% in the MSDS. The dosage info on the package implies about 75% average. Sometimes they even specify 100% in the MSDS, because they only have to list hazardous materials (but they can list all materials if they want to), but the dosage info indicates the presence of fillers. The package will say that the active ingredients are 100% CC.

That's why I always verify the CH level after adding CC.

Do you have an overflow, or do you have to drain manually? And automatic or manual water top-up?

So, if you really want to get to the bottom of it, you can do some thorough testing and logging. Or just accept it for what it is, if it doesn't cause problems...

 

[screamingwalls]

Nice data collection. Looks like in early summer the losses are a bit higher than later in the year?

In my pool, the water volume can vary about 5000l in the range where the skimmer can work, that can lead to differences of about 25ppm in CH. Draining water after heavy rain from too much to workable skimmer levels can wash 10-20ppm of CH out the drain, depending on how much rain there was, and if I expected more rain to come or I wanted to leave room for evaporation.

When calculating the CH rise by a CC addition with PoolMath, you need to consider that these products are not always 100% CC, they often contain fillers. I have seen pool store products that specify the CC content as 60%-100% in the MSDS. The dosage info on the package implies about 75% average. Sometimes they even specify 100% in the MSDS, because they only have to list hazardous materials (but they can list all materials if they want to), but the dosage info indicates the presence of fillers. The package will say that the active ingredients are 100% CC.

That's why I always verify the CH level after adding CC.

Do you have an overflow, or do you have to drain manually? And automatic or manual water top-up?

So, if you really want to get to the bottom of it, you can do some thorough testing and logging. Or just accept it for what it is, if it doesn't cause problems...

I don't have an overflow or an autofill. I just noticed my last data set had the wrong year (it's supposed to be 2019 and 2020).

One of the things I wanted to confirm (which I think the responders have) is that evaporation plus refill will not result in a CH drop - if anything it will result in a CH rise. So at least I can rule that out.

I should check the CH right after adding CC - I think you're onto something there. It looks like the CC product I have been buying is not what it seems. I've been buying Snow Joe MELT50CC Melt Calcium Chloride Crystals Ice Melter - I was buying from Walmart, not Amazon but if I had read the Amazon comments I would have seen that some people claim this is actually mostly salt. I can't tell from the packaging how much is salt and how much is CC - I was thinking it was around 93% CC based on some memory I had of reading that somewhere. But the label on the bag does list Sodium Chloride first in the list of ingredients. The MSDS says the percentage of CC may be anywhere from 60 - 100%.

So, checking the CH after adding CC would've spotted this. I think the product is at least some of my problem if not most of it. My 2020 data could line up with just bad product if you assume a little errror in the testing and my 2019 results could be bad product plus water removal. So I'm going with that explanation for now I think. I'll finish out the bag of CC I have (I don't think a little salt matters much), checking the CH afterward, and then find a better product in the future.

I'll still plan to do a leak test - we finally are getting some visitors this year, so I'll wait until they are gone. When it's a little quieter I'll try to get that test done just to see if there is a leak.

Thanks to everyone who responded.