Best Guess Table

Is "Ben's Best Guess Table" too conservative for some residential pools? I've seen more than a few people here claim to keep their chlorine at levels significantly lower than the Best Guess Table calls for for a given level of CYA with good results (no algae, no known cases of pathogen transmission through the pool water, etc.). Are they just lucky or might it be safe to keep lower levels of chlorine in certain circumstances? Is Ben's table heavily based on his experience with commercial pools? Is there a safety factor built-in to the table?

Without knowing and controlling for the myriad variables (bather load, accident-prone kid load, leaves and other vegetable matter, sunlight, etc.) that affect chlorine consumption, there would obviously be a good bit of guesswork involved in refining the table at all. However, given all the concern about safe chlorine levels it may be something worthy of further discussion.

I'm not advocating that anyone run lower levels of chlorine than Ben recommends; I'm just suggesting that based on some (admittedly sketchy) anecdotal evidence, some further discussion/inquiry might by in order. Short of that, should the table be treated as more of a starting point or guideline than an absolute? That may even have been Ben's original intent but almost everyone here (me included) now treats it as Gospel instead of as a guide.

(Just some food for thought and discussion; I'm not looking to start an argument.)

I know from experience that with an SWG, you can keep it much lower than the table. My CYA is now at 70 ppm and the chlorine level at 2 ppm without any problems but I have not had a dead cow fall in the pool yet.:D

My bather load is very low, much lower I'm sure than what Ben's guide is estimating, but I still keep my levels based on the guide. Heck, It costs me 12.00 a month in CL, I'm not going to mess with it. If I'm going to get that frugal (And I could, I live in New England) I should pack it up. :)

For acedemic purposes, It may be interesting to find out (I'm not volunteering), but if you get that close to the edge you would probably find it is too varied to have any public value.

Wayne

I would think that given the wide variation of pool conditions (different climates, trees vs. no trees, bugs, rain yada yada yada) these numbers probably work well no matter what. If you tried to build a table that took all of these factors into consideration, it would take a lot of work and it would probably be huge.

I would also think that Ben's table allows for some wiggle room - if you don't get to test for a day or two or have more people in your pool one day or there's a rainstorm, whatever, then it won't cause your pool to go south whereas if you keep it right at the stable point then any little disturbance could put you over the edge.

Also it seems to be one of Ben's goals to make pool care easy for people so rather than have people figure out their minimum chlorine level, here's a universal guide that is reasonable and works. Given that 4 ppm of chlorine is considered acceptable in drinking water, these numbers don't seem like much of a concern.

Peter

I just posted something similar to this question elsewhere today. I have had my pool for 8-9 years and have CYA around 30-40 and chlorine at 1-2ppm and have never had any algea or other problems and the water is always crystal clear. I guess it is like gas mileage, your mileage may vary.

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Originally Posted by Wayne LaBanca

My bather load is very low, much lower I'm sure than what Ben's guide is estimating, but I still keep my levels based on the guide. Heck, It costs me 12.00 a month in CL, I'm not going to mess with it. If I'm going to get that frugal (And I could, I live in New England) I should pack it up. :)
Wayne, Cost wasn't my concern. I was really just wondering about the basis for the tabel and whether it might lead to unecessary overchlorination of some pools.

For acedemic purposes, It may be interesting to find out (I'm not volunteering), but if you get that close to the edge you would probably find it is too varied to have any public value.

It would be interesting to collect and analyze some empirical data (e.g. daily chlorine level and CYA, hours of direct sunlight per day, bather loads, microbe counts, algae outbreaks, etc.) from a large number of residential pools and try to find the correlations. It's way over my head from the technical, monetary, and time standpoints. I guess that's why Ben came up with the table.:)

Wayne

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Please don't get me wrong. I think the Best Guess Table is great and I'm confident that the worst it might do is cause some people to overchlorinate their pools somewhat and probably not harmfully. I don't worry very much (really, at all) about the overchlorination issue in my pool, but it's obvious from reading some of the posts on this forum and others that there are plenty of people who worry about it a great deal. If there's a safety factor built into the table maybe people could be more confident in playing around with the levels a little more without too much risk of infections and algae infestations. On the flip side of that, the shock levels in the Table don't seem to have much wiggle room; people who fail to reach those levels, even by a little, often seem to have trouble clearing algae.

For me, the table is for no fuss. Follow it and it is unlikely you will have any problems.

However, for those willing to take some risks with an occasional algae bloom, they can push the limits a bit and see what happens.:eek:

I've noticed that whenever I've let it drift toward the low end of the scale, inevitably thats when I'm most succeptable to problems. Our pool has a cya of 50 , so the range 3-6ppm . I can let it go to around 2 ~ 2.5 without worrying too much , but I cant leave it there for long without CC raising or algea taking off

It may interest some of you to know that one of the forum members, Richard F. (not his forum ID), who has both a strong interest in this topic, and a degree in chemistry, has been investigating it rather intensively over the past year or so. Many of the results he's achieved are extremely technical, but it now looks like we may be able to use some of his work to update the Best Guess Table, early next season.

