Pool eating FC!

[Whjr15]

Hi guys! Just wanted to say, again, how awesome this site is. It’s truly an invaluable resource!

Anyway, after having my pool up for about a month now (24’ round AG 52”), with the help of this site, I’ve been keeping it maintained without the major hassle I went into it expecting. However, I've come across something that I would like advice on.

This morning (about 7 hours ago, from this posting), I added 10% bleach to achieve a FC of 21. (Pool math app must’ve glitched out on me, because I was shooting for 7!) The problem that I’m observing is that, a little over 7 hours later, with no swimming, the FC dropped to 15.5. Is an FC loss of 5.5 in 7 hours normal? From what I’ve been reading on here, I suspect not. Water is also brilliantly sparkling, with no visible indication that anything is wrong.

Other relevant info:
CYA: 40
CC: 0

Thanks!

 

[ajw22]

FC loss is not linear. The higher the FC is the greater the rate of loss. You are ok.

 

[AimeeH]

Also, did you test is at 21 or just that you added enough chlorine to assume it was up to 21?

 

[jblizzle]

With a CYA of 40ppm, I would not be surprised if you lost up to 50% of the FC to the sun in 1 day if you have a lot of exposure.

 

[Whjr15]

I tested it using the tf-100 kit. The pool math app sometimes gives me drastically different suggested amounts, and I’m not sure why. It’s never been THIS big of a discrepancy, but I’ve always caught it & double checked before adding anything.