Unexpected Results

[Swampwoman]

Thanks for the explanation, Matt.

So barring chloride from the fill water (maybe worth a test of tap) JohnSimion's salt increase could only have come fom MA because evaporation without salt addition and no chloride in fill would mean salt should decrease, not increase, right?

But it would take 20 gallons of MA to net that increase in that amount of time...eg about 3 gal a month.

And his salt has increased by 17% over 7 mos or a cumulative 2.4 per cent per mo...

Alternately, if somehow salt is in fill water from softener, would this Calc of evaporation roughly also explain the salt increase?

Re: Unexpected Results
A rise of 125 ppm over 7 months would only be 18 ppm per month but even with 17" per month evaporation (that's normal July pan evaporation in Las Vegas) this would be 9.9 feet of evaporation over 7 months so with 4.5 foot average pool depth that's 2.2 times pool volume so the fill water would have to be 125/2.2 = 57 ppm CH to possibly explain the result based on fill water and that assumes the full evaporation rate over the entire 7 months which is unlikely.

Don't worry...I won't quit my day job. Can't make math work

 

[JoyfulNoise]

Has the city water supply been tested?

Almost all municipal water supplies deliver water with some chloride in it. Some water districts are better than others at lowering chloride levels but you could easily have city water with well over 200ppm chloride in it and never taste the difference. Even water up to 2000ppm has no discernible saline taste.

Since the OP lives in Vegas, and as chem geek has pointed out, the pan evaporation rates are huge for an entire year (almost the entire pool volume can evaporate away in one year). So one would have to check the chloride levels in the fill water to know if it is a significant contributor.

[EDIT]
Fixed sample size error.

Since the K-1766 titrant is designed to measure 200ppm/drop for a 10ml sample, you'd need to do the test on a much larger water sample to get finer detail. If you used 100ml of pool water and added, perhaps 2-3 drops of the potassium chromate indicator, then the titrant would yield ~ 20ppm/drop. At that range, one should be able to check the municipal supply.

[END-EDIT]

Also, remember that most of the Taylor tests have an error of +/-1 drop when the drop count is below 10 drops. After 10 drops, you usually get +/-10% of the final reading. So for water with 4000ppm chloride, the error is +/-400ppm. It's probably a little better than that, but not by much.


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[johnsimion]

Out of curiosity, how are you getting those CYA numbers? The CYA test in the TF-100 is graduated by 10's.

Dom

I use my old ColorQ for the CYA test because having run side-by-side tests with it and the Taylor kit, the results from the ColorQ have always matched the Taylor for this ONE test only. The Taylor CYA test is very difficult for me to read (as in, "Do I see the black dot or are my eyes deceiving me?" The ColorQ of course is very simple to read, and since I always got the same results for CYA, I figured it would be okay to use the ColorQ for this one test only. TFP sent me standard samples that convinced me of the accuracy of the Taylor test for everything else and that's why I only use the ColorQ for this one test.

 

[johnsimion]

Thanks for the explanation, Matt.

So barring chloride from the fill water (maybe worth a test of tap) JohnSimion's salt increase could only have come fom MA because evaporation without salt addition and no chloride in fill would mean salt should decrease, not increase, right?

But it would take 20 gallons of MA to net that increase in that amount of time...eg about 3 gal a month.

And his salt has increased by 17% over 7 mos or a cumulative 2.4 per cent per mo...

Alternately, if somehow salt is in fill water from softener, would this Calc of evaporation roughly also explain the salt increase?



Don't worry...I won't quit my day job. Can't make math work

I haven't tested the municipal water supply for salt content, but it is feasible that that there might be some. It's also possible that I'm putting more MA into the pool than I think. My pool seems to laugh at the PoolMath recommendations for the amount of MA to lower the PH and so I had indeed taken to adding acid by the gallon. But really I cannot believe I ever added 3 gallons in ANY month. Maybe 2, but doubtful I ever added 3. Dunno. I have to stop worry so much about such things.

Just one other note. Joyful Noise referred to the "K-1766 titrant is designed to measure 200ppm/drop for a 25ml sample." I just checked this, and my own K-1766 test uses a 10 ml sample (not 25 ml) but it also measures 200 ppm/drop. Thus when I say 4000 ppm, that's 20 drops with my particular test. So I'm not quite sure how to relate to the accuracy information.

I guess as long as my SWG is happy, I should be happy.

 

[JoyfulNoise]

Just one other note. Joyful Noise referred to the "K-1766 titrant is designed to measure 200ppm/drop for a 25ml sample." I just checked this, and my own K-1766 test uses a 10 ml sample (not 25 ml) but it also measures 200 ppm/drop. Thus when I say 4000 ppm, that's 20 drops with my particular test. So I'm not quite sure how to relate to the accuracy information.

I guess as long as my SWG is happy, I should be happy.

Thanks for catching that! You are quite right, it's a 10ml sample, not 25ml. I was thinking of a different test. I'll edit my post.


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[chem geek]

The salt test only tests for chloride and a water softener will not change the amount of chloride so if there is chloride in the fill water then it will get into the pool. The 290 ppm of water hardness before going through the water softener just means the sum of calcium and magnesium (in calcium carbonate units) is 290 ppm. It does not say whether such hardness is associated only with carbonates (measured as Total Alkalinity) or if some is from chloride. The easiest way to know for sure is to test the fill water for the salt level with the Taylor chloride salt test.

According to this report, the Colorado River from Lake Mead is high in calcium sulfate and bicarbonate, but is also high in sodium chloride. Specifically, the chloride level reported as sodium chloride varies from 34*(58.44/35.453) = 56 ppm to 98*(58.44/35.453) = 162 ppm. So given the 2.2 times pool volume evaporation/refill I calculated before, this says that salt levels could increase by 123 to 356 ppm. That's still not the 600 ppm increase you saw but could explain part of that increase.

 

[Swampwoman]

^Actually, that DOES rather explain it, because it looks after more than half the increase and the rest can be accounted for in the MA diet over 7 mos...eg maybe 10 gallons at 30 ppm!