Ironically, it now looks like the primary changes will be to INCREASE the shock dosages, which are low, if you look at the theoretical HOCl levels achieved. I need to do some liner testing with those high levels before I make those change, however.

Also, please keep in mind, that what I was trying to accomplish with the BGT was a "works for almost everybody / is safe for almost everybody" guide. Inherently, this means creating a guide that is NOT perfectly optimized for each pool, but rather one that is GOOD ENOUGH for almost all pools.

There are several issues to keep in mind.

First, prudent operational conservatism requires the biggest safety margins be maintained adjacent to high risk boundaries, rather than low risk bondaries. Overdosing with chlorine approaches a low risk boundary: the consequences of doing with 6 ppm instead of 3 ppm are almost always trivial. But underdosing with chlorine approaches a high risk boundary. The consequences of maintaining 1 ppm instead of 2 ppm, can be an expensive (in time, money, chemicals and 'stress') episode of algae. It can also result in infection. Consequently, prudent analysis will lead to operational guidelines that err in the direction of overdosing more frequently than in the direction of underdosing.

Second, it is also operational prudent to consider common forms of operator failure, and to set up guidelines, where possible, that minimize the consequences of such failures. With pools, whether home or commercial, 'missed dosing' represents one of the most common forms of operator failure. One possible solution is the use of a feeder. In the past, I've recommended the Rainbow RB-320 as a functional feeder with tolerable 'negatives'. More recently, I've concluded that SWG's are a superior alternative, for those who can afford them. But, high Cl / high CYA represents an operational alternative that is accessible to virtually everyone.

With respect to both issues identified above, I think what I've done, and recommended, largely 'gets it right'. (For this reason, I'd prefer to see less 'how to optimize for low chlorine use' discussion in the general forum sections. I think such discussions tend to lead to pool failure episodes for newbies. Here, in the China Shop, it's fine.)

But, there's a third issue, that of anti-chlorine paranoia. This fear distorts people's perceptions of where the real risks are, causing many (even some of you, apparently) to place a value on low chlorine levels that, in my opinion, is contrary the value that would be attributed as a result of any real systematic analysis of the relative risks and benefits. I've just realized -- actually, when I was posting on the chlorine paranoia yesterday -- that I have not adequately communicated anywhere on my site the information people need to help them make that analysis, at either a 'popular' or a technical level. This is something I need to fix. I'll try to do so this winter.

Meanwhile, I'd encourage some of you to consider what sort of weight should be placed on the benefits of low chlorine (possible reduced costs; reduced suit / liner fading; possible reduced risk of long term effects) versus the costs (low margin of error; increased risk of algae; increased risk of acute infections; increased complication of care guides). I tend to believe that the only substantial realized benefit to optimized low chlorine is what might be called 'tinkerer's satisfaction'. By this, I mean the satisfaction gained by someone who tunes up their PC, and then runs a test to validate that it now benchmarks at 18,700 whatevers instead of the 16,980 whatevers it was running before. The functional benefit of this sort of improvement is virtually nil, but the psychic satisfaction is high.

Please understand that I see nothing wrong with pursuing 'tinkerer's satisfaction': I've done so many times myself. But, the reality is such optimization is NOT what most newbies here need. Instead, they need help in achieving 'good-enough', reliable, and trouble free operation. They can (as some of you have) learn to 'tinker' later, once they've gotten the basics down.

I'd ask all of y'all here to consider adopting such a point of view, when you post in the general forum . . . for the sake of people who just want their pool to work.

Thanks,

Ben Powell
"PoolDoc"

Ben
If I may put my 2 cents in here.
I also posted in the other forum you mentioned. Most pathogens can live in non chlorinated water. The levels of suggested chlorine allow for rapid kill times. This becomes important if an infected person enters your pool to prevent the spread of the infection. It is even more important of you have a person with a lower immunity. Children under the age of 3 to 5 are at higher risk due to an undeveloped immune system. Some thing that will give an adult a good case of gut cramps could in essence kill a child.
With the emergence of antibiotic resistant pathogens we are now seeing in the lab total kill of these pathogens is not optional.

I am not as concerned about the growth of algae in my pool. The appearance is a small part of the reason I feel the levels of chlorine should be kept levels mentioned in the best guess chart. I have said it once and will again if you have algae it’s the least of your problems. You may have a killer in your pool as well hiding in the algae.

I tend to weigh all decisions concerning the health of the persons in my house very carefully. I looked at the risk of exposure to chlorine levels in the best guess chart verses exposure to the pathogens that can be found in the pool environment. I feel that the exposure to chlorine levels given the best guess chart are lower risk than exposure to pathogens.

