When determining pool volume with a chemical test which test is best?

[NickJ]

I have a pool with a very complex shape (hopper style) and received very detailed instructions on using the TA test and precisely adding baking soda (Sodium bicarbonate) to raise TA or acid to lower it and using the new TA (total alkalinity) achieved to back figure the pool volume but I don't understand how this can work. I appreciate the instructions but something must be missing since no specific starting PH or TA is required. The tests do suggest running the pump at least 3 hours after addition but in my experience depending upon the PH or TA of the water initially one impacts the other. In my case acid demand seems high and its impact on TA seems lower than expected. It would seem using an addition of calcium chloride and measuring the increase in calcium hardness would be immune to such interaction and a better test for measuring pool volume so I hope someone will explain this to me.

But recently I did have far less increase in CH after adding over 40 pounds of calcium chloride so I would appreciate instruction on what I may have done wrong. I dumped it directly into the pool and brushed away any flakes that reached the bottom. It appeared to have totally dissolved. I did not back flush the filter for 5 days but I did run my Dolphin pool cleaner. I now wonder if the vacuum bag of my Dolphin cleaner could possibly have collected un-dissolved calcium chloride I could not see but as soluble as it is supposed to be I would have thought the massive amount of water moving through the bag would have finished dissolving any it might have picked up.

I did do this during a SLAM with elevated chlorine if that can have any impact. Thanks

 

[HermanTX]

You have to know a starting point for any chemical before you add something to change it. Then the difference in the 2 values can provide an estimate of pool volume based on what you added. I would test 30-60 mins between the 2 tests if doing pH or TA test because MA mixes quickly.
pH and TA have the least error potential - for example each drop of reagent for TA is 10ppm - for pH it becomes more subjective using a color comparator and deciding 0.2 changes.
With CH, each drop is 25ppm using a 10ml sample.
Salt is a +/- 200ppm per drop
Adding chlorine can be challenging because either UV or unseen algae can be affect it.

Adding CH directly to the pool is preferred so that all looks correct. It can take 48 hrs to register because going through the filter. Based on PM calculation, 40 lbs in 28000 would increase CH by 150ppm - so 125 to 175 would be expected. You could use a 25ml sample to get to a +/-10ppm if you wish.

Hope this helps.

 

[NickJ]

You said "40 lbs in 28000 would increase CH by 150ppm - so 125 to 175 would be expected." Was that supposed to be 275? Typos seem to creep in but I always assume I might have misinterpreted. Thanks

 

[HermanTX]

You said "40 lbs in 28000 would increase CH by 150ppm - so 125 to 175 would be expected." Was that supposed to be 275? Typos seem to creep in but I always assume I might have misinterpreted. Thanks

No - according to my pool math app - it shows 40lbs of calcium chloride in 28,000 gals will only raise CH by 154ppm
PoolMath
I double checked it on the web version of PM.
Maybe @Mdragger88 or @Newdude can double check me.

 

[NickJ]

This year I purchased the $150 TFT Pro testing kit but have always used the Taylor 2006 in the past. The Pro kit oddly says to use the 10 ML sample with 25 multiplier you mentioned above whereas the Taylor 2006 first says to use a 25 ML sample with a multiplier of 10 and then in small print says if you expect very high CH you "can" use a sample o f 10 ML. I also don't like the Pro kit's PH test. It too is FAR less accurate with no acid demand test. I guess with Pool Math you are not supposed to not need it. The 44 ML sample size of the Taylor compared to 10 ML in the Pro kit's PH test alone tells you it can't possibly be as accurate.

 

[Newdude]

Maybe @Mdragger88 or @Newdude can double check me.

You're good for Calcium Chloride.

40 lbs of Calcium Chloride Dihydrate is 117 in 28k gallons.

then in small print says if you expect very high CH you "can" use a sample o f 10 ML

Nobody needs to know their CH down to 10 ppm. 25 ppm is plenty close enough.

It too is FAR less accurate with no acid demand test. I guess with Pool Math you are not supposed to not need it.

I've never had or used the acid demand test. If I'm particularly out of range for Ph, multiple doses may be needed from 8.2 back to a 7, but we recommend only dosing Ph by .4 at a time when large corrections are needed anyway. Having a better idea that the Ph is 8.6(?) really doesn't matter when looking at it as we do.

The 44 ML sample size of the Taylor compared to 10 ML in the Pro kit's PH test alone tells you it can't possibly be as accurate.

The important take away is that it's accurate enough. Any of the other tests in the test kit are accurate to +/- 1 drop, which while not perfect, is plenty good enough for what we need to do.

 

[NickJ]

Sorry, I see what I misinterpreted: You were referring to the "increase in CH only" but I assumed you had read my earlier post and were using my starting CH and were saying the added calcium chloride should have raised it from 125 to 175. If the 42 or so pounds had raised my CH by 150 I would have at least 270 but I only got 220 which is why I think I must have done something wrong. Sorry my misinterpretation of what you were telling me caused you so much trouble.

Do you think my theory to explain this of the Dolphin pool cleaner grabbing un-dissolved calcium chloride would explain it? Thousands of gallons are forced through its cloth bag but I guess it is possible that dis not dissolve all it took out of the water. I have 40 pounds arriving tomorrow and will make sure this time to backflush first and NOT run the pool clearer for at least 48 hours.

