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Tri-chlor Vs. Di-chlor


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I have been using Tri-chlor tablets and granules for years and they seem to work fine except of course the CYA problems. I have recently found that Di-chlor granules are cheaper and I'm thinking about switching, whats the difference between the two? And is there a downside to using di-chlor granules along with the Tri-chlor tablets? Also, whats the maximum chlorine level for a pool that is safe to swim in?

Thanks for the help!

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Dichlor increases CYA at an even faster rate than Trichlor so you should not be using it unless you are intentionally trying to increase the CYA level. Also, when accounting for the actual amount of chlorine (FC) you get, Trichlor is less expensive than Dichlor, even accounting for the pH Up you need with Trichlor. You cannot compare product costs by weight since they contain different concentrations of chlorine. See this post for a cost comparison of chlorine sources. Generally speaking, chlorinating liquid or bleach are the least expensive, Cal-Hypo is close (sometimes cheaper, depending on where you get it), then Trichlor, then Dichlor and finally Lithium Hypochlorite which is very expensive.

Remember the following chemical rules of fact that are independent of concentration or size of pool:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

You should read the Pool School for more information about managing your pool, as well as the appropriate FC level for the CYA level that will prevent algae growth without the need for a supplemental algaecide or phosphate remover or regular shocking.

Because most of the chlorine binds to CYA, the FC alone does not tell you what is safe to swim in. There are pools in desert areas managed by pool services that use a high 100 ppm CYA level and bring up the FC to 14 ppm each week so that it ends up at around 4 ppm the following week when they come back. As high as 14 ppm FC sounds, in the presence of 100 ppm CYA it is technically equivalent (same hypochlorous acid concentration) as 0.13 ppm FC with no CYA which is far lower than found in most indoor pools (which typically don't have CYA and have 1-2 ppm FC). The only issue with high FC would be if you were to drink a lot of water -- something that you do not normally do with pool water (and even then the EPA limit is 4 ppm FC for drinking 6-8 quarts every day).

If you want to prevent algae in a pool using Trichlor where the CYA climbs and you aren't proportionately increasing the FC level, then you should use an algaecide or phosphate remover without fail, such as PolyQuat 60 weekly. If you use a copper-based algaecide, then you should be careful not to overdose or let the pH rise too high or you can get copper staining, especially in plaster pools. If you use chlorinating liquid or bleach as your primary source of chlorine, then you will need to add it every day or two unless you have a pool cover in which case you could add it twice a week. That's what I do, adding 12.5% chlorinating liquid twice a week to my 16,000 gallon pool at around $15 per month and that's it with crystal clear water and no need to shock the pool because the FC/CYA ratio is consistently maintained above the required minimum. The pH is very stable so I only add a small amount of acid every month or two. I also added 50 ppm Borates to the pool this year (didn't have it for previous 6 years) and like it -- it slightly lowered the chlorine demand, is an insurance policy since it's a mild algaecide, and seems to give an extra shimmer to the water (probably due to lowering water surface tension).

Richard

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Dichlor increases CYA at an even faster rate than Trichlor so you should not be using it unless you are intentionally trying to increase the CYA level. Also, when accounting for the actual amount of chlorine (FC) you get, Trichlor is less expensive than Dichlor, even accounting for the pH Up you need with Trichlor. You cannot compare product costs by weight since they contain different concentrations of chlorine. See this post for a cost comparison of chlorine sources. Generally speaking, chlorinating liquid or bleach are the least expensive, Cal-Hypo is close (sometimes cheaper, depending on where you get it), then Trichlor, then Dichlor and finally Lithium Hypochlorite which is very expensive.

Remember the following chemical rules of fact that are independent of concentration or size of pool:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

You should read the Pool School for more information about managing your pool, as well as the appropriate FC level for the CYA level that will prevent algae growth without the need for a supplemental algaecide or phosphate remover or regular shocking.

Because most of the chlorine binds to CYA, the FC alone does not tell you what is safe to swim in. There are pools in desert areas managed by pool services that use a high 100 ppm CYA level and bring up the FC to 14 ppm each week so that it ends up at around 4 ppm the following week when they come back. As high as 14 ppm FC sounds, in the presence of 100 ppm CYA it is technically equivalent (same hypochlorous acid concentration) as 0.13 ppm FC with no CYA which is far lower than found in most indoor pools (which typically don't have CYA and have 1-2 ppm FC). The only issue with high FC would be if you were to drink a lot of water -- something that you do not normally do with pool water (and even then the EPA limit is 4 ppm FC for drinking 6-8 quarts every day).

