Pool Math app assumes calcium chloride is 90% pure?

mgtfp

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Mar 5, 2020
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Melbourne, Australia
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Calcium Hardness increaser is Calcium Chloride, CaCl2, but Calcium Hardness in the pool water gets measured in units of ppm Calcium Carbonate, CaCO3. Considering the molar weights of these molecules, you have to add 110g of CaCl2 to effectively add 100g of CaCO3 to the water. That's probably what you noticed in the App.

On top of that, you have to consider the purity of the specific product you are using, quite often they (even the "premium" ones from the pool store) contain only 70%-90% of CaCl2, the rest are fillers. PoolMath doesn't know about that, PoolMath only tells you how much CaCl2 to add, without potential fillers in the specific product you are using.

And welcome to TFP! You found the right place for all things pool-related...
 

Noober

Bronze Supporter
Oct 23, 2020
2
Melbourne, Australia
Ah! That’s much clearer, thanks. Figured other places and instructions on the pack were simplifying.

What had me double checking is I was at 130 CH (using Clear Choice too) and I added 5kg which brought it up to 350. But that would suggest the volume of the pool is much lower than I thought (completely new house and pool to me). I hoped the adjustment would let me hone in on the volume... super confusing.

Good news is that from finding it cloudy, filthy, broken SWG cell, high filter pressures - now dialed in, crystal clear water, heated for the first time yesterday and swimming today.

Incidentally do you recommend the pH tester you’re using? The rainbow test is infuriating.
 

revitup

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Bronze Supporter
Nov 30, 2019
362
Pawleys Island, SC
I think I speak for all when I say we like our PH60. I very rarely have to cal it. Check it occasionally against a standard 7.0 solution. It pretty much always reads on the money.
 

JamesW

TFP Expert
Mar 2, 2011
21,509
Calcium chloride is available as calcium chloride anhydrous (no water molecules attached to the calcium chloride) or as calcium chloride dihydrate (two water molecules attached to each calcium chloride molecule).

Calcium chloride dihydrate is 76% by weight calcium chloride and 24% water.

If the app doesn't allow you to select which calcium chloride you are using, it might be taking the average of 76% and 100%, which is 88%.
 

mgtfp

Bronze Supporter
Mar 5, 2020
561
Melbourne, Australia
Pool Size
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Chlorine
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Astral VX 7T
Calcium chloride is available as calcium chloride anhydrous (no water molecules attached to the calcium chloride) or as calcium chloride dihydrate (two water molecules attached to each calcium chloride molecule).

Calcium chloride dihydrate is 76% by weight calcium chloride and 24% water.

If the app doesn't allow you to select which calcium chloride you are using, it might be taking the average of 76% and 100%, which is 88%.
PoolMath does let you chose between anhydrous and dihydrate.

In case of anhydrous, it tells you to add about 111g per 1000l to raise CH by 100ppm - this is due to the difference in molecular weight of calcium chloride and calcium carbonate (the molecular weight of Cl is actually closer to 35.5 than 35, with two Cl, this adds about another g to the 110g I was mentioning first).

In case of dihydrate, PoolMath tells you to add about 147g, this is basically the above anhydrous value divided by 0.76 to account for the water content.

I guess that just accounts for the "fillers" I was mentioning, i.e. when the instructions on a product bag tell you to add 1.3kg per 10000l to raise CH by 100ppm, then this is indicating that the product contains mainly dihydrate - the "fillers" are just water. I assume that even anhydrous will turn over time into dihydrate in an opened bag in a humid environment.

Thanks for clarifying this, James.
 
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mgtfp

Bronze Supporter
Mar 5, 2020
561
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
Incidentally do you recommend the pH tester you’re using? The rainbow test is infuriating.
Yes, I am very happy with my Apera pH60. Just make sure to keep it calibrated (when it was new, I was quite OCD about that, but I settled now on a monthly calibration). Make sure to work "clean", i.e. rinse the sensor a couple of times with pool water before making the measurement. I also always store it in storage solution, I found that like that the readings are more reproduceable - especially the time it takes until I can take a reproduceable reading. And when rinsing the sensor with distilled water and drying it with a paper towel before putting into storage solution, you can actually reuse the storage solution. I always keep the provided cap filled with storage storage solution and use my CCL pH test-tube for my water testing.

Play a little with the waiting time before taking the reading. If you just want to replace the colour based test with the same accuracy (about 0.1-0.2), then you can take the reading after about 30sec.

If you want to make use of the two digits of the PH60 (compared to the 1 digit of the pH20), then you should wait a couple of minutes - probably not required for daily pool maintenance, but nice to have when trying to "understand" your pool (especially when being at home due to lockdown...). With the pH60, I actually see the pH fluctuations over the course of the day when chlorine is being created by my SWG and when it's being used by organics and UV. And how over the whole cycle (by measuring pH always at the same time), the SWG-chlorination is actually pH-neutral (as long as your TA is low enough to minimize pH-drift by CO2 outgassing).

In the end, I decided to get the pH60 (and not the pH20), because the sensor head can be replaced. I like the idea of being able to fix something rather than throwing the whole thing out (including its packaging).
 
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