Chlorine Demand (What is it, and why you care)

[Nitro]

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!

[Vinny]

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.

[Nitro]

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?

[TubsAndCues]

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!

[In Canada eh]

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

[Nitro]

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.

[Nitro]

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.

[In Canada eh]

Nitro

You go with that I'll go with what I have learned in the last 7 years


Good Luck

[Nitro]

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.

[In Canada eh]

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

[Spatech_tuo]

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.

[Nitro]

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.

[In Canada eh]

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

[Nitro]

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)

[In Canada eh]

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