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  1. hi again:


    another question:

    are borates advisable both for when TA should be relatively low (swg, bleach) and also when relatively high (dichlor or trichlor), or is there any situation where they should be avoided?


    thx !

    1. chem geek

      chem geek

      There are no downsides to using borates unless you have a dog drinking lots of water -- more of a problem for pools than spas (obviously).  However, borates are most useful when you have a rising pH problem so aren't typically used when Trichlor is used.  They are more useful when hypochlorite sources of chorine are used.

  2. hi chem geek:

    am new to the forum but have followed this and other forums for a few months.

    first of all, thank you for your amazing content and insights. your posts are a welcome beacon of light compared with the often non sensical approach pushed by industry. as a medical scientist and someone who understands inorganic chem, i appreciate your quantitative and evidence-based approach. and i appreciate insights by waterbear, jasonlion, nitro and other informative folks.

    i have a spa at home: 325 gallon hot spring + ace system, ozonator +, pump on 24/7, 102F

    and one at a vacation rental property: 280 gallon nordic hot tub, no salt, unsure if have ozonator (trying to verify with dealer), pump on for 2 hrs every 8 hrs, keep at 85F when house unoccupied

    trying to get the sweet spot with chlorine dosing and water chemistry with the different systems.

    hot spring advises a very low chlorine routine dosing (dichlor only: 2 tsp for shock, 0.7 tsp for routine weekly boosting or 'as needed' for 325 gallons, by my reading), presumably because of the extra chlorine provided by the ace cell

    looks like i need much more chlorine for the nordic which has no ace; am using BBB. one challenge there is trying to avoid pool service more than once a week as i cant be there often, and keep FC up with no more than weekly service. am tempted to use trichlor tabs in a dispenser with slow release and run the TA high (100-120). but when i am there i would like to use bleach which would require TA of 50. any way to mix and match trichlor and bleach in a spa depending on when one is on site or staying several days vs it is sitting idle for a week? recently tested after the spa company dumped and filled and was concerned that the FC was 0 (surprise !). so i want to take control as much as possible but do still need them. 

    also would like to understand bimodal interaction of ozone with active chlorine. it reacts with/consumes active chlorine but has independent organic waste oxidative effects, so ozone depletes FC between soaks but but it lowers FC demand during soaks from what i understand. for the 'rule of thumb' of 3.5 oz of 8.25% bleach for every person hour at 104, you have posted in the past that that amount is halved in the presence of ozone. was that a typo? shouldnt the addition after a soak be double in the presence of ozone?

    is the rule of thumb intended to raise FC by 5 or by 7 ppm?

    how fast will a shock amount of bleach (40% of cya level) work before safe to go in? what upper limit of FC do you use as safe to go in? i know you like the smell of fresh (non combined) chlorine, which i agree with.

    and ppm targets based on the FC/CYA relation are great for verifying target levels, but how can one translate ppm to volume dosing, say for shock amounts of bleach in 280 or 325 gal?

    any difference in speed of bacterial killing / raising of FC between bleach, dichlor, or trichlor in a dispenser? i would think trichlor is slower...

    also why is the rule of thumb independent of spa volume? would think the absolute amount of bleach to raise FC a fixed amount has to relate to spa volume...

    and dose salt alter the rule of thumb, since you have shown that at least for pools, algae is suppressed at lower FC/CYA ratios (4.5%) with salt than without (7.5%)? therefore, with salt does one need less than the 3.5 oz/person-hour?

    any pearls of wisdom on differences in water chemistry i should look out for between hot spring and nordic. the nordic manual is completely silent on water chemistry recs; they defer to dealers entirely, who i find unconcerned with sky high CYA levels; they dont encouraging testing for CYA (what you dont know wont hurt you), and their soution is just dump/fill often, even weekly for heavily used spas...


    many thanks !





    1. chem geek

      chem geek

      It was not a typo.  When ozone is used then it oxidizes bather waste so after a soak roughly half the usual amount of chlorine is needed since ozone will oxidize some bather waste so that chlorine doesn't have to do so.  It's in between soaks (after the 12-24 hours after a soak) where there is no bather load that the ozone has no bather waste to oxidize so will oxidize chlorine and deplete it thereby roughly doubling chlorine demand in between soaks.

      Trichlor tends to dissolve too quickly in hot spa water.  If you use a special dispenser where you can turn it way down then maybe you can get it to work, but it's tricky to use in a spa.  And yes, in that case you'd need the TA higher to help compensate for the acidity of Trichlor.

      The rule of thumb for chlorine usage is to add enough to oxidize the bather waste.  It has nothing to do with maintaining a certain FC level.  If you add enough chlorine to oxidize bather waste then you should end up with the same FC level you started at the beginning of the soak.  So you can decide what that is, but really just add whatever you need to so that you end up with roughly 2 ppm FC 24 hours later.  Of course that assumes you are adding chlorine every day after that, such as soaking most days.  If you aren't, then you might target a higher FC so that you don't have to add chlorine every day.

      Use PoolMath to calculate dosages.  All chlorine sources produce IDENTICAL hypochlorous acid so have identical kill times if the FC/CYA ratio is the same.  What is different is that Trichlor and Dichlor will build up CYA making chlorine less effective because the FC/CYA ratio will drop unless you intentionally proportionately raise the FC level to keep the FC/CYA ratio constant.

      The rule-of-thumb is independent of spa volume because the rule-of-thumb is NOT an FC concentration target but rather a chemical addition amount and that is the amount of chemical to oxidize bather waste.  It doesn't matter what the volume of water is since your bather waste is a fixed amount based on how long you soak and doesn't matter how much water there is when you soak.  You sweat a certain amount so introduce a certain amount of ammonia, urea, etc. and it takes a certain amount of chlorine to oxidize it.  The amount of water is irrelevant to that.

      As for salt, it is not clear if the salt itself inhibits algae some but in a spa this is irrelevant since algae is not an issue due to the spa being covered (dark) and very hot.  It's bacteria that is relevant and that's about disinfection and may not be improved with salt.

      The pool and spa industry does not understand nor talk about CYA buildup and its effects.  It has little to do with the specific spa manufacturer.

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