Chlorine Per Bather Hour?

[Parawood]

I have read on this forum the recommendation to use 5-7 oz of bleach per bather/hour regardless of tub size.

I am finding this to be way too much. I have a 140 gallon hot tub. It also has an ozonator. Per the Pool Calculator, if I add 6 oz of bleach, it will raise the FC by 21 ppm. I is not unusual for my wife an I to spend 1 1/2 hours under the stars. This would equate to 15 - 21 oz of bleach added after a long soak.

Questions:

1. Am I understanding this correctly?

2. How is the ozonator affecting this?

3. Will a high chlorine level contribute to an elevated CC (combined chlorine)?

TIA

Parawood

[chem geek]

Parawood,

No one said this was an absolute number that applies to every situation -- it is a rough rule-of-thumb. Also, the number is 5 fluid ounces of 6% bleach per person-hour, not 5-7. Also, this is without supplemental oxidation and an ozonator is supplemental oxidation so it would be expected to not need as much bleach. Also, the assumption for the amount of sweat is based on a hot tub of 100-104F (closer to 104F). I doubt you are spending 1-1/2 hours in a 104F tub, at least not the entire time at that temp. What is the actual hot tub temperature?

So in your case, you could start by adding 10 ounces of 6% bleach and see what sort of chlorine residual you get at the start of your next soak (the next day?). Then, adjust up or down the amount of bleach you add after your soak so that you get to a measurable residual (1-2 ppm FC) before your next soak. If your ozonator is good, then the amount of bleach you will need may be around half the expected amount or perhaps somewhat less, but this assumes soaking every day. If you don't soak every day and especially if you go a while without soaking, then the ozonator will use up chlorine faster than not having an ozonator.

Richard

[hrustar]

For what it's worth, I have a 425 gallon hot tub with ozonator and I add about 3 oz a day if I don't use it and 4 or 5 ounces if used. I find that my chlorine dips to about 1 ppm, so it's low when i go in which I like.

Not sure why you'd want a FC of 21, that's way too high. I like mine to max out around 4 or 5 unless I'm going away for a vacation, than I bring it up to 12 or a bit higher.

[chem geek]

Not sure why you'd want a FC of 21, that's way too high. I like mine to max out around 4 or 5 unless I'm going away for a vacation, than I bring it up to 12 or a bit higher.

He was calculating for the bather load (using high-end numbers and not accounting for an ozonator, etc.), independent of spa size. His spa is small so adding enough chlorine to handle the bather load has the FC go much higher at least until that sweat and urine gets oxidized so probably only for a few hours after adding the chlorine. It's the right calculation if there wasn't an ozonator and the water was hot (close to 104F) and 5 (instead of 7) fluid ounces per bather-hour were used. There's another user on this forum from a while back that also had a rather small tub and didn't have an ozonator and definitely needed the rule-of-thumb amounts.

By the way, for such a small spa and using bleach, I strongly recommend using 50 ppm Borates to keep the pH more stable; otherwise it will swing way up when you add the bleach and then come back down as the chlorine gets used up. The borates (say, from Gentle Spa) will reduce the pH swing substantially.

[Parawood]

For what it's worth, I have a 425 gallon hot tub with ozonator and I add about 3 oz a day if I don't use it and 4 or 5 ounces if used. I find that my chlorine dips to about 1 ppm, so it's low when i go in which I like.

Not sure why you'd want a FC of 21, that's way too high. I like mine to max out around 4 or 5 unless I'm going away for a vacation, than I bring it up to 12 or a bit higher.

Hrustar and Richard,

Thanks for the replies. The temp is set at 101F. The example was for adding 6 oz of 6% bleach which would raise the FC by 21ppm according to the Pool Calculator. I guess the ozonator is really doing it's job as I only add 1 1/2 oz if not used and 3oz if used for 1 hr. I generally end up with a FC of 1-2 ppm the next day. I would say we give it fairly heavy use.

I have been bothered by the recommendations for using 5 oz per bather/hour in such a small tub. I guess if I can keep a residual of chlorine til the next day, I am fine.

I found when using heavier doses, I would get an increased CC reading. Does the higher level of chlorine more readily convert to an increased CC if it is not utilized and burnt off down to a FC of 1 - 2?

Thanks,

Karl

[chem geek]

I have been bothered by the recommendations for using 5 oz per bather/hour in such a small tub. I guess if I can keep a residual of chlorine til the next day, I am fine.

I found when using heavier doses, I would get an increased CC reading. Does the higher level of chlorine more readily convert to an increased CC if it is not utilized and burnt off down to a FC of 1 - 2?