Bottom line is that I will tinker with my computer, I will tinker with my truck, I will even tinker with my toys:D , but I will not tinker with my family’s health.
Steve

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Originally Posted by medvampire

Ben
If I may put my 2 cents in here.
I also posted in the other forum you mentioned. Most pathogens can live in non chlorinated water. The levels of suggested chlorine allow for rapid kill times. This becomes important if an infected person enters your pool to prevent the spread of the infection. It is even more important of you have a person with a lower immunity. Children under the age of 3 to 5 are at higher risk due to an undeveloped immune system. Some thing that will give an adult a good case of gut cramps could in essence kill a child. With the emergence of antibiotic resistant pathogens we are now seeing in the lab total kill of these pathogens is not optional.
Steve

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That's a point I hadn't considered, but should. People forget that pools are -- to some degree -- not just a swimming hole, but a community toilet. Adequate chlorine and proper filtration can make them safe-to-swim-in community toilets, but can't eliminate the sources of risk.

It's worth noting that all the serious E. coli infections that resulted from waterpark use a few years back, all apparently occurred in water features that were maintaining minimum levels of chlorine.

Ben

Does pH play into the BGT in any way? I've been successfully running my pool with low CYA (~30ppm) pH (~7.2) and chlorine (~1.5ppm). But after adjusting the alkalinity in an attempt to reduce acid use, the pool still 'insists' on settling at 7.6 pH when I don't add acid each day. Which, if I leave it there, makes me think I may want to bump up the chlorine to adjust for the effect of the higher pH.

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Originally Posted by aquarium

Does pH play into the BGT in any way? I've been successfully running my pool with low CYA (~30ppm) pH (~7.2) and chlorine (~1.5ppm). But after adjusting the alkalinity in an attempt to reduce acid use, the pool still 'insists' on settling at 7.6 pH when I don't add acid each day. Which, if I leave it there, makes me think I may want to bump up the chlorine to adjust for the effect of the higher pH.

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Ben's table says to have your chlorine at 3-6 ppm when your CYA is at 30-50 ppm and yes, his table is somewhat pH dependent but not by a lot if your pH is somewhere near 7.5

The following table shows the amount of disinfecting chlorine (HOCl) for your pool and Ben's table with the CYA at 30.

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(CYA=30)
FC      pH    HOCl
1.5    7.2    0.023
1.5    7.6    0.019
3      7.2    0.052
3      7.6    0.043

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You can see that the change in pH from 7.2 to 7.6 lowers the disinfecting chlorine (HOCl) by around 17%, but this is small compared to your levels of disinfection vs. what Ben recommends. Until we know more about minimum levels for disinfection and algae prevention, I would stick with Ben's recommendations and increase your FC level to at least 3. I wouldn't worry so much about the pH at 7.6 -- just don't let it keep drifting.

My guess is that your drift in pH is due to outgassing carbon dioxide (from carbonate) since this occurs almost 3 times faster at a pH of 7.2 compared to a 7.6 pH (see Ben's procedure for lowering alkalinity and you'll notice how lowering the pH is a key component of that procedure). If you have any water features (waterfall, etc.) or upward pointing jets (that break the surface), that would exacerbate the problem. If the pH continues to drift up, you might check your alkalinity level which might be too low. In fact, if you've been adding acid without also increasing alkalinity, your alkalinity might be too low. Higher alkalinity won't change the amount of acid you will need to add, but it will slow down the rise in pH so that you won't need to add the acid as frequently (i.e. instead of some acid every day, you'll add more acid once or twice a week).

Richard

Thanks,

I lowered the alkalinity even more to 80ppm, and now the pH will hold steady at 7.2. Correct me if I am wrong, but it seems to me that there may be a CO2 reaction taking place at the water surface/atmosphere interface that sets the pH dependent on the alkalinity. (We also have 400ppm calcium hardness.)

In any case I'm running the pool at 7.2 pH, 1.5-2ppm chlorine, ~30ppm CYA and 80ppm alkalinity. Clear blue all the way. :D

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Originally Posted by aquarium

Thanks,

I lowered the alkalinity even more to 80ppm, and now the pH will hold steady at 7.2. Correct me if I am wrong, but it seems to me that there may be a CO2 reaction taking place at the water surface/atmosphere interface that sets the pH dependent on the alkalinity. (We also have 400ppm calcium hardness.)

In any case I'm running the pool at 7.2 pH, 1.5-2ppm chlorine, ~30ppm CYA and 80ppm alkalinity. Clear blue all the way. :D

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Well, now I'm a bit confused. It sounds like you added acid to lower the alkalinity and doing so will also lower your pH as well. However, you probably didn't change anything that was your original cause for the increasing pH over time.

