(NOT URGENT :) ) Ideal Predicted Chlorine loss rate

[jblizzle]

Dave, I am having deja vu here

Maybe we will actually come to an agreement ....

 

[chem geek]

The amount of non-sunlight loss is roughly proportional to the unbound chlorine level so roughly to the FC/CYA ratio. So at the minimum FC/CYA ratio the loss will be less than 1.0 ppm FC and while that means you're likely to pass the OCLT it also means that if you have a smaller but still higher-than-normal chlorine demand you may not measure it reliably at a regular FC/CYA level. The other way to handle this would be to have a smaller OCLT requirement when closer to the minimum FC/CYA ratio, but 0.5 ppm is the error limit when using a 10 ml water sample (and yes, as Dave points out one could use a 25 ml water sample in this case).

Here are the three facts that need to be reconciled for non-sunlight loss measurement:

• Normal loss amount is proportional to the active chlorine level (or possibly the hypochlorite ion level if pH is high and oxidation of CYA is the dominant loss) so roughly related to the FC/CYA ratio.
• Normal loss is temperature dependent roughly doubling every 13ºF.
• Relative test error is (at least) +/- 5% of FC level or +/- 1 drop, whichever is greater.

At a regular non-SLAM FC/CYA ratio, the normal overnight loss should be very small, on the order of less than 0.2 ppm and therefore too low to measure given the accuracy of the FAS-DPD test even using a 25 ml chlorine sample. One can certainly do an OCLT and will be able to detect unusually large losses, but will miss smaller losses.

At a SLAM FC/CYA ratio, the 1 ppm FC criteria is very tight even for accurately measured loss and furthermore the FC level is high enough when the CYA is 50 ppm or more that the error in measurement may exceed this 1 ppm threshold. Instead of using an absolute 1 ppm FC threshold, it would be better to have the criteria vary so at half-SLAM (20% FC/CYA ratio) it would be 1.0 ppm FC, at regular SLAM it would be 1.5 ppm FC, and at yellow/mustard SLAM it would be 3.0 ppm FC. However, the testing error of (at least) 5% of the FC may exceed these numbers when the CYA level is higher so the larger of the two would be used. And yes, this makes this more complicated but would reduce the frustrating "my OCLT keeps varying and I can't consistently pass" problem.

I'm not sure we want to address the temperature dependence and instead use closer to worst-case at somewhat higher temperatures closer to 90ºF (which is my best-guess above) and perhaps note that the OCLT on cold water is not as definitive.

Again, remember the main purpose of the OCLT is to detect unusually high chlorine demand and to know roughly when such demand has been met from elevated chlorine levels. It is a "looser" criteria than seeing visible algae or having cloudy green water. Similarly, having the water be cloudy but not green (especially over days) is not as stringent a criteria either since filtration/circulation is more important for clearing a pool once the algae is all dead.

 

[BuckeyeChris]

Just a poll for those reading: how many people do an OCLT when at maintenance, because they suspect something, versus how many do the test only when finishing a SLAM?

Like I said, if you are slamming at high FC, it might be counter productive to come down to 10 to do the OCLT. But people just checking the loss due to suspicion or just as part of their monthly routine, might not want to come all the way up to higher levels just for that. There are two different scenarios when people do OCLT, I'm not sure there is one method to satisfy both needs unless we ditch the hard "1ppm" and go to a sliding scale.

 

[Texas Splash]

Well, my head is about to explode, but for some strange reason I am enjoying this thread more than usual since I have contributed to the OLCT questions more than once. I think the OLCT is a great tool in general to use when diagnosing unusual FC loss. No doubt it can be tricky based on all the variables listed above (current FC, testing methods/standardization, etc). In my limited experience here, I find CG's comments about OLCT based on specific FC ranges in post #22 quite interesting and can personally relate to that concept as I experienced those same OLCT variances based on different FC levels.
Perhaps my biggest concern (aside from standardization of OLCTs) is for the "noobies". There are so many people who struggle with doing a simple pH test let-alone differentiating between varying degrees of FC levels. For those of us who are more "in-the-know" and can relate, it's fine. But for the common pool owner it may be an overwhelming task.

So I have been using this standard phrase along with the OLCT page link:

When performing the OLCT, remember that it must be done when the sun has dropped – no sunlight on the pool. About 8 hours total is recommended. Also, your FC level should be elevated more than normal. Starting (bedtime) FC could be close to your SLAM level, but no lower than 10. Anything less and you may not receive an accurate portrayal of FC loss (by percentage), even if it’s only 1 ppm.

