New pool owner, trying to get my chemicals balanced

[Watermom]

but I'm not sure what else could have caused it, other than not putting as much chlorine in daily or every other day in that rainy period?

Yep. That'll do it. We see it all the time. Usually it is people who come onto the forum and tell us that the rain caused their algae which isn't the case. Usually it is because people aren't as diligent about keeping tabs on their pool when it rains.

As long as you are aware and watching to keep your chlorine levels in the proper range, you'll be fine.

Hope this helps!

[mitchryan912]

Yep. That'll do it. We see it all the time. Usually it is people who come onto the forum and tell us that the rain caused their algae which isn't the case. Usually it is because people aren't as diligent about keeping tabs on their pool when it rains.

As long as you are aware and watching to keep your chlorine levels in the proper range, you'll be fine.

Hope this helps!

I guess I got complacent over the course of the first 2 weeks of July where it stayed above 3.0 ppm, where I didn't really add too much chlorine except for a 2.5 gallon shock on the 1st, some amount on the 9th (probably a quart or two), and then some before a big pool party on the 12th.

I'll keep a better eye on it now so I don't come back with algae problems in the future!

[mitchryan912]

I went back and tracked the weather in the timeframe my FC levels were quite stable, and I think I can conclude 2 things:

1. FC stayed stable because there were a TON of cloudy & partly cloudy days here in the beginning of the month.
2. The chlorine I've been using since the 1st has been some bargain deal chlorine from Menards that was on sale for $1.99/gallon.

I'm guessing that the former helped considerably, but the latter... I think the 12.5% chlorine I've been using lately isn't actually the 12.5% that it's advertised as. I put in 1 gallon earlier today, and it only raised FC up from 2.0 to 4.5 ppm. I added another 2 qts about an hour ago, and will be testing it before I go to bed and then again in the AM, to see if there is anything funny going on.

EDIT: Just checked FC at night before bed, and it's up to 8.0 ppm, which is in line with what you'd expect with 10% chlorine (2.0 to 8.0 via 6 quarts.)

[Watermom]

You will definitely use less chlorine on cloudy days. And, I agree with you that 1.5 gallons (6 quarts) would raise your chlorine by about 6ppm if it were 10% sodium hypo. It is pretty typical for many of the bottles of liquid chlorine to not be the strength they are advertised. That will be especially true if they sit on a shelf for awhile in a hot store. That is one reason we like Walmart's generic bleach. They have a fast turnover and the bottles are in an air conditioned store which further slows the break down.

[mitchryan912]

My morning chlorine level was at 7.5ppm, so that seems like a reasonable amount of chlorine loss, especially if there might have been anything trying to bloom when the chlorine dropped low. We're expected to hit 90 degrees for the first time this year, so I'll be watching it like a hawk and probably doing a second test this evening to see how it's changed over the day.

I think I'll be going back to the pool store for 12.5% after I use up the remaining 3 gallon jugs I bought at Menards. I'm not even sure that the jugs I used previously were 10%, considering how ineffective at least one of them was. Roughly 2.5 gallons out of Midwest's 5 gallon jugs brought me from .5ppm to 5ppm 2 days later (and both measurements were done at Midwest, so who knows how accurate those were.)

This brings up a question I've been wondering about the "Best Guess" method: assuming a pool with 40 CYA, is <4 ppm FC pretty much equal to a pool with 0ppm CYA and 0 FC? If this isn't clear, does the CYA effectively bind up a lot of the chlorine such that it's there, but not always available until the CYA-chlorine bond separates? I guess I'm just curious why there is a minimum of 3ppm when CYA is between 30-50ppm?

[PoolDoc]

I need to update the Best Guess page, to use a more % based dosing.

However you cannot follow the dose curve all the way down to 0.1 FC at 0 CYA, because of the fragility of very low FC residuals: any sunlight, and almost any chlorine demand will eradicate the residual, leaving the pool unsanitized.

Also, chlorine + UV is much more effective at destroying organics and complex chloramines than chlorine alone.

The kinetics of the Cl-Cya molecules is such that, when you have a HIGH CYA level, there's is a functionally limitless reserve of 'true' FC available. This is a factor I need explain more clearly on the Best Guess page, too.

[BigDave]

... does the CYA effectively bind up a lot of the chlorine such that it's there, but not always available until the CYA-chlorine bond separates? ...

Pretty much exactly. In a pool with chlorine and CYA most (almost all) of the chlorine will be bound to the CYA and some will be available for oxidising stuff in the pool (also available or destruction by the sun's UV). There's an equillibrium between the available chlorine and the CYA bound chlorine - as the available chlorine is consumed more is released from the CYA. The CYA can be thought of as chlorine storage, releasing available chlorine when and where it's being used. The equilibrium point depends on the ratio of chlorine to CYA, if that ratio's small then here will be very little chlorine available for cleaning the water.

@PoolDoc: I think you meant unsanitized in the second sentece of your post.