OCLT ppm loss rate per hour

[kar88]

Have seen a couple of references to "6-8 hours" to allow this test to run. Should I assume from this that an hourly free chlorine loss rate of 1/8 to 1/6 ppm per hour (or less) is a passing result?

It seems a target of 1 ppm "overnight" is a bit vague, considering this could easily be applied to anything from a 6 hour to a 10+ hour test?

Apologies if I've missed this being stated in terms of an hourly ppm loss rate elsewhere! If not, could that bit be clarified in the test description?

Many thanks!

 

[ILSBJ]

I think you are not able to measure such small differences accurately enough (1 hour loss)

 

[Donldson]

Welcome to TFP.

If there are no organics in the water then there should be virtually no loss of chlorine if there is no sunlight on the pool. Shouldn't make a difference if it's 6 or 8 hours as long as there's no sun hitting the water.

And as mentioned, 0.15 ppm FC is far too small to be accurately measured. You're getting a bit too in the weeds for a simple test to confirm the organic load on the water is near zero.

 

[kar88]

Not suggesting to run the test for an hour or two, necessarily.

Assume a 10 hour test and a difference of 1 ppm. Is that a pass or fail?

If 1 ppm or more in six hours is a fail, then perhaps the number for 10 hours should be 10/6 = 1.67? Significant difference.

 

[Newdude]

It *might* matter a couple of times a year with heavy debris falling in the early or late season, but most times it doesn't. Although 1 ppm loss overnight is a technical pass, I personally don't want to pass with 'C's get degrees'. I want to pass with flying colors and no questions whatsoever. As a side note, I had 80 oaks and all kinds of organic debris blowing in the old pool in heavy times and I never once saw the FC dip from it. Fresh debris, even the dead brown leaves in the fall, don't consume a noticeable amount like something well into decaying would, such as the same debris that had all winter to fester under the cover. There are some kinds of sap or gunk that consume FC, but if you don't already knowingly battle it, it's likely a non issue.

I think you're trying to hard to pass, which leaves you open to lose the leg up in the battle when warranted. You always want to heed the testing giving you the early warning before it gets out of hand. On the off chance you SLAM'd for no reason, it'd likely be over the first night. The majority of times it was in fact needed and you'd save big with a short SLAM.

My $0.02.

 

[Getitrite]

Have seen a couple of references to "6-8 hours" to allow this test to run. Should I assume from this that an hourly free chlorine loss rate of 1/8 to 1/6 ppm per hour (or less) is a passing result?

It seems a target of 1 ppm "overnight" is a bit vague, considering this could easily be applied to anything from a 6 hour to a 10+ hour test?

Apologies if I've missed this being stated in terms of an hourly ppm loss rate elsewhere! If not, could that bit be clarified in the test description?

Many thanks!

References to a 6-8 hour test are Not referenced in the Overnight FC Loss Test (OCLT) procedure.

There is no target of 1ppm "overnight". The test criteria is: "If your FC level remained the same, or went down by 1.0 or less" ( when following the test procedure).

The NAME of the procedure should not be confused with the steps taken to execute the test.

If the elapsed time when there is no direct sun on the pool from the evening to morning is 1 hour, 5 hours, 10 hours, or whatever, that is the duration of the "overnight" step of the process. No need to break it down any further . . .

 

[kar88]

So this denies the existence of something like a chlorine consumption rate. And the target is not a target.

Ok. I get that there is some art here, and local circumstances. But this seems like a missed opportunity to capture implicit assumptions.

I ain't buying that 1 hour and 10 hours are equivalent, but sure, I can live with a ballpark test!

In my case - 0.5 ppm in 9 hours. Good enough, perhaps?

 

[Newdude]

In my case - 0.5 ppm in 9 hours. Good enough, perhaps?

That's GREAT.