I will watch this thread and adjust that verbiage as needed based on what the TFP experts agree upon.

 

[jimbowrench]

Wouldn't it make sense to have an acceptable drop level for an OCLT at a FC level to see/confirm you have a problem. Raise the FC to 10ppm and a drop greater the 1 ppm overnight indicates a problem.

Then with a SLAM have a different parameter? Holding to drop of <1 ppm at a 34 FC SLAM level seems a bit tedious.

I recently was struggling with a issue and thought my SWG cell was failing. I added bleach to raise to 10ppm FC and did an OCLT which pointed out I had an issue with the water not the SWG. Completed a SLAM, and now the water is crystal clear with very low SWG % and short pump run time.

 

[chem geek]

The problem with specifying just an FC level is that the rate of FC loss will depend on the CYA level. If the CYA is very high, then the rate of normal loss will be lower (of course if there's algae growth then the loss can still be high, but we're mostly concerned about a criteria when one is done). Also, most people do an OCLT to determine the end of a SLAM so the FC level will generally be higher than 10 ppm.

Nevertheless, for a non-SLAM check saying to raise the FC to 10 ppm is reasonable since presumably the CYA level will be in a typical normal 30-80 ppm range.

 

[jimbowrench]

Yeah understood about the CYA level. I know when I did the last SLAM it becomes visually apparent regardless of the FC drop whether there is success or not. Cloudiness or not...... I think the pass/ fail criteria of a SLAM is more about how the pool looks rather than a OCLT result. I have not dealt with an extreme situation with algae but have to think that appearance of the water has to account for something. SLAM until the water looks good!
From a relative newbie to TFP I think this stuff rocks! and I really appreciate Chem Geek's information albeit way over my head is very useful. Thanks to all!

 

[duraleigh]

The problem with specifying just an FC level is that the rate of FC loss will depend on the CYA level.

I can't get my head wrapped around that. Are you saying the over night drop in FC is influenced by the CYA level? How much?

 

[chem geek]

The overnight drop is related to the unbound chlorine level so is proportional to the FC/CYA ratio. So you can't just say it's related only to FC or only to CYA. This is just like the Chlorine / CYA Chart and trying to say that algae prevention only depends on the FC level or only on the CYA level -- it doesn't. It depends on the FC/CYA ratio. The same is true for the amount of chlorine loss overnight. It depends on the FC/CYA ratio and is roughly proportional to that ratio except that at higher ratios above 20% things get non-linear so a regular SLAM is around 10 times higher in unbound chlorine than regular non-SWG minimum while a yellow/mustard SLAM is around 23 times higher than regular non-SWG minimum.

 

[duraleigh]

It seems to me that the question has morphed into questioning the validity of the OCLT in general.

So does that mean our OCLT test is bogus? I am trying hard to understand just where we are going with this.

The question started out as to whether the OCLT was valid at any FC value In other words, if you do an OCLT on a pool at 10 ppm, do you get the same results if your FC was only 4 ppm.....or if your FC was 20 ppm.


The answer seems to be, "No, it would be different" but can we decide or calculate how MUCH different?

 

[Crackerjack4u]

Excellent thread. Very informative and interesting reading. I'll be continuing to follow it.

 

[chem geek]

Nobody is saying the test is bogus in terms of having no value measuring chlorine loss overnight. It's just about whether the 1 ppm loss criteria makes sense at ANY FC and CYA combination no matter how low the FC/CYA ratio. It doesn't. And the criteria may be too tight at SLAM levels when FC is higher (which implies when CYA is higher) due to accuracy limits of the test.

The thread didn't morph. It was all about predicting the chlorine loss rate and the reason was specifically due to the OCLT.

 

[BuckeyeChris]

Perhaps when slamming, you keep slamming until your ocl is less than 3pmm (?), then you drift down to 10 and do a 1ppm loss test.

 

[pabeader]

What's wrong with using a percent loss??? With a sliding scale for the %. Which would lead back to a variable ppm based on current FC/CYA...

 

[chem geek]

The % loss would be OK for the SLAM levels, but you still need a lower FC/CYA ratio limit as well. The % loss criteria should be as high as the limit of relative measurement accuracy so that's roughly 5% of the FC level so 1 ppm at 20 ppm but 2 ppm at 40 ppm.

