The Sun

[PoolDoc]

I'm not quite sure what your question is.

If you are asking, "Is it important to know your FC level, rather than your TC level (total chlorine)?", the answers is, "Not really, unless you are having problems.". As you note, on outdoor pools that are working well, FC pretty much equals TC.

If you are asking, "Is it important to stabilize, since actual FC (un-stabilized hypochlorous acid) levels will be very low if you don't stabilize, and also if you do?", the answer is "yes, very important". There are two reasons.

However the answer to this question is pretty likely very complex very quickly, and push this thread into the China Shop. So, hold onto your hat!

Let me set up some abbreviations, first:
Hypochlorous acid => HOCl
Unstabilized free chlorine (HOCl + -OCl) => UFC
Stabilized free chlorine (all the Cl + CYA species that are in high rate equilibrium with the HOCl level) => SFC
Free chlorine (UFC + SFC) =>
DPD Free chlorine => DFC

OK.

As you seem to suspect, once you add stabilizer, your FC level goes from being mostly UFC to being mostly SFC, and your actual HOCl level is usually very low. Chem_Geek has a pretty heavy duty spread sheet that can allow you to analytically estimate your actual HOCl level, given your pH, your CYA level, your pool's temp, and your DFC level.

So, the question arises, why not just run an UNstabilized pool with 0.5 ppm FC and 0.3 ppm HOCl (@ pH around 7.2), where FC ~= UFC, instead of running a stabilized pool with DFC = 10ppm, but HOCl = 0.3 ppm?

There are TWO major reasons why you should not do this.

FIRST, if you run a pool -- in full sunlight -- with UFC = 1.0 ppm and CYA = 0 ppm, you won't usually be able to maintain a sanitary or algae-free pool.

Here's why:

Normally, when you test, you are sampling from the deck. But, if you sample from the MIDDLE of the pool with very low stabilizer, you will find that the UFC is significantly lower, even 0.

When I first started servicing commercial swimming pools years ago, most of them were trying to operate low chlorine, low stabilizer pools (FC < 2ppm, CYA < 20 ppm). It took me a few years to figure it out, but I noticed that L-shaped large pools operated this way often had large permanent black algae populations in the angle of the 'L', where the pool was relatively deep and far from circulation. When I sampled the chlorine in this zone, I found that FC levels that might be 2.0 at the pool edge, would be 0.5 or less in the middle.

In New York State, where years ago some bromine promoters got the code writers to require use of bromine (which CANNOT be stabilized) on outdoor commercial pools . . . they had a major problem with surface algae, on pools that have gutter return systems. But, on pools with old style (and EXCELLENT) in floor return systems, there's no such problem.

What was happening? By the time the chlorinated water had migrated from the return areas to mid pool . . . the chlorine was mostly or completely gone. In small pools, with pumps running 24/7 and reasonably located returns and main drains, it's possible to run an unstabilized pool without problems. In large pools, with multiple dispersed in-floor returns, and well-balanced circulations systems (both rare!), it's also possible.

BOTTOM LINE? Without stabilizer, you cannot reliably maintain a good chlorine residual THROUGHOUT the pool, except on pools with exceptionally good circulation systems which operate 24/7.

SECOND, if you attempt to run a low UFC + low CYA pool, you will not have sufficient chlorine present in pool water zones to maintain sanitation or control algae.

For example, most the chlorine resistant algae avoids being 'bleached' by creating a 'biofilm' with an outer poly-saccharide (slime) coating, that functions as a sacrificial 'shield', using up the chlorine before it can penetrate to the actual living parts of the algal biofilm.

So, when one of these shields is attached by chlorine in a pool with 0.5 ppm FC, it is sacrifices itself and consumes the chlorine in the 'zone' around the algae. For the attack to continue, FC from other areas of the pool has to physically be moved from where it is, to where the algae is.

BUT, when one of these shields is attacked by a high chlorine high CYA pool, even though there may be only 0.3 ppm HOCl present where the algae is, as the algae 'shield' is sacrificed to consume that chlorine, new HOCl forms almost instantaneously from the OTHER 9.7 ppm SFC present in the area. The NEW HOCl level may only be 0.29 ppm, since you've lost that 0.3 HOCl, but it's still close. And when that is consumed by the shield, a second NEW HOCl level will form, maybe 0.28 ppm.

And so on, till almost all the 9.7 SFC present has been converted to HOCl.

So, the net result is, the high Cl high CYA pool has almost 20x as much chlorine INSTANTLY availabgle to go to work on a problem, compared to the low Cl low CYA pool.

