Hot Tub Forum
Original => Hot Tub Forum => Topic started by: Nitro on April 22, 2009, 12:52:22 pm
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Chlorine Demand is the single most important measurement you can make, to determine how well your tub is sainitized.
Any organic "stuff" (dead or alive) in your tub, Chlorine will kill and oxidize. The more "stuff" in your tub, the more Chlorine is needed. The question becomes, is there a measurement of how much "stuff" there is in your tub. Yes, Chlorine Demand (CD)! CD is basically how much Chlorine (sanitizer) your tub is using. CD will tell you how much Chlorine is being used, and therefore tell you how much "stuff" is in your tub.
Now that we know what CD is, how do we measure it? That's the easy part.
1st, Shock your tub to around 10ppm FC, and make note of where it's at.
2nd, Cover and let sit for 24hrs. DON'T use the tub!
3rd, The next day, test FC again.
Now, take the 2nd measurement and divide it by the 1st measurement. This will give you a percentage of how much Chlorine is left in your tub after one day. Obviously the higher percentage the better. 100% means your tub is not using ANY Chlorine (not likely). 0% means your tub used ALL the Chlorine you put in it (NOT GOOD).
To take it one step further you can calculate CD, which is One minus the Percentage you calculated. That will give you the percentage of Chlorine your tub is using. Obviously the LOWER the better. If it's 0%, that means your tub is using 0% of the Chlorine (again not likely). If it's 100%, that means your tub used 100% of the Chlorine you put in (again NOT GOOD).
Measuring CD is the easy part. Interpreting it is a little trickier. It's best to first measure CD on a fresh refill, BEFORE you use the tub. That way you have a good Baseline (BL). My BL is ~25%. That means, when I shock my tub to 10ppm FC, the next day it is ~7.5ppm. After you start using your tub, the CD will increase. The trick is to try to keep it as close to your BL as possible.
For example, let's say you use the tub (4 people for an hour). After you get out, you shock the tub to 10ppm. The next day you check FC and it's 2ppm. That means your CD is 80%. That's too high and means you didn't use enough Chlorine the night before. Not a problem, just shock to 10ppm again. The next day check your FC. It should be above 5ppm. If not, keep shocking until it is. If you're not using enough Chlorine, your CD will increase and be more difficult to get back down.
So what's a good CD then. Here's my recommendation.
~25% is Ideal. This should be your Baseline.
~50% is Ok. This is probably where most people are at.
~75% is Poor. This means "something" is using up chlorine too fast. You need to shock.
100% is Bad. This could mean you just had a Hot Tub Party, you've been using way too little Chlorine, OR you have "something" actively growing (a bug) in your tub. The former two mean you should start shocking. The latter means you need to decontaminate your tub.
NEVER let your CD get to 100%, or you'll have problems. Also, the older your water gets, the more difficult it will be to lower the CD. If you find it difficult to lower your CD, it could mean you're due for a water change. It could also mean your filter is dirty and needs to be cleaned. Make sure you keep you filter clean, or it will use up Chlorine. MPS will help Chlorine oxidize waste, and therefore lower your CD. So if you find it difficult to lower CD, try shocking with MPS.
Lastly, if you have an Ozonator it will raise CD by about 25%. Therefore the lowest you'll probably be able to get it is 50%. However, the Ozonator will help oxidize waste, so the CD shouldn't rise as much.
Hope that helps you keep you water cleaner.
Happy Tubing! :)
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I think that there's something missing here ... the effects of heat on chlorine.
I agree that chlorine shouldn't disappear quickly but it is possible to lose a substantial amount of free chlorine due to the water's temperature. Chlorine at 90º should last longer than at 104º. 24 hours is a long time for chlorine to be in the tub. Adding 10 PPM after 4 people get in (which may be under-dosing, BTW) and expect to have 5 PPM 24 hours later is a wrong assumption. The fact that you have any chlorine in the tub 10 hours later is a sign that the chlorine has done it's job whether you originally put in 10 PPM or 1 PPM. Having any chlorine in the tub 2 days later is something that I think is wishful thinking.
In a pool, you measure chlorine demand by the amount of chlorine the water used up overnight without the sun shining on the water and dissipating the chlorine. To assume you have a chlorine demand because the sun destroyed the chlorine would be false and I think that this is the same with the heat of the water.
The correct answer IMO is to dose the tub close to or over 3 PPM measured 20 minutes after putting it in EVERY DAY. If you have a chlorine demand then the chlorine will be used up quickly and 20 minutes later it is gone. If it does disappear then the correct procedure is to shock the tub to get rid of the chlorine demand at that point.
And chlorine demand goes up as the CYA level goes up since chlorine becomes less effective as the CYA goes up.
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Vinny,
I think you're missing the point here. The GOAL is to keep Chlorine Demand as low as possible. Obviously CD will rise during use. However, the key is where you LEAVE it. Your IDLE CD is what we're interested in. You measure that with nobody using the tub. Your Idle CD will rise over time if you aren't using enough sanitizer. If you keep your Idle CD low, that means you're using enough.
My Idle CD is 25%, the same as it was 5 months ago, when I last refilled it. I can shock my tub to 12 ppm FC, leave my tub for 7 days and it will still have 1 ppm FC left.
So the question is, what is your Idle CD?
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IMO, I think the bigger point here is not to scare people with CD. CD is really only an issue when you are not able to maintain a chlorine residual after a shock.
