Residual Chlorine Value?

[chemistrydropout]

20x40 IG vinyl liner 25000 gals no Swg or heater

Last night at 10:00pm
TC= 2
FC= 2
CC= close to 0
PH=7.5
Alk=70-80
Ch= very low aquachem doesn't test this value well
Cya= Between 20-30 say 25
Water temp= 90

Due to lots of rain lately the chlorine values have been very hard to maintain. Some evenings my chlorine values are virtually non-existent at .5 and 1. This is after 100 degree days and then 1" rainfalls and such. Last night I noticed the liner does not have the squeaky clean feel that it has been having.
1. My question is about residual chlorine value. I think I know the answer but just want to make sure. Best Guess Table says I need to maintain a value of between 2-5. When adding chlorine in the evening do I shoot for a value high enough so that the next evening I end the day with at least this much free chlorine ppm? OR do I just add enough chlorine to reach a target FC of say 4 or 5 at that time?
2. I guess the real question is does bringing the FC up to say 4 or 5 in the afternoon protect my pool or should I be attempting to allow for the 2 or 3 ppm loss each day and try to have a "residing chlorine value" 24/7 of at least 2 or 3?

Does this make sense to anyone?
Thanks in advance!!!

[cwstnsko]

I would add enough each night to have the FC value you want the next evening. Allowing the FC to drop very low just opens the door to problems. Holding the FC values a little higher through the night and early morning hours just provides that much more time for the chlorine to do it's job.

[aylad]

You should be attempting to allow for the expected loss and still have a minimum of 2 or 3 after the loss-in other words, never let it get below that 2 or 3 minimum.

Janet

[chemistrydropout]

Thanks! That's kind of what I figured and was expecting but it has been very difficult to accomplish this lately with hot sunny days and afternoon t'storms. Normal days even hot days I was experiencing 2 to 3 ppm loss but this rainy hot has been a pretty big problem with me being new to the pool maintenance thing. Saturday I got over 3 inches of rain and that was a real problem especially after I had added fill water just that morning. PH was off the colorameter, alk was low, cya had dropped some, and the chlorine was almost non-existent.

Should I be expecting to see these lower cya levels due to dilution or is it because I am at the very bottom of the range of my testing capabilities for cya? Everyone says you add cya once a season generally!!

[chem geek]

Saturday I got over 3 inches of rain and that was a real problem especially after I had added fill water just that morning. PH was off the colorameter, alk was low, cya had dropped some, and the chlorine was almost non-existent.

Should I be expecting to see these lower cya levels due to dilution or is it because I am at the very bottom of the range of my testing capabilities for cya? Everyone says you add cya once a season generally!!

If you were adding fill water to make up for water that had evaporated from the pool, then you haven't had any net change in pool water chemical amounts [EDIT]except for adding whatever is in your fill water[END-EDIT]. When pool water evaporates, everything in the pool gets more concentrated, but when you add make up water you dilute it back to where it was [EDIT] except for adding what was in your make-up water[END-EDIT].

It is only the rains that are of a sufficient amount to raise the pool water level so that it spills over (into a drain, or otherwise) that cause a true net dilution of the chemicals in your pool. And yes, this is a problem you will have to deal with by adding chemicals (including CYA, sodium bicarb for alkalinity and even calcium chloride) at least once during your rainy season (depending on how much rain you get -- my guess is that once mid-season might be enough).

As for the CYA test, yes it is hard to measure since most tests only go down to 30 (I've seen some go down to 20) so you are really guessing when you still see the black dot and you've completely filled the tube all the way to the top during the test. The good news is that if you have less CYA than you think you have, you end up with more disinfecting chlorine than the minimum needed (assuming you follow Ben's chart for total chlorine levels); the bad news being that less CYA will have you lose chlorine at a faster rate [EDIT]from sunlight[END-EDIT].

Richard

[waterbear]

One thing to take into account with rain water is that there is acidic rain and it will lower the pH and have an effect of your TA. It might be interesting to collect a sample of rain water and test it.

[chemistrydropout]

Thanks for the assistance.