Daily Chlorine Goal

[BoatingDave]

hey after reading this forum I have decided to move to liquid chlorine as my main chlorine source. no more trichlor for me....

I have a hypothetical daily chlorine requirement question (ignoring shocking, assuming no daily change in bather load, no change in weather, no change in CYA, etc.) and I want to keep my FC ~4ppm:

Assuming good water balance, and a goal FC of ~4, I seem to recall reading on this forum that if FC goes below 2 someone suggested that you should add enough liquid chlorine to bring the FC to around 6, but if the FC was in the >2 but <4 range they would add enough chlorine to only go up to 4ppm.

Is there a rough "rule of thumb" that I should follow, or should I always just add enough to bring it to 4 and not overshoot?

Thanks, hopefully this question makes sense..

[polyvue]

hey after reading this forum I have decided to move to liquid chlorine as my main chlorine source. no more trichlor for me....

I have a hypothetical daily chlorine requirement question (ignoring shocking, assuming no daily change in bather load, no change in weather, no change in CYA, etc.) and I want to keep my FC ~4ppm:

Assuming good water balance, and a goal FC of ~4, I seem to recall reading on this forum that if FC goes below 2 someone suggested that you should add enough liquid chlorine to bring the FC to around 6, but if the FC was in the >2 but <4 range they would add enough chlorine to only go up to 4ppm.

Is there a rough "rule of thumb" that I should follow, or should I always just add enough to bring it to 4 and not overshoot?

Thanks, hopefully this question makes sense..

Every pool is different. Chlorine demand can vary greatly -- it's affected by the angle and duration of sunlight on the water (time of year), environmental pollution (dust, bird droppings,etc), the amount of cyanuric acid (CYA) in the water, pumping time and flow rate (determining how much of the water is filtered), not to mention the type of filter media and how clean it is, effectiveness of pool cleaner..... I think the best approach would be to set goals for CYA and Free Chlorine, and pump/filter time, then experiment to see how much and how often you need to add chlorine. In the middle of summer with lots of swimmers you'll be adding it every day I imagine. But if you use a pool cover (another variable!) you may find yourself needing to add chlorine only once or twice a week.

[PaulR]

People have different approaches. Basically, there is a minimum FC below which you'll start to have problems; the key thing is to stay above that at all times. The minimum is largely based on your CYA but ought to be tuned by your experience.

One approach is to figure out how much you have to add on whatever schedule you have, so that you're still above minimum the next time you add. In this approach you're mostly thinking things like "a quart a day" or "half gallon today" and not worrying too much about how high FC gets in between. If you find you're dipping below the minimum, then you up the amount you use each time.

Another approach is to pick an FC target and always bring FC up to that point, however much it takes. In this approach you're mostly thinking things like "need 1ppm today" or "need 2ppm today" and don't care whether the amount of bleach varies each time. If you find you're dipping below the minimum, then you up your target FC.

The goal is to find an approach that works for you and keeps the pool clean.

--paulr

[Fatboy]

Not to high jack this thread but I'm also considering switching over to using 6% bleach for my chlorine source but I have a cost $$ concern.

According to the Pool School on "How to chlorinate" is says...

"Usually you must add about 2-3 ppm daily to keep the pool properly sanitized."

So running the numbers in the Pool Calculator it says that I must add (2) 96oz jugs of 6% bleach to achieve this daily goal. I know there are a lot of variables that will effect this but WOW! Doing the math that's going to cost me about $5.00 a day or nearly $2,000 per year!! How is this less expensive than using Dichlor tablets and shock. I normally spend maybe $200-$250 / year for my chemicals.

Am I missing something here?

[BoatingDave]

Not to high jack this thread but I'm also considering switching over to using 6% bleach for my chlorine source but I have a cost $$ concern.

According to the Pool School on "How to chlorinate" is says...

"Usually you must add about 2-3 ppm daily to keep the pool properly sanitized."

So running the numbers in the Pool Calculator it says that I must add (2) 96oz jugs of 6% bleach to achieve this daily goal. I know there are a lot of variables that will effect this but WOW! Doing the math that's going to cost me about $5.00 a day or nearly $2,000 per year!! How is this less expensive than using Dichlor tablets and shock. I normally spend maybe $200-$250 / year for my chemicals.

Am I missing something here?

Assuming a CYA level around 30-60 I doubt you would need to add that much bleach for a standard size pool. You won't have to add about 2-3 ppm daily, you want to MAINTAIN the level of around 2-3 (or four)ppm. Depending on the size of your pool, your CYA level, how much sunlight it gets you will probably have some residual chlorine from the prior day, so it likely is not going to be zero ppm each day. (Nor would you want that!) How big is your pool and do you have a huge bather load?

[PaulR]

How much your pool loses each day depends on a number of factors. 2ppm is common during the swim season. If you're in a climate where it doesn't freeze, and you don't "close" the pool for the winter (e.g. where I am), still the temp drops a lot and chlorine consumption goes down considerably in the winter. I just peeked at my logs, and last January I was losing 0.5ppm per week. Of course I have a solar cover which helps.

--paulr

[chem geek]

I have a 16,000 gallon pool shown here and here that has a chlorine demand of around 1 ppm FC per day. I have a mostly opaque automatic cover, but the pool is used (open) for 1-2 hours every day and longer on weekends. The pool is also kept warm at around 88ºF. I get 12.5% chlorinating liquid from my local pool store for $3.00 per gallon and I add chlorine twice a week to the pool. I also add a very small amount of acid every month or two. That's it. No clarifiers, no flocculants, no algaecide, no phosphate remover (in spite of having 2000-3000 ppb phosphates), no metal ions, no weekly shocking. Just $15 per month in chemical costs.