I ain't buying that 1 hour and 10 hours are equivalent,

They aren't. Typically 6 to 8 hours and 8 to 10 hours would produce similar results. With 1ppm allowed to oxidize, (bather waste or whatever environmental debris was in there) it wouldn't continue to lose FC without constantly being replaced like an algae bloom would. So IMO, you'd lose the first ppm rather quickly if it truly wasn't algae, then lose nothing after. 99% of locals have 6+ hours of darkness but even somewhere like Alaska with a short 'night' (when it does start to get dark again) would see more than 1ppm loss once the bloom festered a little.

 

[kar88]

Thank you! That's the insight I was missing. Most of the chlorine is likely to be consumed early. So the rate varies during the test.

Would be interesting to verify that in practice, though making for a poor night's sleep..

Appreciate the input from all. One year pool owner and TFP follower from the start.

 

[Newdude]

Most of the chlorine is likely to be consumed early. So the rate varies during the test.

Under a clear 'pass' situation that a little of something besides algae was in the pool, then yes. Once it's gone it doesn't replicate like algae and the there's no further loss. Obviously some common sense comes into play and any OCLT would likely fail after a blowout party with lots of residuals to consume. But for the most part, a 6 hour and 10 hour test would usually both show the same small loss if it wasn't algae.

 

[wireform]

On the other hand I'll agree maybe that the 1ppm loss is early on but that is still not definitive enough till there are 6 or 8 hours after that when there would be a good chance to loose more. If you run with your 1ppm loss at that point and have all 3 criteria taken care of, that's when you could see a algae bloom fester two weeks down the road.

 

[Newdude]

Absolutely. And also why I personally am not happy about a 1 ppm loss, with plenty of time overnight. I'd end up paying closer attention to it, likely with several more OCLTs to ensure I continued to lose only 1, and what did I really save myself at that point ?

SLAM first, ask questions later.

 

[mgtfp]

Have seen a couple of references to "6-8 hours" to allow this test to run. Should I assume from this that an hourly free chlorine loss rate of 1/8 to 1/6 ppm per hour (or less) is a passing result?

As already said, we can't test with such a precision anyway, so there is no point in defining such criteria.

That aside, it's not that simple, you can't just divide the total loss by the number of hours to get the loss rate. You have to look at chlorine loss in the same way as calculating changes to a bank account due to interest rates.

Chemical reaction rates are proportional to the number of available reaction partners, that means that relative chlorine loss (∆FC/FC) is a better measure.

You could for example say that a certain UV intensity results in a chlorine loss rate of ∆FC/FC = 5% per hour (just an arbitrary number).

You start for example with FC 10ppm. After an hour you will have lost 0.5ppm and end up at 9.5ppm. In the next hour you will lose 0.475ppm and end up at 9.025ppm and so on. The FC after a time t can be calculated with the formula:
FC = FC₀ (1-p)^t
With FC₀ the starting FC, p the relative loss per hour (e.g. p=5%=0.05) and t the time in hours.

This is already a bit academic, because the UV intensity is not constant throughout the day, and it will be a lot more beneficial for the pool owner to just work with a chlorine loss to UV per day, like ∆FC/FC = 20% over a whole day caused by the total UV load over that period. That number will change with cloud coverage on a daily basis, so you may even be more interested in a weekly average. Then this number changes with the seasons.

With algae it gets even more complex, because algae reproduces, you therefore have two competing rates, an algae reproduction rate and an algae kill rate. Only if the kill rate is faster then the reproduction rate you have a chance of winning the battle.

Let's say you have a swamp, and at normal target FC levels your reproduction rate is faster than the kill rate. Then, after an hour, you will have more algae than you started with, and therefore your hourly relative chlorine loss rate ∆FC/FC will speed up.

Then you have to consider all the chlorine losses due to chlorine oxidising things that are independent from algae. And draw a line somewhere. This line will of course be in a grey area. If you pass with flying colours with no or maybe just 0.5ppm loss, then you have a clear pass. If you fail with losing most of your FC then you have a clear fail. And inbetween? If in doubt, slam a bit longer and see if things improve.

I hope this will be enough to show the motivation behind TFP's simple 1ppm criterion.