Chlorine demand in a hot tub is a very simple fix. Drain and refill.
I respect your opinion and knowledge, Nitro, but I think you are making an issue out of something when it doesn't need to be.
And Vinny is right, the heat of the tub alone will dissipate the chlorine faster than most people think.
However, chlorine usage is a great thing to discuss, just as any other sanitizer would be. Personally, I still do not understand why the majority of the people on this board choose chlorine over bromine in their tubs anyway. Bromine is a much better sanitizer in a spa; pools, however, are the opposite.
Like I said, I do appreciate your knowledge and willingness to help, but there are many other things that I think should be discussed before this.
Thanks!
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Nitro
I have spent the last 7 years in the municipal water treatment industry and believe me there is no such thing as "idle chlorine demand"
A tub left alone with no bathers will have an increasing demand even if you were able to maintain a perfect 1 ppm in the water.
There is simply no way to idle the chlorine demand without constant monitoring and adjustment to the free chlorine residual. Something that is just not cost effective in a hot tub. Chlorine demand is constantly changing due to the number of variables that effect it
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IMO, I think the bigger point here is not to scare people with CD. CD is really only an issue when you are not able to maintain a chlorine residual after a shock.
Chlorine demand in a hot tub is a very simple fix. Drain and refill.
I respect your opinion and knowledge, Nitro, but I think you are making an issue out of something when it doesn't need to be.
And Vinny is right, the heat of the tub alone will dissipate the chlorine faster than most people think.
However, chlorine usage is a great thing to discuss, just as any other sanitizer would be. Personally, I still do not understand why the majority of the people on this board choose chlorine over bromine in their tubs anyway. Bromine is a much better sanitizer in a spa; pools, however, are the opposite.
Like I said, I do appreciate your knowledge and willingness to help, but there are many other things that I think should be discussed before this.
Thanks!
Heat does effect CD a little, but how often do you change the temp in your hot tub, and by how much? A few degrees will not make much difference. Besides, the point is to keep CD low, for your tub, whatever that is.
If you feel Bromine is better, that's your opinion. I'm just saying checking CD is helpful for Chlorine users.
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Nitro
I have spent the last 7 years in the municipal water treatment industry and believe me there is no such thing as "idle chlorine demand"
I beg to differ. There IS such a thing as Idle Chlorine Demand. It will rise if there isn't enough sanitizer being used, and it will stay low if there is enough sanitizer being used.
A tub left alone with no bathers will have an increasing demand even if you were able to maintain a perfect 1 ppm in the water.
Sorry Incorrect! With no bathers, FC drops non-linear. That means it drops by a percent (whatever that is). It does NOT drop by a flat value. So if your CD is 50%, you will have HALF the FC you had the day before. Again, this assumes you have static water i.e. not being used.
There is simply no way to idle the chlorine demand without constant monitoring and adjustment to the free chlorine residual. Something that is just not cost effective in a hot tub. Chlorine demand is constantly changing due to the number of variables that effect it
I'm not saying CD doesn't change (get higher). It does when you use the tub. However, there DOES exist a Baseline Chlorine Demand for your tub, and the closer you stay to that level the better. When I say IDLE Chlorine Demand, I mean the average CD your tub is at when it's not used. In other words a day or two after use.
As long as you have Chlorine in your tub, and it's not being used, the CD will decrease (slowly). OTOH, if you're constantly using the tub, and you're not using enough Chlorine, your CD will increase until it becomes 100% i.e. Chlorine Lock. My point is, you won't know it UNLESS you track your CD. Therefore, it's actually more useful to check how much Chlorine your tub is using during non use (i.e. Idle Chlorine Demand) rather than just checking the Chlorine level.
If you don't believe me, check it out yourself. It's easy, just wait a day after you use your tub, shock to 10 ppm, wait a day, check FC again and calculate. Mine is 25%, and has been since I refilled my water 5 months ago, EXCEPT for one time when I had a hot tub party, and didn't use enough Chlorine (never dropping below 1 ppm though). It went up to 50% for a week, until I got it back down by shocking. The next two hot tub parties I had, I used more Chlorine and my CD stayed low.
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Nitro
You go with that I'll go with what I have learned in the last 7 years
Good Luck
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Nitro
You go with that I'll go with what I have learned in the last 7 years
Good Luck
The time spent doing something means nothing, if you don't keep an open mind and continue to learn.
If you ever do feel the urge to test your CD, let us know what you find.
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Nitro
I don't know what you do for a living but I do know what I do.
Your information is wrong
There is no way you can idle the chlorine demand without using distilled water. It is just not possible
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The time spent doing something means nothing, if you don't keep an open mind and continue to learn.
It goes both ways. In Canada can take his field knowledge and enhance it with more theory and if you had the field experience he had it would alter your approach to water care as well.
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Nitro
I don't know what you do for a living but I do know what I do.
Your information is wrong
There is no way you can idle the chlorine demand without using distilled water. It is just not possible
My information is correct. You may be misinterpreting it though.
I don't know what you mean by "Idle the CD". CD does change depending on the amount of use. But if there is no use it will stay relatively constant (decreasing slowly, until either it hits the Baseline, or the Chlorine runs out).
My point is if you use enough chlorine during and after use, the CD will go back down to the baseline faster, and stay there. If you're not using enough Chlorine, you will fall behind and your CD will keep getting higher over time, eventually becoming 100%. I've tested and confirmed this in my tub and others.
