CYA drop?

[JBrooks]

I've been maintaining my new pool by myself for about 6 weeks. After some initial weird things happening, things have settled down and have been pretty stable. For the last week or so I noticed FC dropping quicker than usual, so I've been supplementing with liquid chlorine. I figured the FC drop was because I dropped my pump down to 8 hours a day and chlorine generator to 30% (after running 12 hours a day/100% by the PB), so I raised the generator to 40% but it hasn't really helped. I've been testing FC and PH every 1-2 days. Just added LC yesterday.

Today is the first time I've tested everything in about 2 weeks and noticed my CYA dropped to essentially zero. Just to be sure, I tested twice (see pic - not even in full sun). The kit is brand new. Previously, it had been consistently around 35 for a few weeks according to my test kit, although the pool store tested my water and said it was 100 ppm

We've been getting a lot of rain. I would say I've had to drain off 6-8 inches of pool water in the last 1-2 weeks. Is it safe to assume the drop in CYA is due to this water run off? Or is there another explanation? I have never added pool stabilizer - I'm assuming my PB did. I just bought a 7 lb bag of granular pool stabilizer at the pool store but I haven't added any yet. The pool calculator says I need to add 6 lbs, 6 oz. to reach CYA of 70.

FC 6.5
PH 7.6
TA 100
CH 275
CYA ~0

 

[willygee]

My pool and location similar to yours...8 inch drop and then rain topped it off brought my CYA down from 100 to 80. I keep FC in the 6 to 10ppm range. Water is xtal clear. PH does climb after these Fla rains..a little MA cures that.

 

[JBrooks]

My pool and location similar to yours...8 inch drop and then rain topped it off brought my CYA down from 100 to 80. I keep FC in the 6 to 10ppm range. Water is xtal clear. PH does climb after these Fla rains..a little MA cures that.

Yeah the water is completely clear. The PH likes to rise to 8.0 so I add MA to bring it back down to 7.6. I've added about 1 gallon over the last 4-5 weeks.

So I'm guessing it's just all the draining that's causing the essentially zero CYA now. It's just odd that the pool store said my stabilizer was 100 ppm when I was measuring 35 with my kit. Now I'm measuring zero, so I'm taking another pool sample back tomorrow to see what they measure. It's just unsettling when the numbers are so drastically different between what I measure and what they measure.

 

[an1vrsy]

Trust your testing. You have a great kit. Try the instructions for the Extended CYA Test

 

[AK-]

On a SW system you should keep your CYA on 60-79 (see FC/CYA Levels).

Keep in mind you cannot really test CYA below 20.

A lower CYA will lead to more chlorine loss to UV. On my pool with CYA at 30 was losing 5-6 ppm per day on peak season (heated pool), by increasing CYA to 40 my chlorine loss dropped to around 4ppm per day.

 

[Mdragger88]

Trust your testing- like mentioned above there could be some cya, just too low to measure so take that into account before adding. Add about 1/2 of what PoolMath calls for via the sock method then retest in a day or so. Doing it this way will ensure you don’t overshoot your target & have to drain. As soon as the sock is started chlorinate to the new corresponding cya level according to the FC/CYA Levels.

 

[JBrooks]

OK thanks everyone. I'll try adding half of what the pool calculator says then retest after 24-48 hours.