Very High FC Depletion from Sunlight - How'd my CYA drop?

[RoberTX]

CYA still dropping quickly

So here's what's happened since my last post.

8/5 - added 4 lbs of CYA

8/13

FC 4.0
CC 0.0
pH 7.8
TA 110
CH 260
CYA 55

8/22

FC 7.0
CC 0.0
pH 7.5
CYA 40

So my CYA has dropped by about 15 ppm in 17 days - barely been a trace of rain. It's been hot, 100+ every day. A leak is not suspect, as my CH level remains solid at 260.

My daily chlorine depletion has been better since I added the 4 lbs of CYA on 8/5, but now I'm starting to notice more demand again (as expected due to my CYA lowering).

It has also been somewhat diffiicult to keep my pH down. It typically creeps up from 7.5 to 7.8 every three days, then I add about 32 oz of MA to bring it back down to 7.5

Right now, I'm thinking I will add another 4 lbs of CYA to target close to 60 ppm. I would then anticipate the CYA to reduce to about 40 again in about 3 weeks. Doesn't having to add 4 lbs CYA every three weeks seem a little unusual? :?

I would really like to figure out the cause of such a drastic reduction in CYA. Any ideas?

 

[JasonLion]

Yes, adding 4 lbs of CYA every three weeks is extremely unusual. About the only thing that could explain it is a water leak. You mentioned that you thought a leak was impossible because your CH level went up, but that could happen if you have fairly high CH fill water. You should test the CH level of your fill water to verify this. A leak is by far the most likely cause.

Your PH drift is much more common. That is being caused by a relatively high TA level. If you let your TA go down to around 70 your PH should be much more stable.

 

[RoberTX]

My tap water CH is 120 ppm maximum (average 102), per the city of Arlington. The few times I've tested it, it's been under 120. My pool CH is always exactly 260, it doesn't go up or down. If I had a leak, wouldn't my CH be decreasing? Further, I've left my autofill off, had no rain, and I don't see the water level dropping more than normal evaporation amount.

Thanks,
Robert

 

[257WbyMag]

If I had a leak, wouldn't my CH be decreasing?

No, unless you are topping your pool off with distilled water.

Every time you add tap water to your pool to top it off, assuming that you don't have a water softener, you are adding calcium to your pool. You've tested your fill water and you know it's in there.

 

[RoberTX]

OK... if I take 8 oz of pool water (CH 260) and 8 oz of tap water (CH 120 max), and mix them together, what would the CH of the mixture be? (260 ppm + 120 ppm) / 2 = 380/2 ppm = 190 ppm. 190 ppm < 260 ppm, so replacing some of my pool water with a lesser CH tap water reduces the pool CH. Obviously (even with a small leak) if I am replacing pool water with tap water, my CH would reduce by some amount. Evaporation is different, since the calcium stays in the pool. My simple averaging above assumes the the CH is due to completely disolved calcium deposits, and those deposits would exit the pool with the leak.