Again, if you feel the urge to test your CD, let us know what you find. If you want to go further, test CD once. Don't use the tub. Then test it again the next day or two and compare the results. They should be pretty close. However the only way to know your baseline CD for sure, is to test it on a fresh fill.
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My information is correct. You may be misinterpreting it though.
I don't know what you mean by "Idle the CD". CD does change depending on the amount of use. But if there is no use it will stay relatively constant (decreasing slowly, until either it hits the Baseline, or the Chlorine runs out).
My point is if you use enough chlorine during and after use, the CD will go back down to the baseline faster, and stay there. If you're not using enough Chlorine, you will fall behind and your CD will keep getting higher over time, eventually becoming 100%. I've tested and confirmed this in my tub and others.
Again, if you feel the urge to test your CD, let us know what you find. If you want to go further, test CD once. Don't use the tub. Then test it again the next day or two and compare the results. They should be pretty close. However the only way to know your baseline CD for sure, is to test it on a fresh fill.
OK I'll explain
CD does change depending on the amount of use. But if there is no use it will stay relatively constant (decreasing slowly, until either it hits the Baseline, or the Chlorine runs out).
Here is where your error is. Demand will change regardless of use unless the water in the tub is distilled. Given that most tubs use tap water to fill them up with, your demand will increase slowly even if there is no use, otherwise you would never run out of free chlorine. All it takes is for 2 "bugs" to get into your water and your theory goes out the window. Remember that not all hot tub "bugs" are destroyed by chlorine
demand=dosage rate-free chlorine
My point is if you use enough chlorine during and after use, the CD will go back down to the baseline faster, and stay there. If you're not using enough Chlorine, you will fall behind and your CD will keep getting higher over time, eventually becoming 100%. I've tested and confirmed this in my tub and others.
Your demand will never get down to the baseline and stay there in pool or spa water, it's just not possible to ever kill or oxidize all of the critters in the water.
You did not give your test a long enough time to incubate. Do your test and then wait 48 hours with no use whatsoever and then tell me if you have any chlorine demand. If you have then you have not "idled your demand" you have just dropped it for a short period
Again, if you feel the urge to test your CD, let us know what you find. If you want to go further, test CD once. Don't use the tub. Then test it again the next day or two and compare the results. They should be pretty close. However the only way to know your baseline CD for sure, is to test it on a fresh fill.
I can point you to numerous articles, water treatment experts, water plant operators, environmental engineers and lab staff that will all tell you your information is incorrect
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In Canada,
I still think you're misunderstanding me. What I'm calling Chlorine Demand is the RATE OF CHANGE that FC decreases.
On a fresh fill, after shocking, your tub will have a Baseline Chlorine Demand. This Demand is there because of "stuff" getting into the tub, outgassing etc. With no use this CD will stay CONSTANT, as long as you have Chlorine in the water. It will NOT increase. The Demand will stay the same, unless something CHANGES. i.e. People soak in the tub, a dog pees in it, the FC drops to zero allowing stuff to take over etc.
However, after use in order to get the Demand back to the Baseline, it takes more Chlorine than usual. If you add enough Chlorine, your CD will drop back to Baseline. If you don't add enough Chlorine, your CD will stay higher the next time you soak. That's my point. If you don't use enough Chlorine, your CD will get higher, eventually reaching 100%, or you change the water.
My CD is always 25% (has been for 5 months) when I'm not using the tub. I've tested this many times. I can shock my tub to 12 ppm FC. The next day FC will be 9ppm, next day 6.5ppm, next 5ppm, then 4ppm, 3, 2, 1.5, 1 etc. The FC drops at a CONTANT RATE with no use. Again, this is what I'm calling Chlorine Demand. Yesterday, my FC was 4 ppm, today it's 3. I just tested it.
Have YOU ever tested the Rate of Change of FC (i.e. Chorine Demand)???
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On a fresh fill, after shocking, your tub will have a Baseline Chlorine Demand. This Demand is there because of "stuff" getting into the tub, outgassing etc. With no use this CD will stay CONSTANT, as long as you have Chlorine in the water. It will NOT increase. The Demand will stay the same, unless something CHANGES. i.e. People soak in the tub, a dog pees in it, the FC drops to zero allowing stuff to take over etc.
This is where I disagree.
On a fresh fill with tap water the demand will slowly increase even with no use. Even if you shock and chlorinate there is no way you can kill every single "bug" in the water. As long as there are 2 they will multiply exponentially, meaning 2 becomes 4, 4 becomes 8, etc etc.. So even with no use your demand will increase. You are making the assumption that the water is pristine (distilled) after a fill and shock and that is just not the case
My CD is always 25% (has been for 5 months) when I'm not using the tub. I've tested this many times. I can shock my tub to 12 ppm FC. The next day FC will be 9ppm, next day 6.5ppm, next 5ppm, then 4ppm, 3, 2, 1.5, 1 etc. The FC drops at a CONTANT RATE with no use. Again, this is what I'm calling Chlorine Demand. Yesterday, my FC was 4 ppm, today it's 3. I just tested it.
You have just proved my point. Your free chlorine level drops even with no use, therefor your chlorine demand has increased. Dropping from 12ppm on day one to 9ppm on day 2 is equal to a demand of 3ppm. Let this process go until the chlorine reaches zero then it will start to climb back up because you have leftover critters in your water. Your chlorine demand is constantly changing, one day its 3 next it is 2.5 then a week later it is 12 again. I think you may be misusing the term demand
Have YOU ever tested the Rate of Change of FC (i.e. Chlorine Demand)???