Yes, you are correct that there may be CO2 being released at the water surface, but this normally doesn't happen (at least not very quickly) unless 1) your pH is low, 2) your alkalinity is extremely high, and/or 3) you have water features that aerate your water. Your alkalinity is not high so let's look at something else. Just so you know, when the alkalinity (carbonate) in the water is released as CO2 into the air, it raises the pH without changing the alkalinity (as measured by the test kit). I know that sounds weird, but it's the way the chemistry works. When you add acid to restore the pH back to where it was, then that lowers the alkalinity, so the NET of this combination of outgassing of CO2 and adding acid is a lowering of alkalinity with no change in pH. If I've hopelessly confused you, I apologize.:confused:

If you have been cycling up in pH, then adding acid (which lowers pH and alkalinity) and getting to lower pH, how has your alkalinity been moving -- measuring it at the same pH at two points in time? If you seem to be bouncing between a low pH, lower alkalinity and high pH, higher alkalinity, with a net lowering of alkalinity over time, then the CO2 outgassing is most likely what's going on in which case you should see if your water is getting aerated (check your pool jets, see if you have any fountains, water slides, a very windy day, etc.). If, on the other hand, you are bouncing in pH while the alkalinity stays the same (when measured at the same pH), then something in your pool is making the pH rise or you are adding something to your pool to make it rise (though liquid chlorine or bleach makes the pH rise when you add it, the pH goes back down again as the chlorine gets used up).

At this point, it sounds like you are stable so "if it ain't broke, don't fix it" is probably the rule to follow. Personally, I have found an alkalinity level of 80 to be a bit on the low side (I'm usually at 100) and the pH swings more than I would like as a result. I also run my pool at a pH of around 7.4-7.5. If you decide to increase your alkalinity, you might want to do so next time you dilute your pool water since your calcium hardness is on the high side.

Richard

"aquarium", I must apologize. Though an alkalinity of 100 and pH of 7.4-7.5 works fine for my pool, your pool may be very different and need different parameters. What I do know is that you will outgas more CO2 if you have lower pH and/or higher alkalinity so if you want to keep your pH closer to 7.2, then you will need to keep your alkalinity down so that the amount of aeration in your pool (which may not easily be changed) has the pH rise slowly enough for your tastes.

This whole issue has given me the thought of another graph I can construct that will show the relative CO2 outgas rates as a function of alkalinity and pH. I cannot give absolute rates since that's a physical aeration process that will be different for everyone's pool, but at least I can show the relative effects. I'll put this in the China Shop under my Pool Water Chemistry thread.

Richard

No, I aerated the water and simultaneously added acid to intentionally push down the alkalinity, which has resulted in the water stabilizing at the lower pH as well.

I'm not sure about the chemistry at the water surface, but all other things being equal, it appears that the recommended alkalinity range of 80-120ppm will yield the recommended pH range of 7.2-7.8 in a fixed relationship. 80ppm yields 7.2 pH, 120ppm yields 7.8 pH, etc. (at least in my pool :D )

EDIT:

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Originally Posted by chem geek

Though an alkalinity of 100 and pH of 7.4-7.5 works fine for my pool, your pool may be very different and need different parameters.

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Which is consistent with my experience of the relationship between pH and alkalinity through that range. END EDIT

BTW, I want to run the pool close to 7.2 pH in order to get the most benefit from the bleach. You said:

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At a pH of 7.5, FC of 3.0 and CYA of 30.0 (and TDS of 550, Temp of 80ºF), the HOCl is 0.045. At a pH of 7.2, with everything else the same, the HOCl is 0.052 which is not a huge change, but is still about a 15% change.

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Is there a reason NOT to run the pool at 7.2 pH?

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Originally Posted by aquarium

No, I aerated the water and simultaneously added acid to intentionally push down the alkalinity, which has resulted in the water stabilizing at the lower pH as well.
:
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BTW, I want to run the pool close to 7.2 pH in order to get the most benefit from the bleach.
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Is there a reason NOT to run the pool at 7.2 pH?

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OK, at least now I understand what happened. As for running the pool at the lower pH to get more chlorine benefit, that is true, but it's a relatively small benefit compared to what you can simply get by adding more chlorine. Usually you set your pH for other factors including not stinging the eyes, not risking rusting of metals, etc.

I speculate that the biggest factor in the lower pH will be a tendency to outgas more CO2, even though you've lowered your alkalinity. A pH of 7.2 combined with an alkalinity of 80 (assuming CYA is 30, though this isn't a huge factor) produces exactly the same rate of outgassing as a pH of 7.4 combined with an alkalinity of 120 which is the same as pH 7.5 with alk. 150. You can wander over to the Pool Water Chemistry thread in The China Shop to see a graph of this (it's the last graph in the first post in that thread). I don't want to post it here for risk of scaring people into banning me from ever posting outside The China Shop again:eek:

Richard