Karl,

Yes, the ultimate goal is to use only enough chlorine so you never get to zero and end up with a decent residual before your next soak (or before the next time you add chlorine, if you don't soak often). And yes, it does seem that your ozonator is doing an excellent job of taking care of your bather waste. 3 fluid ounces for 1 hour (for 2 people?) would be 30% of the normally required usage (if you meant per person-hour, then that would be 60% of the normal usage if no ozonator). The 1-1/2 fluid ounces (per day?) when not used is possibly a little higher than it would be for tubs without an ozonator, but that's the small tradeoff where an ozonator works really well when you use the spa regularly. The 100F temp may be helping a little bit and you might not be sweating as much as "average".

As for CC, this is a temporary reading and it will depend a lot on when you are measuring it. When you add chlorine after a soak, the chlorine combines almost immediately with the ammonia in your sweat (and urine) to form monochloramine that measures as CC. After that, the chlorine more slowly combines with urea to form monochlorourea (and possibly other chlorourea forms) that also measures as CC. Between these two, if there were no ozonator it would be a very high CC level (perhaps 10 ppm in a small hot tub such as yours, mostly as chlorourea). With a higher chlorine level, the chlorine probably combines with urea more quickly for a higher CC reading sooner. Over the next hours, these compounds get more fully oxidized and the CC drops. If you use a lower amount of chlorine, then less CC is formed initially, but there is likely more urea hanging around. That urea probably gets oxidized by your ozonator before it can combine with chlorine to form CC.

The above would explain a higher CC sometime after the soak, but were you measuring higher CC before your next soak when you were using higher chlorine levels?

Richard

[Parawood]

I have been bothered by the recommendations for using 5 oz per bather/hour in such a small tub. I guess if I can keep a residual of chlorine til the next day, I am fine.

I found when using heavier doses, I would get an increased CC reading. Does the higher level of chlorine more readily convert to an increased CC if it is not utilized and burnt off down to a FC of 1 - 2?

Karl,

Yes, the ultimate goal is to use only enough chlorine so you never get to zero and end up with a decent residual before your next soak (or before the next time you add chlorine, if you don't soak often). And yes, it does seem that your ozonator is doing an excellent job of taking care of your bather waste. 3 fluid ounces for 1 hour (for 2 people?) would be 30% of the normally required usage (if you meant per person-hour, then that would be 60% of the normal usage if no ozonator). The 1-1/2 fluid ounces (per day?) when not used is possibly a little higher than it would be for tubs without an ozonator, but that's the small tradeoff where an ozonator works really well when you use the spa regularly. The 100F temp may be helping a little bit and you might not be sweating as much as "average".

As for CC, this is a temporary reading and it will depend a lot on when you are measuring it. When you add chlorine after a soak, the chlorine combines almost immediately with the ammonia in your sweat (and urine) to form monochloramine that measures as CC. After that, the chlorine more slowly combines with urea to form monochlorourea (and possibly other chlorourea forms) that also measures as CC. Between these two, if there were no ozonator it would be a very high CC level (perhaps 10 ppm in a small hot tub such as yours, mostly as chlorourea). With a higher chlorine level, the chlorine probably combines with urea more quickly for a higher CC reading sooner. Over the next hours, these compounds get more fully oxidized and the CC drops. If you use a lower amount of chlorine, then less CC is formed initially, but there is likely more urea hanging around. That urea probably gets oxidized by your ozonator before it can combine with chlorine to form CC.

The above would explain a higher CC sometime after the soak, but were you measuring higher CC before your next soak when you were using higher chlorine levels?

Richard

Richard,

Generally measure before a soak in the afternoon. Usually been > 12 hours after adding chlorine.

Today, FC= 5.5, CC= .2 I had added 3 oz bleach last night after a combined 1 hr soak.

Regarding Hrustar's reply above, would there be any benefit to turning the ozonator off when going on a vacation?

Thanks again,

Karl

[chem geek]

Well, then I don't have a good answer for why the CC was higher measured after 12 hours when you used more chlorine. I know why certain kinds of CC would be higher, such as nitrogen trichloride, but the CC is mostly measuring monochloramine and chloroureas and perhaps some other chlorine with organics. Maybe some organics are slow to form CC so having lower chlorine levels gives the ozonator a chance to oxidize those organics but once chlorine attaches to them then the ozonator isn't able to get rid of it as easily -- that's just speculation on my part.

As for the ozonator and being on vacation, it's a catch-22. If you leave it on, then it will keep the water from clouding but will also use up the chlorine faster. If you turn off the ozonator, then the chlorine will last longer, which is good, but if for whatever reason the chlorine did get to zero then you have no real backup -- at least if that happens with the ozonator running then the ozonator is providing some backup, albeit not a fast kill (depending on turnover time and it doesn't do much for anything on spa surfaces). I'd probably turn off the ozonator and give a good dose of chlorine with enough such that the normal "no bather load" chlorine demand (usually as a %) has it last through the vacation with some ample safety margin -- you could see what your chlorine demand with "no bather load" is with the ozonator on vs. off to help make that decision.