keeping your water clean; chlorine and CYA relationship

Hi Dan! I will answer what I know.

TC=total chlorine, FC=free chlorine, CC=combined Cl.

FC is the good stuff that is working to keep your pool sanitized. CC is "used-up" chlorine, a waste product that smells strongly and causes irritation of eyes and skin. CC levels should be kept below 0.5, pref below 0.2. TC=FC+CC.

I use laundry bleach as my chlorine source. Pros: cheap and easy to use, relatively slow to degrade in storage. Cons: bulky; requires frequent testing of water and addition of more. Liquid chlorine from the pool store can often be had cheaply, but due to its higher concentration of Cl, it breaks down more quickly in storage. It's less bulky than bleach but still rather unwieldy.

Trichlor and dichlor are stabilized; good if you need to add CYA to your pool, bad if you don't. Convenient to use in tablet form; doesn't require such frequent monitoring as liquid. High level of available Cl; lots of bang for the buck.

Cal hypo is more often used as shock than sanitizer; it tends to cloud the water. Raises CH. Rather hazardous to store.

Lithium hypo: reputed to be an excellent product; most expensive of the usually available chlorine sources.

[Chorine gas is another but is not usually available to residential pool owners. It's cheap but requires special equipment to store and introduce into the water. Suitable for large commercial pools. Very acidic.]

Some of these tend to have more effect on pH than others, but my personal experience is only with bleach, so I don't recall which ones. I know that liquid chlorine tends NOT to cause pH fluctuation.

All of these chlorine sources break down to hypochlorous acid once they're added to the water. Sources with higher concentrations of chlorine tend to be less stable [edit: in storage, that is, not during use].

The desired Cl level depends on your CYA level. The higher your CYA, the more FC you need. High CYA levels may require an FC of 10 or more for adequate sanitization. Can't say I know much about the inner workings of the Cl/CYA relationship, but there is a chart floating around. . . I believe 30-50 is typically quoted as an ideal CYA range, unless you have a SWCG, which usually recommends a higher CYA level. Cl about 3-5 for that CYA range is good, I think. Maybe 12-15 for shock.

The knowledgeable folks around here can correct any mistakes I've made, and provide those "inner workings" details for you.

Jules

"Breakpoint" is ususally calulated by multiplying the measured amount of combinind chloramines by a factor of 10 and then raising your FC by that amount above what it currently is. For example, your FC is 4 ppm and your CC is 1.5 ppm. You multibly 1.5 by 10 to get 15 so you have to raise your pool 15 ppm above the 4 ppm or to 19 ppm. This doesn't always work because other factors that effect breakpoint include the amount of sunlight the pool gets(UV light is an important part of the breakpoint equation) and exactly which chloramines are presents (mono and di chloramine are easy to break down. Many of the higher ones are not.) Also the CYA level will have an effect. The table that Duraleigh posted is a good guide that has proven it's effectivness in most cases.

Some additional thoughts:

A pool should be uncovered while shocking and if you do use a cover it should be off the pool during the day normally to help prevent a buildup of combined chloramines.

"Shock" is a verb, not a noun! It is something you do to a pool by raising the FC level to achieve breakpoint, NOT a product that you put in it that is labeled Shock.

Cal-Hypo is the "bad boy" of chlorine chemicals in terms of being a fire hazard because once it gets started it is self-feeding and even explosive (under some conditions). Mixing Cal-Hypo with Trichlor, Acid, Ammonia or many other chemicals is bad, bad, bad. The higher concentrations of Cal-Hypo (e.g. 73%+) are particularly dangerous which is why it is usually sold in lower concentrations (e.g. 65% or 48%).

It is also not a good idea to add liquid chlorine (bleach or chlorinating liquid) to Trichlor for similar reasons though Trichlor is less of a fire hazard than Cal-Hypo.