If you want to prevent algae in a pool using Trichlor where the CYA climbs and you aren't proportionately increasing the FC level, then you should use an algaecide or phosphate remover without fail, such as PolyQuat 60 weekly. If you use a copper-based algaecide, then you should be careful not to overdose or let the pH rise too high or you can get copper staining, especially in plaster pools. If you use chlorinating liquid or bleach as your primary source of chlorine, then you will need to add it every day or two unless you have a pool cover in which case you could add it twice a week. That's what I do, adding 12.5% chlorinating liquid twice a week to my 16,000 gallon pool at around $15 per month and that's it with crystal clear water and no need to shock the pool because the FC/CYA ratio is consistently maintained above the required minimum. The pH is very stable so I only add a small amount of acid every month or two. I also added 50 ppm Borates to the pool this year (didn't have it for previous 6 years) and like it -- it slightly lowered the chlorine demand, is an insurance policy since it's a mild algaecide, and seems to give an extra shimmer to the water (probably due to lowering water surface tension).

Richard

Great! I appreciate the help. On the Borates, how often do you add it?

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That's what I do, adding 12.5% chlorinating liquid twice a week to my 16,000 gallon pool at around $15 per month

I'm amazed you can get along with twice a week. Right now, my CYA is still at 63, but I add 2 quarts in the evening to my 20k gallon pool. It is fine int he morning, but by noon, I'm at .5 PPM. I add another 2 quarts at noon, and back down to .5 by 6PM. I actually shocked up to 10PPM a couple of nights ago as was back down to .5 by noon the next day. I guess all of this 100 deg weather here in Texas is really taking its toll on my chlorine budget!

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The Borates are added just once and then you only add more if there is dilution from splash-out, backwashing, rain overflow, etc.

As for the twice a week chlorine, that's because I have an opaque electric safety cover so my chlorine usage is very low at 1 ppm FC or below per day. Without a pool cover, one usually needs to add chlorine every day or two, even with a higher CYA level. One can use a really high CYA and a larger FC swing to get to once a week, but that's more extreme and what some pool services do. For example, some have the CYA at 100 ppm and raise the FC to 14 ppm where in a week it drops to around 4 ppm at which point they raise it again.

You do seem to be losing more FC than normal. At your CYA level, the loss is usually closer to 1/3rd up to perhaps 1/2 of the FC per day. Something doesn't sound right. I'd check the overnight FC drop. It should be <= 1 ppm. If it drops more than that, then you've got something consuming chlorine in your pool (i.e. it's not just sunlight) and should shock the pool to 40% of the CYA level (around 25 ppm FC) and keep it there until your overnight drop becomes 1 ppm or less (or at least 2 ppm or less). Perhaps you've got nascent algae growth or the CYA reading isn't accurate -- it sounds like the number came from a pool store and they are notorious for inaccurate readings, especially with CYA. You should get your own good test kit: either the Taylor K-2006 you can get at a good online price here or the TF100 from tftestkits.net here with the latter kit having 36% more volume of reagents so is less expensive per test.

Richard

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I think my test kit is K2005, but it still has the CYA test... I'm showing about 65 myself (that is a very subjective test, but I squint real hard and put in drops til I can't see the dot even while squinting!). Yesterday was a cloudy day and only about 92 here and I still had chlorine at the end of the day so that was much better. I think that in the summer, I'm just going to have to view the pool like my dog and feed it every day!

The local YMCA actually had to close their pool yesterday morning because it tested for zero chlorine... I think it's just something about this hot sunny weather we've been having in Texas.

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I think my test kit is K2005, but it still has the CYA test... I'm showing about 65 myself (that is a very subjective test, but I squint real hard and put in drops til I can't see the dot even while squinting!). Yesterday was a cloudy day and only about 92 here and I still had chlorine at the end of the day so that was much better. I think that in the summer, I'm just going to have to view the pool like my dog and feed it every day!

The local YMCA actually had to close their pool yesterday morning because it tested for zero chlorine... I think it's just something about this hot sunny weather we've been having in Texas.

I'm thinking your right also, this direct sunlight and high temps is hard to deal with. Do you use a chlorinator, it seems to help me a bunch!

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I have a chlorinator, but even with it turned up and feeding 2 tabs / day into the pool, it was all I could do to keep 1-2PPM in the pool. I was getting uncomfortable with the amount of CYA that would put in the pool (I had just drained it to drop my CYA down), so I turned it off and went to just the liquid chlorine. I'll turn the auto-feed back on when I head out on vacation though.

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  • 12 years later...

Trichlor is the least expensive source of chlorine between the two.

Trichlor has more chlorine by weight and less CYA by weight than "pure" (anhydrous) Dichlor. But the chlorine makes the Trichlor weigh more (per molecule) so the net effect is that in 10,000 gallons 1 pound of Trichlor gives 11.0 ppm FC and 6.7 ppm CYA while 1 pound of Dichlor gives 6.6 ppm FC and 6.0 ppm CYA. HOWEVER, this is for Dichlor that is DIHYDRATE. Part of the inefficiency of dichlor is that you are buying water with it (in "dry" form as it is a hydrated crystal with the Dichlor). If you were able to find ANHYDROUS Dichlor, then one pound would give 7.7 ppm FC and 7.0 ppm CYA. So with anhydrous Dichlor, you get a little more CYA per pound than with Trichlor. But anhydrous Dichlor is far less common -- most is the dihydrate form.

I hope this helps.🙂

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