Best Guess Table

[PoolDoc]

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"

[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.

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 , but I will not tinker with my family’s health.
Steve

[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.
Steve

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

[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.

[chem geek]

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.

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.

(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

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

[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.

[chem geek]

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.

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.

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