I do every day but never in my tub because I know it changes. I dose at a rate that will give me 3-5ppm 20 minutes after adding chlorine. I know that demand will have that at about 1ppm 24 hours later therefor I have a chlorine demand of about 2-4ppm every 24 hours
Chlorine demand is equal to the total chlorine minus the free chlorine
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In Canada,
I believe we are just arguing over semantics. I think you are misinterpreting the word Chlorine Demand. Chlorine Demand, is the RATE at which Chlorine is being used. I'm just specifing it as a percentage. It will never be 0%, unless (like you say) it's distilled water in a vacum. If it's 100%, it means there is so much "stuff" in the tub, as soon as you add Chlorine, it gets used up.
Here is the formula again: Chlorine Demand = 1 - (Day2 FC Reading) / (Day1 FC Reading)
I really think if you reread my original post, you will understand what I mean. I hope so, because my fingers are getting tired from all this typing. :)
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Based on my informal education (I'm self taught in this stuff ... OK I did have chemistry 20+ years ago). At spa water temp, bacteria doubles every 20 minutes, there are a lot of 20 minutes in a 24 hour period - 72 to be exact. Assume that chlorine residue stays up above 1 PPM in a 24 hour period, I would think that bacteria can stay in check.
But the air that we breathe has bacteria in it as does the water we drink and the surfaces we sit on, lay on or touch. If one bacteria got into a sterilized water (which BTW we aren't sterilizing the water) that 1 bacteria would become thousands within a day.
To say that a body of water that isn't completely sealed off to the outside world is staying in a non changing state is wrong ... all you need is a drip of water from the cover that has bacteria growing on it to get into the spa water and bingo you have a soup growing. We aren't talking bottled water (Poland Springs is my favorite) we're talking spa water.
And I have had a tub with 0, yes that's zero, chlorine demand when my tub had chlorine lock - not a d**n thing would make my chlorine go down. :o
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In Canada,
I believe we are just arguing over semantics. I think you are misinterpreting the word Chlorine Demand. Chlorine Demand, is the RATE at which Chlorine is being used. I'm just specifing it as a percentage. It will never be 0%, unless (like you say) it's distilled water in a vacum. If it's 100%, it means there is so much "stuff" in the tub, as soon as you add Chlorine, it gets used up.
Here is the formula again: Chlorine Demand = 1 - (Day2 FC Reading) / (Day1 FC Reading)
I really think if you reread my original post, you will understand what I mean. I hope so, because my fingers are getting tired from all this typing. :)
We may be confusing the way you are using the term "demand" In every textbook I have seen chlorine demand is defined as
Total Chlorine minus-Free Chlorine=Chlorine Demand
Like Vinny just said in his post. Demand will increase over a given time due to the exponential rate at which the "bugs" multiply. I believe your original post claimed that demand would drop over a period and that was the point I was debating
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Based on my informal education (I'm self taught in this To say that a body of water that isn't completely sealed off to the outside world is staying in a non changing state is wrong ... all you need is a drip of water from the cover that has bacteria growing on it to get into the spa water and bingo you have a soup growing. We aren't talking bottled water (Poland Springs is my favorite) we're talking spa water.
Vinny, I never said the tub is in a "non-changing state". I said the RATE of Chlorine usage (i.e. Demand) will stay constant without use. What will change is the FC. It will decrease CONSTANTLY.
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Vinny, I never said the tub is in a "non-changing state". I said the RATE of Chlorine usage (i.e. Demand) will stay constant without use. What will change is the FC. It will decrease CONSTANTLY.
No you didn't say that but it's being implyed as if nothing is happening in the water.
The fact that 2 bacteria grows to 4, 8 16, 32, 64, 128, ... is a constant changing state in itself.
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I said the RATE of Chlorine usage (i.e. Demand) will stay constant without use.
Here is where you lose your validity.
If the "bugs " are multiplying at an exponential rate how can your demand be constant? Your argument would work if the tub was using distilled water other then that the chlorine demand will always increase over a period of time, with or without use
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We may be confusing the way you are using the term "demand" In every textbook I have seen chlorine demand is defined as
Total Chlorine minus-Free Chlorine=Chlorine Demand
Like Vinny just said in his post. Demand will increase over a given time due to the exponential rate at which the "bugs" multiply. I believe your original post claimed that demand would drop over a period and that was the point I was debating
Actually, Combined Chlorine (CC) = Total Chlorine - Free Chlorine. CC is created when ammonia and FC combine, hence Combined Chlorine. I believe there is a link between CC and CD. That is, when CC rises, so does CD. However, CD can rise without CC rising.
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If the "bugs " are multiplying at an exponential rate how can your demand be constant? Your argument would work if the tub was using distilled water other then that the chlorine demand will always increase over a period of time, with or without use
Chlorine Demand cannot continue to increase with FC in the water. If it did, you couldn't measure it, because it would continue to rise to infinity. If there were no Chlorine in the tub, then that would happen.
However, you are forgeting there is Free Chlorine in the tub keeping it in check. As the FC keeps it in check, it gets used up. i.e. Converted to Chloride (Salt), or Combined Chlorine (CC) depending on the waste.
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I believe there is a link between CC and CD. That is, when CC rises, so does CD. However, CD can rise without CC rising.
