CYA disappears over the winter

[fcfrey]

Hi Richard.

I love your deep studies of the chemistry in our pools. Having an understanding of the “why” seats the information much more permanently than memorizing the charts. Keep them coming.

Last fall I drained 6480 gallons out and refilled with well water to bring the CYA level from 45 to 30. I stabilized the chemistry in the pool (based on Ben’s BG Chart) for the winter (without shocking) to the following:

FCL 4.5 -- range 2.2 – 5.1
TA 70
PH 7.5
CYA 30 -- Range 25-35 --- NOT a SWG pool

The pool is covered with a mesh Loop Loc cover which allows the rain and fine dirt to pass through (keeps the big lumps out).

Since the winter (so far) has been very mild, I have been throwing the Aquabot in once a week and checking the chemistry. So far, the FCL has dropped very slowly, and I have made 1 adjustment (Less than a gallon of 6% bleach) since November to keep in my range.

I keep accurate rainfall records so I will know how much dilution from rain/overflow and will be able to predict fairly accurately what the CYA should be in the spring when I reopen the pool. If it is significantly different from the predicted level I will know that your theories have come into play.

Once again --- Thanks for sharing your knowledge, I’ll keep you posted.

As a follow-up to Richard's study:

I opened my pool to find crystal clear water and a CYA level of 25 PPM.
I measured 11.28 inches of melted precipitation (5414 gallons in my pool) over the winter and considering the dilution method was the excess water going out the overflow with very little mixing.

This was about what I expected considering I was able to keep the pool clean and treated until mid December when it froze over and then start cleaning and treating again in mid March.

I fully opened the pool last weekend (removed winter cover) and am currently using about a half gallon of 6% bleach per day in full sunshine and no swimmer load. We will see how it goes but I want to keep the CYA as low as I can afford.

[chem geek]

Thanks for the update. In my own pool, I kept the winter rains out (the pool has an automated opaque safety cover and I had the cover pump take the water into a drain, not into the pool. This winter, the CYA dropped from around 20 ppm to perhaps 10 ppm (it's hard to measure that low) and I maintained chlorine levels the entire time. That sort of change isn't very conclusive but it does appear that at least for some, keeping the chlorine level up prevents the CYA from at least completely going away. For those that let the chlorine go to zero, having the CYA go away is more likely. There are exceptions in both cases, but that seems to be the trend.

Richard