BOTTOM LINE? Pools with high Cl and high CYA will, in generally, have far fewer problems, and much better sanitation. This can be overcome, to some degree, by excellent circulation systems operated continuously.


------- I'll await corrections ---------------

[PoolDoc]

Was your question answered? -Ben

[Interloper]

Yes, Ben - thanks! Think the thing that had been causing me the most trouble was understanding how & when chlorine bound up with CYA was released and became available. Sheltering it is all well and good, but unless there was some way for it to come out from that shelter, I was always hazy on what benefit it had.

Now, in a side question -- at what point does the sheltered chlorine become noticeable / unpleasant for humans? With constant recharging via SWCG, is there a ceiling to CYA levels where keeping things that high becomes a bad idea?

[PoolDoc]

Read through the "Best Guess" page (in my signature), which addresses this topic at some length. If you still have a questions, after, ask again.

Ben

[waterbear]

Also, most SWCG's recommend a maximum CYA of 80 or 100 ppm and in actual practice running the CYA at the maximim is better than running it at lower levels. I will explain more below.

What this really seems to mean, to me, is that I don't really need to do anything once I find the power setting on my SWCG which seems to be able to burn off chloramines. Accidental super-chlorination won't be a problem, because Mr. Sun will simply burn off any excess. If I ever want to really "shock" the pool, I'd just dump in a couple gallons of bleach at dusk, just as I did back in my BBB days.

There is so much wrong with that statement that I don't know where to start so I am going to just ramble on below and hope it explains it all! However, I will say this--the proper FC level to maintain is 5% of the recommened manufacturer's maximum CYA level and the CYA should be kept at the mamimum for an outdoor pool. This translates into 4 ppm FC and 80 ppm CYA for the majority of US made SWCGs and 5 ppm FC and 100 ppm CYA for many of the Australian made SWCGs. This is an empirical observation but it has proven true for several years now by members of this and some other pool forums on the internet and has been sufficiently proven to me by my own observations in the (admittedly limited) number of customer's pools with SWCGs (and in my own).

Could be I just don't get it -- but it sure seems like a SWCG becomes a sort of self-regulating device. Like using a brick as cruise control because your car has a regulator set at 55 miles per hour.

As far as why test water with a SWCG, the SWCG is only an autmated way to add chlorine to the water. The amount of chlorine present is adjusted by adjusting the cell output and/or pump run time. IF you have a unit with automatic ORP control of sanitizer level (THIS would be the regulator set at 55 MPH you mention) then you do not need to test FC as often (but electronic controllers are only as good as their calibration, which can and does drift, and they do fail after a period of time and present their own set of maintenace tasks that are more complex, IMHO, than testing and tracking chlorine levels!) If you do not have an ORP controller and electrode then the unit is 'dumb' and just makes chlorine for whatever percentage of pump run time you set it for. It is entirely possible for the FC to be too high or too low. However, once you get it adjusted right it can be pretty stable. I only need to adjust my cell output/ pump run time in the spring and fall (my pool is open year round) since the hotter summer requires more cell on time than the cooler winter to maintain the same FC in my pool.
However, if you have a week or two or rainy weather or have a pool part with a huge bather load then you might need to manually shock the pool, just as you would ANY pool or maybe make some adjustments to run time or cell output before hand if you know the pool party will have 40 people in a 10K pool and you want to start with a higher FC level and maintain it a bit higher for the party to 'keep up', just like in any pool.
You still need to do regular testing of pH, TA, CH, CYA, and Salt (particularly if you have a backwashing sand or DE filter or the pool has an autofill device). The SWCG does not automate other water balance issues (with the possible exception of those units that come with add on pH controller/electrode and use either CO2 gas injection or acid feed using a peristaltic dosing pump for pH control. However, once again these have their own set of calibration/maintenance issues and, from my experience using automated pH control on commercial pools I can tell you it is MUCH easer to test pH and dose the proper amount of acid to adjust pH downward than to get these type of units calibrated and adjusted. If your test kit has an acid demand test this becomes even easier (even though the genera feeling on the forum is that acid demand tests are not useful). If the concern is overdosing on acid becasue of bad testing technique using a demand test, well, I really cannot comment other then to say work on your testing techniquie, it ain't rocket science! Also, just because you have pH automatin does not mean that you don't have to test pH. Testing the pH is the ONLY way to know if the controller is operating properly and if it is not then you need to take corrective action manually and then readjust/recalibrate/repair the pH system!