You do realize I do this for a living don't you. Your theories are not getting any better
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You do realize I do this for a living don't you. Your theories are not getting any better
Didn't they use chloramines in drinking water for a while as a low level sanitizer?
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Chlorine Demand cannot continue to increase with FC in the water. If it did, you couldn't measure it, because it would continue to rise to infinity. If there were no Chlorine in the tub, then that would happen.
However, you are forgeting there is Free Chlorine in the tub keeping it in check. As the FC keeps it in check, it gets used up. i.e. Converted to Chloride (Salt), or Combined Chlorine (CC) depending on the waste.
Do you believe that 1 ppm in 105 degree water will keep the water free of contaminants? If you do your better go back to school.
If you want I can list out all of the contaminants that are somewhat chlorine resistant and those that are completely chlorine resistant in water with the type of residual a hot tub has, it may take a while though its a long list. Giardia and cryptosporidium come to mind right off the to[ of my head
I believe you should do a little more homework before you advise folks on water care and what constitutes safe water, reading a few pages on the Internet is not the same as having licenses in water treatment
However, you are forgeting there is Free Chlorine in the tub keeping it in check. As the FC keeps it in check, it gets used up. i.e. Converted to Chloride (Salt), or Combined Chlorine (CC) depending on the waste
This one is my personal favourite where are you getting this information
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First off I want to apologize for the tone of my last post. Believe me I think its great that you are trying to help people with water care. Many folks are very intimidated by it.
OK lets try to come up with some terms we can both understand.
Total chlorine=free chlorine + combined chlorine this is also equal to the dosage rate. So if you dose something at 5 ppm and then 12 hour later you have 4ppm and then 24 hours later you have 3ppm. You have a chlorine demand of 2ppm over that 24 hour period.You cannot assume that the water is completely disinfected at that time. In fact its FC level will continue to drop until it reaches zero because there is still a demand.
If the water continues to use FC then we can assume that there is still a demand in that water. Also as the FC drops the ability for the contaminants to multiply increases, so demand also will increase. There are some bacteria that are immune to chlorine in the 1 to 2 ppm levels, That's why some municipal plants are employing ozone or UV as a way to combat these little buggers
So as the FC level drops the demand increases
Once again I apologize for my previous tone
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Didn't they use chloramines in drinking water for a while as a low level sanitizer?
Still do Vinny.
In fact one of the plants I work at uses chloramines because of a very large rural distribution system. It is a very effective sanitizer and will maintain itself for a much longer period in the distribution system. Confuses new operators though because they do not check for FC in the system they are only concerned with Total. If they come across a FC residual it means there is not enough ammonia in the water
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First off I want to apologize for the tone of my last post. Believe me I think its great that you are trying to help people with water care. Many folks are very intimidated by it.
OK lets try to come up with some terms we can both understand.
Total chlorine=free chlorine + combined chlorine this is also equal to the dosage rate. So if you dose something at 5 ppm and then 12 hour later you have 4ppm and then 24 hours later you have 3ppm. You have a chlorine demand of 2ppm over that 24 hour period.You cannot assume that the water is completely disinfected at that time. In fact its FC level will continue to drop until it reaches zero because there is still a demand.
If the water continues to use FC then we can assume that there is still a demand in that water. Also as the FC drops the ability for the contaminants to multiply increases, so demand also will increase. There are some bacteria that are immune to chlorine in the 1 to 2 ppm levels, That's why some municipal plants are employing ozone or UV as a way to combat these little buggers
So as the FC level drops the demand increases
Once again I apologize for my previous tone
I'm not saying 1ppm FC is safe and will even last long. It will only last as long as the Demand level. But as I said earlier, the demand will never be 0%. My minimum is 25%.
I agree with you there is always Chlorine Demand in the water, and it does vary with use, and other factors. All I'm saying is you can measure that demand using my formula. Again, it boils down to semanitics. I'm calling Chlorine Demand the Rate of FC usage specified as a percentage.
Here is the formula: Chlorine Demand = 1 - (Day2 FC Reading) / (Day1 FC Reading)
If you don't agree that this is CD, what would you call this percentage?
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Nitro
I can't make sense of your formula.
I'll just use target numbers, keep in mind they mean nothing. Assumption is a dosage of 5ppm, then 4 ppm 20 minutes later and then 3ppm the next day.
So
1 minus (day 2 FC reading) or 3ppm divided by (day 1 FC reading) or 4 ppm
3divided by 4= .75 minus 1=-.25
or
3(day 2 FC) minus 1=2 divided by 4 (day 1 FC)=.5
I'm confused
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Nitro
I can't make sense of your formula.
I'll just use target numbers, keep in mind they mean nothing. Assumption is a dosage of 5ppm, then 4 ppm 20 minutes later and then 3ppm the next day.
So
1 minus (day 2 FC reading) or 3ppm divided by (day 1 FC reading) or 4 ppm
3divided by 4= .75 minus 1=-.25
or
3(day 2 FC) minus 1=2 divided by 4 (day 1 FC)=.5
I'm confused
According to the rules of arithmetic it would be:
1 - (Day2/Day1)
1 - (3/4)
1 - 0.75
.25
25%
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I think you may be on to something, 25% may be your demand for the first few hours, or maybe even a day. It does make sense that a dose of 5 ppm would be gone in 4 days. The problem is you are assuming that Demand and FC will move together (linear). When actually the demand will increase as FC decreases. So given a diminishing FC residual and a increasing FC demand, I don't think your equation adds up quite right.
