My water is close to the recommended balance and I'm about to starting using my SWCG that I just installed. I drained 60% of my pool water and slowly crept up on my goal values to avoid overshoot. That story is outlined in my thread in Just Getting Started. Along the way, one thing I noticed and remarked upon in my thread but might be of wider interest is that I found that the Pool Calculator significantly underestimated the amount of muriatic acid I needed to bring down my pH, whereas the acid demand test in my Taylor K-2006 seemed to estimate it quite well.

My test kit only measures pH up to 8.0 so I disregard those measurements where from hindsight I know it was clearly higher. But two days ago, I measured a pH of 7.8 (my first measurement clearly below 8.0). Acid demand test showed 1 drop to 7.6 and 2 drops to 7.4. The formula for fl oz of 31.45% muriatic acid used to populate Table E of Taylor's Pool & Spa Water Chemistry handbook is MA (fl oz) = 9.165 * 1e-4 * N * V where N is the number of drops required to reach goal pH and V is the volume of the pool in gallons. (See also the first equation in Wojtowicz's paper here: http://jspsi.poolhelp.com/ARTICLES/JSPS ... p39-56.pdf ). [edited to fix notation in equation for clarity]

Since I was aiming for 7.5 (not realizing at the time that 7.8 is in fact considered "ideal") I used 1.5 drops and 12000 gallons for my pool in the equation and came up with 16.5 oz of muriatic acid. On the other hand, using 12000 gallons in the Pool Calculator, and entering TA = 70 (test performed using speed stirrer) and 0 for borates gave 8.5 oz. This is very nearly a factor of 2 difference!

I decided to split the difference and added 12 oz of MA. (Unrelated but I also added 40 lbs of solar salt). After 24 hours of my pump running, I checked my pH and it was 7.6. From the acid demand test the day before, to get to 7.6 was one drop corresponding to 11 fl oz which was close to what I added, so the acid demand test was as accurate as one might expect from reading the color scales. From the Pool Calculator, adding 12 fl oz should have brought my pH down to 7.4, but my pH clearly did not go that low.

I then added 4 more oz for 16 total of MA and the next day I measured it at 7.5, so the original acid demand test was accurate while Pool Calculator said that 16 oz should have brought it to 7.3. I also reconfirmed my TA is at 70.

What is going on here? I thought of two possibilities for seeming inaccuracy in Pool Calculator estimates for MA. (Note that uncertainty in the volume of my pool should impact both methods in the same way).

1. Borates. I replaced 60% of my pool water and I assume tap water doesn't have borates. (Last night I added 25 lbs of boric acid but this was before I added them). It's possible my old pool guy had added something with borates but as far as I know he only added trichlor or dichlor. I actually measured them at the same time I measured 7.5 pH using the LaMotte's strips. The test strips are hard to read and it seemed to match the 15 ppm square almost as well as the 0 square, so I noted 0-15 for borates. But playing around with the Pool Calculator, for TA 70 and 12K gal pool, to add 16 oz of MA and go from pH 7.8 to 7.5 requires 23 ppm borates and that is clearly higher than I had in my pool at the time, but it can't be ruled out that some small concentration of borates could be part of the issue.

2. CYA. The Wojtowicz paper referenced above provides tables of MA needed to reduce pH to 7.2 for various start values of pH and ppm of TA. What is interesting is that he provides different tables for 50, 100, 150, and 200 ppm of CYA. He also provides a multiple linear regression showing MA needed vs. starting pH, TA, and CYA, and the factor in front of CYA is ~0.13. An example from the table is that is takes 26.4 oz MA to reduce pH from 7.8 to 7.2 at TA 90 with CYA 50 , and 32 fl oz at CYA 100. FWIW my CYA is 75.

I've read several posts claiming that the acid demand test in the K-2006 is superfluous, but since it is a more direct measurement for pH change and thus unaffected by errors in borate (and CYA) measurements, perhaps it should be acknowledged that it is the more accurate way to estimate the need for MA? Personally, I'd rather take the few extra seconds it takes to do the acid demand test and get the correct amount of MA to add the first time, rather than having to add MA twice. Wow that stuff is nasty!

Although it is acknowledged on Pool Calculator that adding CYA will lower pH, as far as I can tell it does not account for CYA level when calculating pH changes (or perhaps it assumes a fixed value). Why not?

I realize that the Pool Calculator has been verified by 1000's of users whereas this post is based on the experience of only one inexperienced pool owner over two days. Big difference. But I will also note that I've read several seemingly experienced posters talk about the need to "get to know your own pool" as far as the amount of MA to add regularly, particularly for SWCG owners dealing with pH drift. The Pool Calculator is a obviously a great tool and it's been very helpful to me. I offer this post as a suggestion for making it more accurate, if it in fact accounting for CYA as well as TA and Borates would do so.