Bather Load Rule of Thumb / Cedar Hot Tub

[undherben]

Hello all,
I've been reading up on chlorine and bather load, and on this forum I see a recurring statement, with an reference by the TFP Expert chem geek:
"A rough rule-of-thumb is that the chlorine demand from bather load in spas is around 7 grams of chlorine per person-hour."

I've searched this forum, but can't find the original post where the 4 grams for pools and 7 grams for spas comes from.

My question is this: Where was this rule of thumb started, or how is it derived?

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This is my situation:

I have a new cedar hot-tub with no liner, and I'm advised to keep the Free Chlorine at 1-2 ppm, and never above 3 ppm until the tannins leach out. Then I can use the Cu-Zn-Ag ionizer for water purification to reduce the chlorine usage. But for now, I'm still seeing tannins in the water, and I'd like to use it now.

Given that it's only 440 gallons, I'm more limited than people with standard spas and hot-tubs in the amount of Chlorine I can add. If I keep a level above 3 ppm, I risk creating wood-pulp.

Translating the 5 fluid ounces of 6% Bleach per person-hour (or really the 3.5 tsp DiChlor 56%) to a 440 gallon volume, that comes out to 5.8 ppm per person hour.

My questions are:
Does that mean I should put in enough to add 2.9 ppm before bathing and another 2.9 after... and hope the period I have FC above 3 ppm is short?
and
Shouldn't I stay below 5 ppm anyway?

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I love the new Cedar Tub and I want to use it (safely) without damaging it.

Thanks in advance.

 

[JoyfulNoise]

Look up any post in the hot tub section by chem geek and you'll find it. I believe that the amounts cited come from actual European studies of commercial/public hot tubs and spas and it is the amount of chlorine one needs to add after soaking to fully oxidize the bather waste generated. Hot tubs and spas have MUCH HIGHER bather waste loads than pools and are also higher temperatures therefore they require more chlorine to oxidize bather waste.

As far as your setup goes, "low chlorine" is a myth propagated by the pool and spa industry nowadays to play on people's emotions about "chemicals" and "toxins" and all that nonsense. See this post - Alternative sanitizers and pools--The Truth!!

The "mineral" systems (Cu-Ag-Zn) are going to do very little to improve the quality of the water in your tub or keep you safe from disease transmission. You still need a proper sanitizer and all of the mineral system suppliers, by law, must state that their products require chlorine or an approved EPA sanitizer in order to operate a hot tub safely. The only "mineral system" that is approved by the EPA for hot tubs is the silver ion & MPS (potassium monopersulfate, aka, non-chlorine shock) system sold under the trade name Nature² by Zodiac (I believe there are other smaller vendors of this system as well). That system is only effective in hot tubs when the water temperature is maintained above 96F and will not provide any effective sanitation if the water temperature is lower. Chlorine and bromine are the only sanitizers that can maintain an appropriate level of residual disinfectant that is effective at all temperatures. While they do take a little bit of effort to setup and maintain, they are the safest route in terms of sanitary water.

Given that you are using an un-lined wooden tub, I'm not sure you'll be able to maintain an appropriate level of chlorine or bromine given how quickly it will react with the organic compounds in the wood. With all of the tannins in the water, your CC levels will be elevated anytime you add chlorine. You'll have to keep an eye on the water quality and just replace the water more frequently.

Good luck with the tub. Let us know how it works out....

 

[undherben]

Thanks for your help.
I have been searching for chem geek, but he shows up everywhere! I was hoping to find the original post where the rule of thumb was established.

I'm not worried about chlorine being too high for our health. I'm just trying to keep the chlorine in a narrow band above 1ppm for sanitation, but below 3ppm to prevent damage to the wood.

I apparently have a number of months still before I can use the ionization system. That will be ineffective until the tannins stop leeching out. So I need to get a handle on the Chlorine Demand of the Cedar Hot Tub itself versus the Bather Load. I do have MPS, but I had been using DiChlor and Bleach.

It's not clear how to perform quantitative water quality testing.

We have been replacing the water pretty frequently! It seems to help with the tannins creating "cedar tea".

It is an un-lined wooden tub. I hope I will be able to figure out the right balance. I can't follow the recipes I read here without understand what's behind them.

I'm getting new filters to see if that will help.

 

[JoyfulNoise]

I can't follow the recipes I read here without understand what's behind them.

I'm not sure what you mean by this...is there something specific you have a question about? Chlorine chemistry is not all that mysterious and most of the ideas we teach here were worked out in the 1970's when chlorinated cyanurates started to get used more widely. Which "recipe" are you having a hard time with?

As for the cedar tannins, those will definitely cause FC demand and keeping the tub at 1ppm could be very challenging. I think with an unlined cedar tub, you're looking at lots of water exchanges no matter what you do as the wood is just going to keep emitting tannins until the concentrations in the wood decrease.

Water testing on your own is easy to do, you just need to get a good test kit. If you're going to use chlorine in your hot tub then you should get a Taylor K-2006 kit as it will measure all of the main parameters that you need. Minerals are not really possible to measure as the quantitative tests for them are either expensive (copper) or not really available outside of an analytical chemistry lab (Zn and Ag). This is one reason why the idea of using "minerals" is really not that good - there's no easy way to test for them, there are no officially determined limits for their efficacy, and overuse leads to all sorts of problems. Chlorine and bromine can at least be tested for in a quantitative way and it's easy to know how to dose a body of water to get results that you need.

 

[undherben]

Sorry I was unclear. I meant I can't follow the cookbook instructions to determine chlorine demand. "Shock your tub to around 10ppm FC" because I'm limited to under 3 ppm.

I'm not finding specific recommendations for unlined wooden tubs, since they are admitted less common!
So I'm trying to understand what's behind what I'm reading. I'm not questioning what's stated here - I'm trying to figure out how to adapt them to my unique situation.

For example, perhaps I can
- test CD within 8 hours instead of 24, for example?
- add the 6 ppm ppm worth of Chlorine in two doses: before bathing (how long to wait?) and after bathing?
- maybe I just switch entirely to MPS? (But how do you test water quality?)

I have the Taylor K-2006 and have been using it... a lot! At least once a day. I ordered more R-0870 DPD powder from TFTestKits. For fun I got the little speed stir, which I'm going to try today.

I found I have a very high chlorine demand with a fresh fill, and tannins are not yet visible. New filters are on order. Once I establish the chlorine demand of the tub with everything fresh (water and filters), and then from a bather-hour, I think I will be more comfortable.

Thanks again!

I'm Reading
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How Long Should One Wait to enter Spa
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