If you were able to totally disinfect your tub at some point in the day and have zero demand then I think you would be right. However its just not possible
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I think you may be on to something, 25% may be your demand for the first few hours, or maybe even a day. It does make sense that a dose of 5 ppm would be gone in 4 days.
:)
The problem is you are assuming that Demand and FC will move together (linear). When actually the demand will increase as FC decreases. So given a diminishing FC residual and a increasing FC demand, I don't think your equation adds up quite right.
Actually the Demand stays the same (relatively) from day to day without use. If I shock my tub to 12, the next day it will be 9. A few days later when my FC is 4, the next day it will be 3. That's my point, CD stays the same, as FC decreases. This is according to my measurements. Even if CD does increase (slightly) as FC drops, it's too small to measure, and doesn't really effect the whole concept.
If you were able to totally disinfect your tub at some point in the day and have zero demand then I think you would be right. However its just not possible
Again, it's not necessary (or even possible) to totally disinfecting the tub, and drop CD to zero. As I said, CD can never be zero. My minimum is 25% (0.25)
BTW, my equation is correct and does come out to 0.25 (1 - 0.75 = 0.25)
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I want to make some comments, but want to tread lightly since things did seem to get personal in this thread though now seem more on-track.
Sanitation, Pathogen Kill Times and Uncontrolled Bacteria Growth
As for saying a tub is sanitary at 1 ppm FC or any level, that's not what anyone should be saying. It's a spectrum of risk since there are a variety of pathogens and as noted, the protozoan oocysts such as Giardia and especially Cryptosporidium are not killed quickly by chlorine. By the way, these protozoan oocysts are like viruses in that they do not multiply without being in a host so they aren't a good example of uncontrolled growth. Bacteria and active protozoa (e.g. amoeba) are better examples of that. These hard-to-kill protozoan oocysts aren't ubiquitous either and generally only enter pools and spas in dangerous amounts from infected persons via a fecal accident, specifically a "loose" accident -- diarrhea. As for most bacteria and viruses, the vast majority of the common ones are relatively easy-to-kill, at least in free form before they form biofilms (for bacteria). I'll talk about the bacteria that causes hot tub itch separately later on.
The CT (chlorine concentration in ppm times time in minutes) value for most heterotrophic bacteria is only 0.04 which is very low. This is why the EPA DIS/TSS-12 (http://www.epa.gov/oppad001/dis_tss_docs/dis-12.htm) laboratory requirement for swimming pool disinfectants (also used for spas) has a 6-log (99.9999%) kill time requirement of 30 seconds or less for two bacterial species (Escherichia coli and Streptococcus faecalis) that is met with chlorine (with no CYA) at about 0.75 ppm FC (though 1 ppm FC is the reference standard used). The 2-log CT for E. coli at pH 7.5 and 25C is around 0.04 while the 2-log CT for S. faecalis at pH 7.5 and 25C is around 0.12. These roughly translate to 6-log CT values of 0.12 and 0.36, respectively. There are strains of even E.coli bacteria that are more resistant to chlorine as noted here (http://www.cdc.gov/ncidod/osr/site/eip/pdf/zhao-2001-chlorine%20inactiv%20of%20e-coli%20O157h7.pdf) though are still killed quickly. In that report, most strains had 7-log reductions in 30 seconds at 0.25 ppm which implies a 2-log CT value of < 0.036 while the resistant strain had 4-log reduction in 1 minute at 0.25 ppm which implies a 2-log CT value of 0.13.
As for bacteria generation (doubling in population) time, it's around 15-60 minutes in normal temperatures, but at hot tub temps it could be faster than that (depending on species). Fortunately, kill rates also increase with temperature and do so even faster than generation rates. However, so long as the chlorine kills the bacteria faster than they reproduce, then instead of an exponential growth of population, you have an exponential decay (kill). So after the initial chlorine usage to kill bacteria that are present, the amount of chlorine used to kill bacteria is very very low if the chlorine level is maintained. This is because there is no spontaneous generation so the only bacteria that needs to get killed is newly introduced bacteria into the tub such as from bather load though in practice there is much more chlorine used up oxidizing ammonia and urea from sweat and urine than from skin bacteria. Water that has been treated and does not have new bather load does not keep using up chlorine through bacteria growth unless the chlorine level drops below that required to prevent such growth -- that is, unless the level drops so that chlorine kills more slowly than the bacteria can grow.
I'm sure you've seen the same effect in water treatment. Once you've killed the pathogens and oxidized the organics in the initial water you don't find that the water continues to use up chlorine at the same rate. If you keep chlorine in the water and kept it in a clean container with clean water, for example, it would last for a while (I'll talk about hot tub temps later) and is how bottled water is made. The problem is that in the distribution system, there are many more opportunities for the chlorine to react. Monochloramine reacts more slowly, so persists longer, which is one reason it's now being used instead of chlorine in the distribution system (the other reason is that it forms fewer disinfection by-products).
Killing bacteria faster than they reproduce is vastly different than what is required for water treatment. In water treatment, exposure times are limited and a fairly massive kill is required in that time, especially because its drinking water. In pool and spa water, you want reasonably fast kill times to prevent person-to-person transmission, but to prevent uncontrolled bacteria growth you technically only need a 50% kill (0.3-log) time faster than the bacteria generation time since chlorine should always be present at certain minimum levels (more on that later about how most people don't do this). It takes a very low level of chlorine to achieve this -- 1 ppm FC would kill faster than 15 minute bacteria doubling for 2-log CT of up to 100. I'm not saying one should use this level of chlorine but rather that it takes an extraordinarily low level to prevent uncontrolled growth of most bacteria. Note that this discussion has only talked about chlorine with no Cyanuric Acid (CYA) in the water.