While the SWCG does automate the addition of chlorine it does add a unique problem with pH control since the constant aeration of the water caused by hydrogen generation at the cell cathode will cause CO2 to outgas faster which leads to the pH rise problem so often seen with salt pools (It is NOT, as often stated, due to the formation of sodium hydroxide since the excess OH- ions are neutalized when the chlorine is reduced back into chloride ions in the process of sanitation/oxidation, an acidic process).
The pH rise in salt pools (or any pools, for that matter, that use an unstabilized chlorine source like sodium hypochlorite--which is what the SWCG produces, calcium hypochlorite, or lithium hypochlorite is primarily from the outgassing of CO2! (We are ignoring any effects from new, curing plaster or aggregate finishes since they are a transitory cause of pH rise in pools.)
There are a few things a pool owner with a SWCG can do to minimize this:

1. Run the CYA at the manufacturer's MAXIMUM level so the cell on time for the desired FC level is as short at possible. This will minimize areation and therefore pH rise.

2. Maintain the FC at 5% of the CYA Much empirical evidence suggests that this is the 'sweet spot' for minimizing algae outbreaks, chloramine production, etc. (Thanks chem geek!)

3. Maintain the TA at 70-80 ppm (and adjust the calcium upward for a plaster or aggregate finish pool if needed to maintain proper water balance). TA is actually a measure of the amount of bicarbonate in the water and the higher the bicarbonate the more CO2 can form. The more CO2 in the water the faster it will outgas, the faster it will outgas the faster the pH rise. This is to our advantage when we are using stabilized chlorine sources like trichlor and, to a lesser extent dichlor, bromine, and MPS, which are both net acidic on use and need to have the constant acid additions neutralized but it works against us with any of the unstabilized chlorine sources and SWCGs which are pH neutral on use (alkaline on being added or created in the pool, acidic on santizing/oxidizing.
Lower TA mean less bicarbonate in the water means less carbonic acid means slower ougassing means slower pH rise.

4. DO NOT DROP THE PH TOO LOW when lowering pH. The lower you put the pH the faster it rises because the lower you put the pH the more bicarbonate you convert into CO2/carbonic acid (operative word here is acid). The more CO2 the faster it will outgas, The faster it tougasses the faster the pH rises. We use this fact to our advantage when we are trying to lower TA by dropping the pH to the lowest save limit to drop the TA (by converting bicarbonate into carbonic acid/CO2) and by then aerating the water to drive off the CO2 (reducing the amount of carbonic acid and causing the pH to rise quickly.
IF we do not drop the pH in a salt pool below about 7.6 and lower it when the pH hits 8.0 we will find that we have a more stable pH for a longer time than if we drop it to a lower value when we add acid.

5. Add borate to 50 ppm. Besides the well documented algaestatic properties of borate it also introduces a secondary borate/boric acid pH bufffer that works in a complimentary way with the bicarbonate/carbonic acid buffer already in the pool. The bicarbonate buffer tends to push the pH upward toward 8.4 while the borate buffer moves the pH downward. Together they keep the pH at around 7.7-7.8 for an extended period of time. Borates can be added inexpensively with 20 mule team borax (sodium tetraborate decahydrate) and muriatic acid or you can use straight boric acid or one of the proprietary borate products (which are either sodium tetraborate pentahydrate or a mixture of mostly boric acid and some sodium tetraborate pentahydrate) from Proteam, Bioguard, Guardex/Omni, or Pool Life (HTH). There is a lot of info already in the forum about using borate and if you have any questions you can always start another thread.

Hope this clears some things up.

Ben, at your discretion, move this to the China shop if you see fit! However, it is just general SWCG advice with the explanation as to why.

[PoolDoc]

Also, just because you have pH automation does not mean that you don't have to test pH. Testing the pH is the ONLY way to know if the controller is operating properly and if it is not then you need to take corrective action manually and then readjust/recalibrate/repair the pH system!

Yes.

This is one of the MAJOR reasons I dislike ORP/pH control systems -- and I was a dealer for a $6,000 set up for years. Calibration is complex, compared to other pool tasks. And it is ESSENTIAL!

I have seen a brand new therapy center DESTROYED over one weekend by a new and mis-calibrated ORP/pH controller and acid feed system (not my installation - I was hired as a post-disaster consultant). Damage exceeded $200,000.

If you over dose acid by hand, you have a little problem. If your pH controller fails, due to poor calibration or for any of a number of other reasons . . . and you have a fresh tank of acid, you can destroy your pool!