low FC to High CYA-does this upset pool water balance?

[duffer-AL]

I used 3" tablets a few months ago but it took 1month to see the rise in cya from 40 to50. To my surprise 1 month later(sept 12,2015) it measured 60. I used the TF100 kit. 2 questions-1. Obviously I can up my FC level, but If I leave it at the present level FC=4 what are the consequences. 2. Why did kit take so long from the time the tablets dissolved to when it registered. I have tested cya about 1-3 days after they dissolved and found no readings.
duffer-1 (Ralph)

 

[tim5055]

Welcome to TFP!

CYA takes awhile to register on tests, so if you checked it a couple of days after the tabs were gone it is possible not all was showing on the test yet. Add to that the error rate built into the test and a change from 50 to 60 is not unreasonable.

Leaving the FC low will make you more prone to get an algae outbreak. AT CYA 60 the minimum FC level should be 5 and you generally want to target slightly above that so that it does not drop low during the day or with a high bather load in the pool.

See the Pool School - Chlorine / CYA Chart

 

[duffer-AL]

Thanks Tim5055, I really appreciate the fast response. Helps a lot. re: potential algae outbreak- would dosing with Polyquat about 8 oz for my 18,500 inground vinyl pool provide some insurance against algae breakout?

 

[tim5055]

Yes. If you want to go with that added expense, then a quat based algecide allows lower FC levels. Is there a reason you just don't up the FC a little?

Remember, the "strength" of the chlorine in the water is moderated by the higher CYA. So, as long as you stay within the ratios listed on the chart I linked then the water "feels" the same. When I took over my pool from the previous owner of the house the CYA was well above 200, 250 is what I'm guessing. I kept my FC in the neighborhood of 20 for most of the first summer as I worked the CYA down. No one noticed the higher FC levels, again because it was kept within the ratios.