Gut check on how much dichlor to add when tub won't be used for several days

[watson524]

Hi all,

We will not be using our hot tub for a period of just over 4 days and will not be able to add anything do it. I believe I've read that with no ozone and no bather waste to oxidize, the FC "should" drop about 25% / day. We have a Nature2 stick in the primary filter and 12 hours of filtration / day. Is it safe to say I should bring the FC level up to about 10 to last the 4 days? I wasn't sure if it was 25% of the initial each day (in which case I need about 12ppm on day 1 so there's about 2ppm left at the end) or if it's 25% of what's left so a 10ppm initial would leave about 3.2ppm at the end.

thanks!

 

[jblizzle]

It is the later, but what is your CYA level? If you are using Nature2, that is making the CYA climb. And if your use dichlor, that is adding even more CYA ... and the required FC level to keep the tub safe is a function of the CYA level.

 

[watson524]

We were at 55 at last check on CYA two weeks ago (I only run that test every 3 - 4 weeks). We normally use Leisure time replenish which is MPS, Dichlor and some other things. We figured we're just over 3 months in so we'll likely drain some out (or total water change if I decide to use some ahh-some) to lower the CYA soon anyway.

Since it's 25% of the remaining, I'll take it up to 10ppm which will add 9ppm CYA if I recall from chemgeek's prior guidance and go with that.

thanks!

 

[chem geek]

If the loss was 25% per day, then 10 ppm FC would be 3.2 ppm FC after 4 days and 1.3 ppm FC after 7 days. You can always do a 24-hour FC loss test to see what your loss rate actually is. You would do such a test away from any bather load so at least one day and preferably two with no bather load before doing the test.

 

[watson524]

Unfortunately, I'm out of time to do a 24 FC loss test with no bather load. I'll just get it up to 10ppm and have to hope for the best. Since we won't actually be in it on day 4 (tho I'll be able to look at it) it's more like a 5 day so it should be down to 2.4'ish if the 25% / day pans out.

Out of curiosity, why is it 25% of the remaining? How does it "know" what's left vs using a set amount each day? I'm thinking it's because it has less to "clean up" as the days go on until it gets too low where the bad stuff wins and causes issues?

 

[chem geek]

If you lower the water temperature, the chlorine will last longer and will likely decay at less than 25%.

As for why the loss is a percentage of what is remaining, this is how chemistry works. The rate of a chemical reaction as well as many physical processes (e.g. outgassing) is proportional to its concentration. This is true for first-order reactions which are the most common. So the rate at which chlorine outgases and reacts with the spa cover or the rate at which it reacts with slow-to-oxidize organics that may be remaining in the filter is proportional to its concentration. When there's twice as much chlorine, it reacts twice as quickly. As the concentration of chlorine drops, its rate of reaction drops proportionately so it keeps dropping by a percentage, not an absolute amount.

Now if the chlorine is reacting rather quickly and is only limited by what it has to react with, then the amount of loss would be absolute and dependent on how much there is to react with. This is how chlorine reacting with bather waste mostly works. It reacts in a number of hours and while it does slow down as it reacts, not only because its concentration drops but so does the bather waste with which it reacts, these reactions are fast enough that for practical purposes the amount of chlorine used over 8-12 hours is mostly dependent on the amount of bather waste (and that's dependent on the bather load -- person-hours).

The reason chlorine outgassing and reacting with the cover or reacting with slow-to-oxidize organics in the filter is different is that the absolute amount of cover material or organics in the filter isn't changing much so the reaction rate depends only on the chlorine concentration and this rate is rather slow. As the spa gets used more, the chlorine loss rate will slowly rise as there is a buildup of slow-to-oxidize material especially if the filters are not thoroughly cleaned. Changing to fresh water has one start over again where the loss is mostly due to outgassing so is largely temperature dependent. I should also note that the rate of chemical reactions themselves is also temperature dependent so the rate of chlorine loss roughly doubles every 13ºF though it depends on the specific reaction (the 13ºF is a rough rule-of-thumb based on what we've seen for the most common chlorine losses).

 

[watson524]

Thanks for that added info chem geek. So I have the tub set at 102. If I drop it to 80deg the usage of FC would fall by half twice? So 6% / day vs 25%? I tried to change the temp on the pool calculator from 102 to 80 but it didn't seem to change the amount of dichlor needed to go from say 1 to 10 at all.

 

[chem geek]

The Pool Calculator calculates what it takes to increase a chemical level by adding chemicals. It does not calculate loss rates.

As for the loss rate at 80ºF, it will probably be about half, but to play it safe I'd figure 15% per day. If you raised the FC to 10 ppm, then you'd still have 3.2 ppm FC after 7 days. That should be reasonable.

 

[marty]

Wait... Dichlor has cyanuric acid? I thought only trichlor 3'' tabs/pucks have that added?
Is Nature2 the same as poolRX/spaRX?

It is the later, but what is your CYA level? If you are using Nature2, that is making the CYA climb. And if your use dichlor, that is adding even more CYA ... and the required FC level to keep the tub safe is a function of the CYA level.

 

[jblizzle]

Dichlor has more CYA in it than Trichlor. The nature 2 uses Trichlor packs in addition to the minerals (metals).

 

[marty]

I was told to use dissolve granular dichlor for wood hot tub(said liquid chlorine and bromine is bad for wood because its a salt, but isn't all types of chlorine?), it has a cover, hate to ad cya for nothing, what do you recommend, Cal hypo, hygrogen peroxide? reson I didn't use trichlor 1'' tabs in floater was because I thought it had a lot of cya...
Thanks

Dichlor has more CYA in it than Trichlor. The nature 2 uses Trichlor packs in addition to the minerals (metals).

 

[jblizzle]

All forms of chlorine add salt to the water. I am not sure if that is truly bad for the wood or not. I would not use hydrogen peroxide.

 

[chem geek]

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 at least 7 ppm.

As for hydrogen peroxide, see this post I wrote in your other thread. Hydrogen peroxide is completely incompatible with chlorine and in fact is sometimes used as a chlorine neutralizer.