The heat in both cases above is mostly from mixing a base (Cal-Hypo, liquid chlorine) with an acid (Trichlor). So adding acid (e.g. Muriatic Acid, dry acid) to a base (pH Up which is sodium carbonate) is also bad though it mostly just bubbles carbon dioxide, but can splatter the acid. In the case of mixing any source of chlorine with ammonia, the heat is from the formation of chloramines.

So in summary, do not mix a strong acid with a strong base and do not mix chlorine with ammonia.

ACID: Trichlor, Muriatic Acid (31.45% hydrochloric acid), Dry Acid (sodium bisulfate), chlorine gas (when dissolved into water)
BASE: Bleach or chlorinating liquid (sodium hypochlorite), Cal-Hypo (calcium hypochlorite), lithium hypochlorite, pH Up (sodium carbonate or washing soda)
CHLORINE: Trichlor, Dichlor, bleach or chlorinating liquid, Cal-Hypo, lithium hypochlorite, chlorine gas

My Pentair SWCG manual says to keep the CYA at 75 and the FC at 1-3ppm. This doesnt jive with the tables listed in this thread to keep your FC between 5-10. Is the Pentair manual incorrect?

The short answer is "yes, the Pentair manual is incorrect". The more complete answer is that pools with SWG systems do not all require the same FC levels as shown in the chart. A few users have found that indeed they need to keep the chlorine levels higher as per the chart, but most do not. However, the recommendation of 1-3 ppm FC at 75 ppm CYA is definitely not enough to keep away algae except at the high end of this range. We have found (through many users on the Pool Forum) that a minimum FC level of 3 ppm at 70-80 ppm CYA will work for most pools. Some pools will get mustard/yellow algae at this level and need to run higher at around 6 ppm FC and one pool got green algae at the lower 3 ppm level.

The reason that SWG pools can operate at a lower FC level and not normally get algae is that they are more continuously dosing the pool with very high localized levels of Free Chlorine. In the SWG cell itself, the FC level is around 80 ppm (and much of this may be disinfecting chlorine, at least briefly) so water that flows between the plates in the cell gets super-chlorinated and any free-floating algae gets killed there. Of course, it takes quite a few passes (turnovers) of the pool water to get most of the water exposed to that high chlorine level (most water flowing through the SWG cell does not go between the plates), but it does seem to take care of much of the algae. The problem is that any algae that gets stuck to pool surfaces (typically in biofilms) will not get exposed to this higher chlorine level and can grow. If you regularly brush your pool, then this will help prevent such algae from taking hold.

Ben' best Guess Chart

duraleigh said:

Jules,

You are modest to a fault. Excellent review of available Cl.

That chart you're referring to looks something like this (AKA Ben's "Best Guess" Chart):

Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
0 ppm . . . . ... . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
10 - 20 ppm . .. . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
30 - 50 ppm . .. . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
60 - 90 ppm . . .. . 5 ppm . . . . . 10 ppm . . .. 20 ppm
100 - 200 ppm ... . 8 ppm . . . . . 15 ppm . . .. 25 ppm

So, what it means, for example, is if your stabilizer (CYA) is, say, 40ppm, you would be wise to keep your FC level 3-6ppm. If you needed to "shock" your pool (obtain breakpoint chlorination) you would probably need to obtain at least 15ppm of free chlorine. That relationship always brings up active discussion and is a good one to become comfortable with for new pool owners.

The process of "shock" or breakpoint chlorination is also quite revealing to new owners.....the actual process is a little over my pay grade, however. (Okay, Waterbear, Chemgeek, Medvampire, I've given you the lead-in.....please take over :lol: )

Click to expand...

Thought I'd throw in the chart I developed from Ben's and some of Richards wisdom --- I covers the "inbetweens".

Hamop78,
Just curious, how did you arrive at the values for the graph. Ben's chart is based on empirical observation in actual pools. When put to the test it works. I am wondering how you arrived at the values you got. Richard has done some calculations of actual chlorine levels at various CYA levels and has investigated the chlorine levels needed to keep algae and bacteria under control. Check here and here for some of this info.