If you wanted to do an OCLT at normal FC/CYA levels (5% or 7.5% ratio), you'd have to use a 25 ml sample size to get enough resolution where even 0.4 ppm FC would be suspect as too high a loss. It's not really practical to do an OCLT at normal levels unless one sees a higher loss which of course is definitive for a problem, but a less severe problem may not get detected.

 

[JoyfulNoise]

When I had originally asked the question, it was to know if there was a formula for calculating how much FC is lost by simple chlorine extinction (hypochlorous acid degrading into chloride). This is what would be similar to what one finds with high strength bleach slowly degrading over time. Chemgeek's response was, to paraphrase, "it's so small at pool FC concentrations that it doesn't matter." So let me hijack back the thread I started with another question regarding chlorine loss....

Since self-extinction of the chlorine is not the driver of an overnight chlorine loss, then what is left are organic contamination (oxidation of organics and ammonia) and biological contamination (algae growth, bacterial growth and biofilm oxidation). I would make the proposition that organic oxidation and ammonia can probably be ignored as any large increase in FC consumption by either of those would have been preceded by something unusual and obvious (huge storm with debris or some kind of ammoniacal compound being released into the water, etc). Or, at the very least, lets just ignore the organic oxidation part as most people, I think, perform OCLT's because they fear an algae outbreak.

So the follow up question I would then ask is, with respect to an overnight chlorine loss and biological contaminants, is there any data to estimate how much chlorine is consumed by a growing algae bloom or other biological contamination?

Algae and bacteria are always present in pool water, the TFP levels are just set high enough so that the kill rate of the population exceeds the reproduction rate. So if the algae is able to grow to a population size where the standard kill rate at the active chlorine levels TFP uses is no longer sufficient to control population growth, then is there any estimate of what that chlorine consumption would look like for a biological population that has achieved "critical mass" in terms of reproduction rate and population size? My guess is that there is no easily available data on that unless someone in the water treatment community has done some comprehensive studies of biological contamination levels and chlorine disinfection.

 

[duraleigh]

The % loss would be OK for the SLAM levels, but you still need a lower FC/CYA ratio limit as well. The % loss criteria should be as high as the limit of relative measurement accuracy so that's roughly 5% of the FC level so 1 ppm at 20 ppm

Then it seems we can have a valid test by requiring the FC to be @ 20 ppm and a 1 ppm loss overnight, is that correct?

 

[Piolin]

::

 

[chem geek]

Then it seems we can have a valid test by requiring the FC to be @ 20 ppm and a 1 ppm loss overnight, is that correct?

Yes, but most people want to do an OCLT when at SLAM levels and that may not be with the FC at 20 ppm. It's often higher at higher CYA levels and that's where people run into problems not passing because their test error is larger than the actual loss which the percentage rule would help alleviate -- that is, no loss greater than 5% of the FC level.

Oversimplifying has consequences. There are now several people who are upset that our standard charts don't tell them how to have their pH under control and they are questioning why we don't just tell them that a lower TA is OK if it makes the pH more stable, but that for plaster pools one needs a higher CH and/or pH target all of which are easily calculated by the CSI done in PoolMath. While this is a different topic, it is related because having people avoid percentages for OCLT because we want ONE NUMBER or a chart with some numbers needs, at a minimum, a caveat that this is for people who go apoplectic at simple division.

If you want to be consistent with what we do for things like the FC/CYA chart, then make a chart for OCLT that does the 5% math on the FC level.

- - - Updated - - -

So the follow up question I would then ask is, with respect to an overnight chlorine loss and biological contaminants, is there any data to estimate how much chlorine is consumed by a growing algae bloom or other biological contamination?

Those losses are also small or should be. Again, the primary loss of chlorine overnight without sunlight in uncovered pools is chlorine oxidation of cyanuric acid. That is going to usually be between 0.5 and 1.0 ppm FC over 12 hours at SLAM levels (an FC that is 40% of the CYA level) but may be higher in some circumstances such as higher temperatures or higher pH or CYA.

 

[Crackerjack4u]

Yelp, I think Piolin has the right idea. I'm getting me a cola, and some popcorn too while I await the next portion of this awesome thread. ::

I'm ashamed to admit that I did have to look up the word "apoplectic," but I'm with ya on everything else . So keep the info flowing.