Calcium level consistently dropping?

[Galun]

I've owned my house with the pool for about 1.5 years. The pool had very old plaster and probably needs a replastering soon. I had been using BBB and testing my own water for about a year now.

I opened the pool over the weekend and did the full TFTest for the first time in about 6 months. I didn't do the full test during winter as I figure I could get the pool into balance during opening week. I do check FC and PH using the quick test every week just to make sure the automated systems are working as intended. Anyways, TA and CYA were basically the same as 6 months ago (80 and 30), but CH level went from 230 to 170. I also had the same experience last year during the swim season as the CH level dropped from about 250 in April last year to 180 when I closed the pool in October. I remember putting in 8lbs (or 15lbs, forgot what pool calc told me) of hardness up chemical into the pool, tested that CH was raised from 180 to 230, and then closed the pool.

Any ideas on where the calcium went? Looks like CH can only go down with reverse osmosis, splash out, and dilution. Not sure how reverse osmosis can occur. There certainly wasn't splash out when the pool was closed. As for dilution, my CYA level had stayed the same. I do lose water and I wasn't sure if it's evaporation or possibly a small leak. But if it's a small crack, why would I lose CH but not CYA?

Any help will be much appreciated. Thanks!

EDIT: Added full stats in case it helps.

FC = 5
CC = 0
PH = 7.5
TA = 80
CH = 170
CYA = 30

I must say that's not too bad as a first test after minimum maintenance for 6 months!

 

[duraleigh]

Do you have an autofill?

 

[Galun]

Do you have an autofill?

No I don't. I use a garden hose to add water when the water drops below the mid point of the skimmer. It takes a few days to a week after a fill before it drops below the line again, and it seems to be dependent on the temperature.

It am fairly certain it's not the case of a noticeable leak masked by the auto fill, since I "top off" manually.

 

[duraleigh]

In that case, I suspect an error in one or more of the tests. CH simply doesn't go away (I know you know that....I am just reinforcing).

Your logic is correct in that CYA should've moved downward, too, so my first suspect would be the CYA test. It's pretty easy to be off by 10ppm or maybe a little more so that, or an error in the CH test, would be the only logical thing left.