FC Dissipation over Time vs. FC concentration

enginerd · Jul 21, 2015

Does FC dissipate faster per unit time the higher your FC level is above the target FC in the Chlorine/CYA table? I'm trying to see if I'll have to use more chemicals if I add chlorine less frequently, but at higher doses.

For example, my pool has a CYA of 50 ppm right now. According to the table, the target FC level is 6 ppm. If I wanted to overshoot this target so I wouldn't have to add chlorine so frequently, will this result in a faster loss in FC per day than if I simply got it to the 6 ppm target? I would assume that FC dissipation would be faster since the FC/CYA ratio would be higher than optimal (11.5%) since there would be less available CYA "sites" for the FC to bind to, but am unsure as to how much faster. Any chemists in the house?

Another (extreme) example: is there an issue to simply raising the FC to shock level and repeat once the FC drops below the target FC?

Does anyone who plots his/her pool chemistry history see any noticeable increase in dissipation at higher FC levels for the same CYA level? A good place to look at is when you had to shock your pool with something that doesn't have CYA in it (e.g., liquid chlorine) and see if the slope of the FC vs. time plot is higher at higher FC levels. I just started the TFPC method and data logging, so don't have enough data to draw conclusions.

I understand it's a good practice to check the levels every (other) day and adjust accordingly, but am curious about the science as well.

Richard320 · Jul 21, 2015

From this thread: Pool Water Chemistry

https://www.troublefreepool.com/~richardfalk/pool/ChlorineLoss.gif

Yes, the higher you go over target, the faster you lose it. It's not a fixed ppm per day.

chem geek · Jul 21, 2015

Most of the chlorine lost to sunlight is actually from the chlorine bound to CYA rather than the unbound chlorine, at least at normal pool pH (at high pH the unbound chlorine loss increases). So the chlorine loss at a constant CYA level is roughly proportional to the FC level.

So you lose the least amount of chlorine if you are able to maintain the minimum FC/CYA level at that level rather than swing higher and have it drop down.

If you wanted to work out more technical details, I can give you chlorine loss rates for hypochlorous acid and hypochlorite ion from which you can calculate their net combined loss, but for the loss of chlorine bound to CYA we don't have specific data on that because there are no scientific reports on that though we do have rough estimates I'll talk about below.

At a pH of 7.5 and a minimum 7.5% FC/CYA ratio, the peak noontime chlorine loss rate is 0.077 ppm FC per hour. At pH 8.0, the loss rate would be 0.189 ppm FC per hour so for 8 hours of equivalent noontime sun in a day at pH 7.5 this is a loss of 0.6 ppm while at pH 8.0 this is a loss of 1.5 ppm. In theory these losses are dependent on the FC/CYA ratio so should not change with CYA level when maintaining that ratio (i.e. when following the chlorine / CYA chart in Pool School), but in practice we do see lower losses at higher CYA levels so there is some sort of CYA (or chlorine bound to CYA) shielding effect of lower depths.

There is additional loss of slow chlorine oxidation of CYA which is roughly 0.2 ppm FC per day. Beyond that there are losses of chlorine oxidation of pool covers and any ammonia or organics in the pool. Every person-hour in a 10,000 gallon pool consumes roughly 0.11 ppm FC. Finally, there are the two factors we don't know in detail which is the loss of chlorine that is bound to CYA and CYA's shielding effect on lower depths that reduces the chlorine sunlight losses I quoted.

Since a higher FC level at a constant CYA (and pH) level results in BOTH a proportionally higher FC/CYA ratio so proportionately higher hypochlorous acid and hypochlorite ion AND a proportionally higher FC so proportionally higher chlorine bound to CYA level (since 97-99% of FC is bound to CYA), the overall chlorine loss will be proportional to FC all else constant. However, as I noted earlier, a higher FC with a proportionally higher CYA level actually loses less chlorine. This post shows this for an SWG example with FC/CYA ratio of around 5% where you can see the declining absolute chlorine loss per hour as the FC and CYA level both climb but their ratio is held constant.

AprilsZoo · Jul 22, 2015

Some people do maintain their pools at CYA levels above the 30-40ppm that is generally recommended.... This typically allows them to dose their pools every couple-3 days instead of daily.... the tradeoff is needing higher baseline FC levels, and requiring higher SLAM levels if that becomes necessary.

But for someone who is unable or unwilling to test and dose everyday, and who understands the ramifications, then it is an option.

enginerd · Jul 23, 2015

Thanks, Richard320. I visited the link and was blown away by the in-depth details.

enginerd · Jul 23, 2015

Thanks, chem geek. This confirmed my suspicion, but you and Richard320's link (of your analysis, I believe) gave me quantitative information to optimize my pool for FC loss vs. chlorine dosage frequency. I am assuming your analysis is purely theoretical, correct? Or is it experimental? I didn't see any error bars on the data points. In any case that thread will be a great help to my understanding of the chemistry behind all this. I'll be munching on it for a while.

A related point that may already be answered elsewhere: is pH 7.5 the optimal for pool maintenance? I've been sitting at pH 7.8 for a while and noticed a lot of your charts here (Pool Water Chemistry) assume a pH of 7.5. Just in case, my stats since yesterday: FC: 8.5 ppm, CC: ~0 ppm, CA: 425 ppm, TA: 140 ppm, CA: 40 ppm.

chem geek · Jul 23, 2015

The chlorine loss numbers for hypochlorous acid and hypochlorite ion in full sunlight are calculated theoretically from absorption spectrum and quantum yield data based on calculations in this spreadsheet, but have been confirmed in experiments without CYA (particularly those at pH 7.5 with roughly 30 minute half-life for chlorine). The rest of the info when CYA is used is hypothetical based roughly on what we see here on the forum though I would use the rough rule-of-thumb that most pools trying to maintain the minimum FC/CYA ratio through manual dosing lose around 2-3 ppm FC per day though some lose less and others lose more (usually not more than 4 ppm). Another best guess for loss rates when using an SWG is in the chart in this post that shows that higher CYA levels have lower absolute FC loss in spite of the proportionally higher FC level. This is based roughly on experiments Mark made in this post and this post.