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Last week I was sitting at:
PH: 7.5
TA: 80
It rained about .5 inches last night and now:
PH: 6.8-7.0
TA: 40
Both tests were done very carefully by me with my fas-dpd kit and last week's numbers were in line with what the pool store test a day earlier showed.
The only thing I did was shock the pool up to 15 ppm to get rid of excess CC, which it accomplished taking my CC from 1.6 to .5.
The low pH can nuke your alk, the trick is finding out what caused your pH to drop. Cal Hypo (if that's what you used) raises pH, so there's something else at work here. 0.5" of rain (assuming 20k gal pool) isn't enough to move the numbers that much I don't think.
If the chlorine level was high due to shocking at 15 ppm FC, then the pH may not be accurate and could falsely read higher than it really is. Nevertheless, even if it were accurate, if the FC drops from 15 to 3, for example, and if 1 ppm of this drop "broke" Combined Chlorine, then the pH would be expected to drop frm 7.5 to 7.04 due to the fact that the process of chlorine breakdown (from sunlight or oxidizing organics or breaking down ammonia/urea or monochloramine) is an acidic process. So the drop in pH is understandable if the FC was dropping along with it.
The above scenario would only have the TA drop from 80 to 70, however. To have the TA drop to 40 would require a lot more acidic processes or additions along with aeration to drive out carbon dioxide. Raindrops can certainly aerate the water, so starting with the above numbers of 7.04 pH and 70 TA and going to 6.9 pH and 40 TA would require the equivalent of 72 fluid ounces of Muriatic Acid in (assumed) 15,000 gallons. If we assume 16x32 feet for the pool area, then 0.5" of rain is around 160 gallons so the required amount of acid in that volume would have a pH of 1.5 while even acid rain doesn't get below 4.5 so the very low TA number you measured is suspect and unexplained.
Richard
In some localities the pH of rainwater can become very low (acid rain). This can lower both pH and TA but I don't think .5" of rain would have THAT much of an effect unless the rain was very corrosive and acidic! Is it possible that the rain washed something acidic into your pool?
Well guys, I searched the archives for info on adding borax. Understanding that the effect on ph was reliant upon the TA I added it in small dosages. Ended up adding just over 4 pounds (which was all I had). It rained a ton last night (likely an inch total) and testing today ended up with PH 7.2 and TA of 50.
I assume I should just continue to correct the ph with borax; then address the TA with baking soda. Is that correct?