Next topic: Cyanuric Acid (CYA) and its effect on chlorine disinfection rates
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Cyanuric Acid (CYA) and its effect on chlorine disinfection rates
Cyanuric Acid (CYA) significantly reduces the active chlorine (hypochlorous acid) concentration by orders of magnitude. CYA is added to water every time stabilized chlorine is used. The following are chemical rules that are independent of concentration of product or volume of water:
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.
The equilibrium between the active chlorine that kills pathogens, hypochlorous acid, and CYA has been known definitively since at least 1974 as described in the paper in this link (http://richardfalk.home.comcast.net/~richardfalk/pool/OBrien.htm). CYA's effect on chlorine effectiveness has been well documented over the years including papers here (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1256554&blobtype=pdf), here (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=377704&blobtype=pdf), here (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1227093&blobtype=pdf), here (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=546955&blobtype=pdf) and here (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=239534&blobtype=pdf).
At pool temperatures, the rough rule of thumb near a pH of 7.5 is that the equivalent FC with no CYA is roughly the FC/CYA ratio. I show a derivation of this here (http://www.troublefreepool.com/post50548.html#p50548). This means that a pool with 3 ppm FC and 30 ppm CYA is technically equivalent in its disinfection rates to a pool (80F) with 0.1 ppm FC and no CYA. At spa temperatures (above 100F) the equivalent is around 0.4 ppm FC with no CYA. A spreadsheet I wrote that calculates all of this is here (http://richardfalk.home.comcast.net/~richardfalk/pool/PoolEquations.xls).
So, put these two pieces of information together. If one uses a 350 gallon hot tub every day and adds around 4 ppm FC each day (which is about right for a single person soaking for a half-hour or two people soaking for 15 minutes at 104F temp), then every month the CYA increases by over 100 ppm. The disinfection rate drops over time. The oxidation rate also drops so one can often see Combined Chlorine (CC) persist longer and even get behind the bather load so see the required chlorine usage go up. After 3 months, an FC of 4 ppm in 300 ppm CYA is equivalent to only 0.06 ppm with no CYA (at spa temps). These are reasons why an alternative approach of using Dichlor (for about a week to get to 20-30 ppm) and then switching to bleach has been proposed. You do want at least some CYA in the water or else the chlorine level is too strong and will break down hot tub covers and other components too quickly (as well as oxidizing skin, hair and swimsuits and also outgassing faster).
Hot Tub Itch
The bacteria that causes hot tub itch/rash is Pseudomonas aeruginosa and it thrives at hot tub temperatures. It's ideal growth temperature is, not surprisingly, 98.6ºF since we are a common host for this bacteria. It is fairly common and often on the skin and our body normally keep its numbers in check. However, if we have any sort of compromised skin (cut, infection, wound, abrasion) and get exposed to overwhelmingly larger numbers of this bacteria, then it becomes opportunistic and causes a rash/itch/bumps as our body responds to fighting this bacteria. See this link (http://www.textbookofbacteriology.net/pseudomonas.html) for good info about this bacteria.
Unfortunately, I could not find any consistent sources on the CT value for this bacteria. One source described the bacteria as relatively easy-to-kill while other sources described it as hard-to-kill. It seems that until it forms biofilms, it's easier to kill but that it very readily forms such biofilms. This link (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=377704&blobtype=pdf) implies a 3-log (99.9% kill) CT of < 0.1 (roughly a 2-log of < 0.07) which is in the range of easy-to-kill bacteria, but that same paper also shows the effects of CYA as I described earlier in this post and also indicates how ammonia and high bather load lowers chlorine effectiveness. This link (http://www.ehponline.org/members/1986/069/69002.PDF) goes into more detail about differing populations, CT values, resistance, etc. This link (http://epa.gov/etv/pubs/02_vs_exceltec.pdf) implies a 4-log CT of 50 (implied 2-log CT of 25) which is obviously pretty high, possibly because it is being used in water treatment and the bacteria may be clumped or in biofilms.
I originally came up with a 4 ppm FC at 20 ppm CYA recommendation based on this data, but at pool temps this is equivalent to around 0.2 ppm FC with no CYA. This would kill faster than bacteria reproduce up to a 2-log CT of around 20. At spa temps, the 4 ppm FC with 20 ppm CYA is equivalent to around 0.75 ppm FC with no CYA so would handle up to a 2-log CT of around 75. This isn't perfect, but is a lot better than what happens with Dichlor-only use.
Next topic: Chlorine Outgassing, Bather Load, "Chlorine Demand"
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Chlorine Outgassing and How Most People Actually Use Chlorine
Unfortunately, at spa temps, chlorine outgasses and smells. During a soak, the ammonia in sweat and urine combines with the chlorine to form monochloramine that smells even more. So most people do not like to soak with sustained higher chlorine levels even though that is what would be required to prevent person-to-person transmission of pathogens. Since people generally soak for less than an hour, there won't be much doubling of bacteria population in that time so the issue isn't uncontrolled bacteria growth. By adding chlorine right after a soak, any bacteria introduced in the tub can be killed, assuming one uses sufficient chlorine to oxidize bather waste and still have a residual of chlorine. There is the unsettling issue of whether hot tub itch bacteria can be killed before forming biofilms or whether adding chlorine after a soak is sufficient to kill bacteria in such biofilms.