 

[HermanTX]

Sorry, I see what I misinterpreted: You were referring to the "increase in CH only" but I assumed you had read my earlier post and were using my starting CH and were saying the added calcium chloride should have raised it from 125 to 175. If the 42 or so pounds had raised my CH by 150 I would have at least 270 but I only got 220 which is why I think I must have done something wrong. Sorry my misinterpretation of what you were telling me caused you so much trouble.

Do you think my theory to explain this of the Dolphin pool cleaner grabbing un-dissolved calcium chloride would explain it? Thousands of gallons are forced through its cloth bag but I guess it is possible that dis not dissolve all it took out of the water. I have 40 pounds arriving tomorrow and will make sure this time to backflush first and NOT run the pool clearer for at least 48 hours.

No issue. Always good to double check calculations.
Your pool volume could be off by a little as well and that affects actual results. We got 2 inches of rain recently added to the pool. I do not have an auto drain so I just left it because next week the sun will evaporate it. So adding chemicals today will be off because 2 inches can be 1000 gals added to pool volume.

As @Newdude stated - we want to be accurate - within reason - we are not determining the glucose in your blood! We are limited by the +/- 1 drop of the reagents we are using.

I do not think your Dolphin pool cleaner grabbed and held the calcium chloride. I routinely run my pressure side cleaner after adding a bag of salt to help mix even after I brush it around. Before adding any more calcium chloride, test again to determine your starting point.

As we always recommend, when increasing CH or CYA, do it in stages because overshooting it means draining water which is more costly and time consuming.

 

[Newdude]

we are not determining the glucose in your blood!

Ironically that's worse. With a blood sugar of 100, the meters are +/- 20, which is the high and low #s (120 or 80) usually needing a correction. You could be dead on perfect and be considering a correction on either end.

It's the diabetes version of testing at Leslie's.

 

[HermanTX]

Ironically that's worse. With a blood sugar of 100, the meters are +/- 20, which is the high and low #s (120 or 80) usually needing a correction. You could be dead on perfect and be considering a correction on either end.

It's the diabetes version of testing at Leslie's.

Obviously I am not a doctor, but I was hoping that glucose test was more precise - oh well - bad analogy.

 

[NickJ]

One thing I did want to comment on:

The title of my post (about determining pool volume chemically) seems to have been lost since many comments seem directed at balancing the water for normal pool use "only".

While I do appreciate those comments, thank you, I am hoping to use the CH test to determine my pool's volume by adding a known weight of calcium chloride (adjusted for the %) and back figuring the pool volume versus the PPM hardness it achieves. I chose CH because it is too low and I need it but it also seems to me it should be more immune to interactions between PH and TA but no one has confirmed that.

When I added calcium chloride (about 43 pounds) previously Pool Math predicted it would raise CH far more than it did (achieved about 100 ppm when 150 was predicted) so I was hoping for some explanation other than my"calculated" pool volume being off that much. I feel sure I did something wrong and wasted almost half of the calcium chloride I added.

I have another 40 pounds coming today (not adjusted for % yet) and I hope to repeat the test measuring everything as precisely as I can. My CH is down to 200 due to back flushing a lot of rain so I can still stand a significant increase. Back when I used powdered chlorine ( several years ago) I had the opposite problem (accumulating too much hardness),

I appreciate any suggestions on what I did wrong and how I should correct it.

 

[Newdude]

Just verify if its calcium chloride or calcium chloride dihydrate and we can see if last time was a fluke.

 

[HermanTX]

The title of my post (about determining pool volume chemically) seems to have been lost since many comments seem directed at balancing the water for normal pool use "only".

While I do appreciate those comments, thank you, I am hoping to use the CH test to determine my pool's volume by adding a known weight of calcium chloride (adjusted for the %) and back figuring the pool volume versus the PPM hardness it achieves. I chose CH because it is too low and I need it but it also seems to me it should be more immune to interactions between PH and TA but no one has confirmed that.

Post #2 addressed what the potential error is with each test.
Since you want to use CH to back calculate the pool volume then I would suggest using the 25ml sample and then each drop is 10ppm of CH.
As noted before based on 28k gals, the CH should have increased by 150ppm over your starting CH level with 40lbs of calcium chloride. Although you did say "over 40 lbs was added" I just used 40 lbs for the calculation. The error would be 125 to 175 ppm. I now have used 43 lbs and the predicted increase for 28k gals would be 166ppm. Since the increase only was 100ppm that implies the volume is greater than 28k gals. Back calculating it would indicate your pool may be closer to 40k gals.
So to obtain more concise calculations I would do the following
Test for CH using 25ml sample - this is the baseline
Get a food scale and measure the amount of calcium chloride (CC for this discussion) added (take into account the weight of the container) - your CC should be 100%.
Add CC directly to pool, then brush it to mix well - keep pump running
Test after 6 hrs and record
Test after 12 hrs and record
Test after 24 hrs and record - if the results of 12 hr and 24 hr are consistent then use the last for the calculation - if not then wait for another 12 hrs and test again
All tests should use the 25ml sample with each drop equaling 10ppm of CH
Once you have consistency the difference in the baseline and ending values will be used to determine the volume based on whatever amount of CC added.
pH and TA do not affect CH testing