So if I didn't have a pool cover, I might have a 2 ppm FC per day chlorine usage and a $30 per month cost. If you live in a hot sunny area (say, Arizona) then you might operate the pool at a higher CYA level with a proportionately higher FC level and this will usually keep the chlorine usage closer to 2 ppm FC per day or somewhat less. If you keep your pool at cooler temps (i.e. not at 88ºF), then the chlorine usage is somewhat lower.

With your 30,000 gallon pool, you would use more chlorine than my pool so at 2 ppm FC and using the chlorinating liquid example the cost is around $60 per month though could be less if your daily chlorine demand is less (say, 1.5 ppm FC instead of 2 ppm FC per day). If you use a pool cover, that cuts down chlorine usage.

You need to shop around for bleach and chlorinating liquid prices in your area. Standard bleach at a grocery store can be expensive. Wal-Mart may be cheaper or a local pool store or big-box store may carry chlorinating liquid at a more reasonable price (be sure to account for container size and % strength differences). You can see some prices (mostly of chlorinating liquid) around the country in this thread. With Trichlor, you also need to account for the pH Up or similar products you need to add since it is very acidic. With chlorinating liquid or bleach, you won't need hardly any pH adjustment whatsoever once you get your TA level in line (usually lower). A cost comparison of chlorine sources I did a while ago is described in this post, but prices of bleach have gone up in some areas while the price of chlorinating liquid in my area actually went down this year (it was high last year).

[BoatingDave]

With chlorinating liquid or bleach, you won't need hardly any pH adjustment whatsoever once you get your TA level in line (usually lower). A cost comparison of chlorine sources I did a while ago is described in this post, but prices of bleach have gone up in some areas while the price of chlorinating liquid in my area actually went down this year (it was high last year).

here's a question. I am adding about 30 oz of 12% liquid chlorine to my pool each day to bring chlorine from about 2.8 up to 4. I have a 25,000 gallon pool. However, it seems that I have to add about 20oz of HCL each day too to keep the pH in line. my TA is 100, CH about 190, pH 7.5. Why do I need so much acid? It seems the clorine liquid is raising my pH a bit each day.....

[chem geek]

See if you can find out the pH of your chlorinating liquid. A good 12% liquid chlorine will have a pH of 12.5 or lower. If it's higher than that, then this will have the pH rise more quickly, but even so shouldn't require daily acid addition. Though the chlorine will raise the pH upon addition, it will drop back down to where it started as the chlorine gets consumed/used since that is an acidic process. The only rise in pH at that point will be from the "excess lye" in the chlorinating liquid and from carbon dioxide outgassing (or from plaster curing).

Most likely, however, is that the pH rise is due to outgassing of carbon dioxide from your pool water since pools are intentionally over-carbonated. A TA of 100 ppm makes that worse. As you add acid, the TA will drop slowly over time (unless you have evaporation and refill with fill water that has normal-to-high TA). You can accelerate the TA lowering process via the procedure outlined in Lowering Total Alkalinity. If you have any waterfalls, spillovers, fountains or other aeration features, that will tend to make the pH rise faster.

You may find that if you lower the TA to 80 ppm or 60 ppm, that the rate of pH rise is lowered and most especially that the amount of acid you need to add is lowered. Use of a pool cover makes a big difference. I have a mostly opaque pool cover and use only 12.5% chlorinating liquid added to my pool twice a week at a rough rate of 1 ppm FC per day equivalent. I only need to add a small amount of acid every month or two.

Basically, TA not only buffers the pH, but is a SOURCE of rising pH in its own right because it is mostly a measure of bicarbonate in the water which is proportional to the amount of carbon dioxide in the water and that level is higher than naturally would occur when in equilibrium with the carbon dioxide in air. This chart shows how over-carbonated the water is at various TA and pH levels. You will notice that the rate of pH rise should slow down as the pH gets higher so at the lower TA level you may find that the pool tends to settle in at, say, 7.6 or 7.7 which is fine.

If your pool plaster is new (within 1 year old), then the curing of that plaster will have the pH rise substantially over time and this effect lessens over time and is worst in the first months but can last for at least a year.

When you were using Trichlor, you needed a higher TA level since Trichlor was acidic and the higher TA level provided more pH buffering and also caused the pH to rise from carbon dioxide outgassing which helped to counteract the pH drop from the Trichlor. You don't want that effect anymore now that you are using a hypochlorite source of chlorine. Lowering the TA is the answer.

[BoatingDave]

If your pool plaster is new (within 1 year old), then the curing of that plaster will have the pH rise substantially over time and this effects lessens over time and is worst in the first months but can last for at least a year.

Ah Ha! Excellent advice. Btw, the plaster is a little under one year old. I thought that it could still be adding to the pH rise, (I was certainly adding acid last year when it was really fresh) but I was hoping that it would be nearly cured by now. I'll bet I am getting the double whammy of the new plaster, plus the relatively high(-ish) TA. I don't have any fountains, though there is a small spillover from my spa to my pool (though I'd be surprised if that was significanlty raising the pH).

If the plaster is still relatively new, should I be aiming for a lower pH (around 7.2 ish) like they suggested last year when it was really fresh?

[chem geek]

No, you do not need to be aiming for a lower pH. That will just frustrate you since the pH rise will be more rapid in that case. I would just lower the TA level (to at least 70 ppm if not 60 ppm to start with) and target a somewhat higher pH of around 7.7 instead of 7.5. See if that's more stable. If it is, then adjust your CH higher to compensate for the saturation index. I'm sure you'll find a reasonable sweet spot. You can distinguish between the pH rise from carbon dioxide outgassing vs. that from the curing of plaster because adding acid will have the TA drop over time from the former while the TA won't drop over time from the latter.