On another forum I did a survey of all the hot tub itch/rash/lung incidents (here (http://www.poolspaforum.com/forum/index.php?showtopic=11064&st=0&p=51033&#entry51033)) and most were due to low or no chlorine (or bromine) for an extended time (many hours to a day or more). Some other incidents were after 1-2 months of Dichlor-only use even with chlorine levels (presumably) maintained. There was one incident where N2 (silver & copper ions) was used but the chlorine level was low. We have since had (just reported this week) one report of a problem with Dichlor-then-bleach that we are looking at to figure out more details (not added to the chart yet). I would say that having N2 is a good insurance policy, especially if one isn't going to consistently maintain a proper chlorine level.
Bather Load and "Chlorine Demand"
Most reactions with chlorine are with hypochlorous acid (for sanitation and oxidation) or with hypochlorite ion (for oxidation -- sometimes) and these are first-order reactions with respect to chlorine which means that the rate of the reaction is proportional to the active chlorine concentration. This means that the reaction rate is an exponential decay as chlorine gets used up (assuming other items are constant; if the other items also decline, then it's second-order and the decay is even faster). This is also true for chlorine outgassing rates. This also means that it is the percentage drop in chlorine that is fairly constant over time (the exception being the fast reactions of oxidation of some bather waste, especially ammonia). So let's look at the ways that chlorine can get used up. I list these in typical order of highest to lowest amount of usage.
- Breakdown from UV in sunlight (generally only with outdoor pools since most spas are covered)
- Oxidation of bather waste -- mostly urea and ammonia from sweat and urine
- Outgassing from spas (i.e. mostly only at hot temperatures and accelerated by aeration)
- Oxidation of materials in the spa (skin, bathing suit, spa materials)
- Oxidation of Cyanuric Acid (CYA)
- Killing (oxidation) of algae (mostly for pools, not for spas)
- Disinfection (oxidation) of pathogens, mostly bacteria
It is incorrect to say that chlorine breaks down at spa temperatures. Yes, chlorine does break down faster at higher temperatures and at higher concentrations, but as you can see from the table at the bottom of this link (http://www.odysseymanufacturing.com/about_product.htm), the half-life of chlorine gets cut by around 1/4 for every doubling in concentration and gets cut by around 1/2 for every 10F temperature increase. The concentration of chlorine in pools and spas is so much lower than that in the concentrated chlorinating liquid (or bleach) shown in the table that effectively its breakdown simply does not happen by temperature in pools and spas. In spas, it is outgassing from the higher temperature and from aeration that results in chlorine loss when the other sources of loss mentioned above are not present. We don't have enough data yet from users of the bleach-then-dichlor method to know the rate of oxidation of CYA by chlorine, but it's not particularly fast in any event.
The term "chlorine demand" (CD) is getting interpreted in different ways. What Nitro is really looking at in his meaning of CD is the 24-hour FC drop in percentage terms when the tub is not in use, and presumably after a day of non-use (i.e. don't start measuring the loss right after a soak since obviously bather load still needs to get oxidized). He is essentially trying to measure a baseline for the non-bather-load sources of chlorine consumption. This represents a baseline for relatively "clean" water. As I've noted earlier, if one uses Dichlor-only and has CYA build up or if one is simply not using enough chlorine to handle the bather load, then one can get behind and build up ammonia (actually monochloramine since this gets formed quickly, but takes longer to oxidize) and urea and other compounds that don't get oxidized quickly enough before the next soak. When this happens, the apparent chlorine demand (usage or consumption) seems to increase. Even if you don't soak for a day, the leftover organics to oxidize result in a higher measured CD. Also as noted earlier, one often measures a higher Combined Chlorine (CC) in this situation.
Next Topic: Conclusion
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Conclusion
Nitro is using CD as an indicator of whether one is using enough chlorine to keep up with bather waste and other sources of chlorine consumption. A rough rule-of-thumb I came up with based on reports of chlorine usage and some articles on the amount of nitrogenous compounds in sweat and urine (see Table 4.1 on p. 62, PDF page 85, here (http://www.who.int/entity/water_sanitation_health/bathing/srwe2full.pdf)) is that every person-hour of soaking at 104F takes around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (MPS) to oxidize the bather waste and have some residual of chlorine leftover in 24 hours.
Perhaps one should call chlorine demand something different such as chlorine usage or chlorine consumption though really it's a baseline with no bather load.
So measuring the amount of non-bather-load chlorine usage over 24 hours can be useful in determining if you are using enough chlorine, need to shock, or should replace the water. It does not have to be done very often.
One final point is the KISS (keep it simple, stupid) principle. Most pool and spa owners don't want to bother with knowing the "why"; they just want to know "what" to do in the simplest most straightforward way. So while all of these discussions are useful for those that want to know the "why" in order to develop a better "what", we shouldn't lose sight of the fact that simplicity is what is desired. Yes, every situation is different, but one needs to distill the essence into some simple rules and then expand upon that with exceptions or further info AFTER someone has mastered the basics.
Richard
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My CD seems to be right about 20%. I have tested it for 24 hour periods, 48 hours periods and even a few 4 and 5 day periods. Each time it comes in very close to 20%
I would call that a pretty constant CD rate?