Quote Originally Posted by MrTodd View Post
Wonderful explanation! I have a question, you said 1/4 gal. per 10,000 gal. for treatment. How much will that lower your PH? Is there a good "rule of thumb"?
Short answer:
NO, it can't be predicted, but it can be measured!
Long answer:
How much effect a given dose of acid has on pool water CAN be calculated in theory. But in actual pool practice it cannot, for multiple reasons:
  1. Incorrect pool volumes -- Most pool owner's estimates of their pool's gallons is INCORRECT by more than 15%. In many cases, the error exceeds 40%!
  2. Testing errors -- in order to calculate the effect of a given dose of acid, you need ACCURATE measurements of the current pH, the carbonate alkalinity, the borate level, the cyanuric acid level, and possibly, other values. But, even with the best available test kit (the K2006) errors often exceed 10%, and exceed 20% with stabilizer values. If the pool owner is using other kits or 'guess strips', individual test value errors may exceed 80%!
  3. Non-linearity -- because of complexities of buffer chemistry, pH change calculation in the presence of weak acids (boric acid / borates; carbonic acid / carbonates; cyanuric acid / cyanurates; etc) the change produced by a given dose does NOT follow a line, or even a smooth curve.
PoolCalculator.com attempts to provide the sort of calculation you ask for, but is notorious among the support team here for producing bizarre and misleading results, even assuming that valid data was provided. Of course, no calculator can out-perform the data it is supplied, and as noted above, pool owners CAN NOT provide the sort of data needed for accurate prediction.

HOWEVER, while pH change cannot be reliably or accurately predicted, it can be easily measured with the acid and base demand tests in the K2006.

Whether that has value or not is debated. My own opinion is that it has NO value for well operated pools, but does have value for a service guy encountering badly operated pools needed urgent correction.

What isn't really debatable is that the most practical approach, that will produce the most consistent results in pool care is using the test-small dose-retest-redose approach, instead of the all-at-once approach espoused by pool stores.