Water Chemistry Question

[Bigralphy]

Hi all,

I'm now at the one month mark of owning a hot tub and I have a question.  I have been using dichlor for sanitation and now my cya is around 200.  My tub is under a roofed gazebo and does not get any sunlight.  I currently have a cc level of 1.0 ppm, fc of 3, pH 7.6 and Ta 100.  The issue I am having is keeping the fc level greater than 0 before we use it the following day. I usually add 1 tbs of dichlor after use but my fc is down to 0 at noon the following day.  I have also tried adding 90 ml of 10% chlorine and it is also at 0 the following day.

It would appear that I need to shock with a higher ppm rate the  I am.  How much liquid chlorine should I add and is this OK to shock with or should I shock with dichlor?

Any responses are appreciated

[chem geek]

Your CYA is getting high enough that the chlorine is less effective.  Technically, the active chorine that actually disinfects and oxidizes your bather waste is proportional to the FC/CYA ratio so as your CYA climbs over time the active chlorine level is getting lower since you aren't changing your FC level.  This is the typical/usual problem with using Dichlor-only because for every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

So yes, you can use chlorinating liquid or bleach to prevent further CYA buildup, but other that water dilution or a water change you can't recover back to your fast oxidation when the CYA was lower unless you use a proportionally higher FC level.  This is why some people use the Dichlor-then-bleach method where they only use Dichlor to build up to around 30-40 ppm CYA and then switch over to using bleach (plus once a month use Dichlor for a day to build up CYA that is slowly lost over time).  To use chlorinating liquid or bleach as the primary source of chlorine, you need to have the TA be lower and use an additional pH buffer such as 50 ppm Borates (usually from boric acid) in order to prevent the pH from rising due to carbon dioxide outgassing.

As for how much chlorinating liquid to use, just add more until you start to get an FC reading (but make sure you aren't bleaching out your chlorine test kit -- if you are using a proper Taylor K-2006 kit then such bleaching out does not happen as you can measure up to 50 ppm with the FAS-DPD test).  Raising the FC to a higher level might get the CC down, but if you use the spa regularly you may be too far behind to be able to recover -- at least not without having the FC so high that you may not like soaking in the spa (because you may smell the chlorine/chloramines).

By the way, 1 tablespoon of Dichlor is only enough to handle 35 person-minutes of soaking in the spa and you refer to "we" so I presume two people are using the spa probably for at least 20 minutes or more.  So you aren't adding enough chlorine anyway.  The rough rule-of-thumb when you do not have an ozonator (do you have one?  I suspect you don't) is that for every person-hour of soaking you need 3-1/2 teaspoons of Dichlor or 3-1/2 teaspoons of 8.25% regular unscented bleach or 7 teaspoons of non-chlorine shock (MPS) to oxidize bather waste.

[Bigralphy]

No ozone and a 2006 test kit.  I will do a water change shortly.  How often would one expect to change the water using dichlor daily and mps once per week with 2 ppl soaking 4 times a week? I would like to try the dichlor bleach method but I am a bit paranoid.  Not sure why as I test the water faithfully each day.  Where does one buy borate?

[Bigralphy]

Forgot to mention that I have uv as well.  Does this cause more chemical to be used?

[chem geek]

If the UV is strong enough it will create additional chlorine demand.

You didn't say how long you soak, but let's say it's for 30 minutes, and you didn't say the size of your spa, so let's say that's 350 gallons.  So 2 people 4 times per week for 30 minutes for each soak would be 2*(4/7)*(30/60) = 0.57 person-hours per day.  The standard Water Replacement Interval (WRI) formula using person-hours and that would apply to Dichlor-only dosing is the following:

WRI = (1/9) x (Spa Size in Gallons) / (# of person-hours/day) = (1/9) * 350 / 0.57 = 68 days

So that would be around 2 months (with the Dichlor-then-bleach method you should be able to go around twice as long between water changes).  Perhaps you are soaking longer than I indicated.  You say your CYA is at 200 ppm so that implies adding 200/0.91 = 220 ppm FC of Dichlor over 30 days so 7.3 ppm FC per day on average.  That would be roughly 3-1/2 teaspoons (a little more than one tablespoon) of Dichlor per day in 350 gallons, yet you said that's all you added after each soak, not every day.  So something isn't adding up.  Is your spa smaller than 350 gallons?

[Bigralphy]

We soak for about 40 minutes usually daily. Tub is 1370 liters. I just had the water tested at the dealer.  They say my ta is 160 and my taylor kit says 100.  There pH was 8.2, mine is 7.5.  They state my combined chlorine is 19.5 ppm, mine says 1ppm.  Now I don't know what to think.  I know the taylor instructions state to hold the dropper completely upside down.  When the store tested they had the bottle vertical applying drops which may result in a smaller droplet.

I am currently shocking the he## out of it and I will see what happens after.

Thanks chem geek.  I appreciate it and will be trying the dichlor bleach method on the next fill which will be I  about 30 days.

Also her cya test showed 100 with a test strip but my drop kit showed much higher

[chem geek]

Do you mean they held the bottle horizontally so the bottle tip was pointing to the side?  That's definitely wrong and yes it does result in a smaller drop (usually) so a falsely high reading.  Pool stores often don't know how to test properly.  Trust your own results.

If your drops don't come out well formed and "hang" on the tip, then use a moist cloth to wipe the tip and get rid of static electricity.  This most commonly occurs with the TA test.

I find it hard to believe that your CC is 19.5 ppm.  I think they are wrong about that.  Test strips are notoriously poor for the CYA test.  I'd just ignore the pool store readings completely -- again, trust your own results.  After all, that's why you spend money on a proper test kit.

Though you haven't reported problems with biofilms or unusually high chlorine demand, I think you should use Ahh-Some just before your next water change.  It cleans out greases, oils, and biofilm from piping and other surfaces.  While properly maintaining a spa with chlorine should prevent such buildup, it's hard to be perfect about such maintenance and when there is no circulation, such as a spa jet not getting used for some time, the chlorine can get used up and bacteria grow there.  Some people use it for each water change.  You can decide for yourself how frequently to use it (i.e. maybe not for every water change).

[Bigralphy]

Yes I meant horizontally.....insert embarrassment hear:(  I am sure they measured incorrectly.  I soaked tonight and had a reading of about 4 fc when we were done.  I will try to find some ahh some but not sure anyone carries it near my parts.  I was also quoted some outrageous prices for replacements for my test kit. It would be 70 bucks for r 870 and r871 refills.

Thanks again chem geek. I appreciate all the information you have provided me and I recently read a couple of your articles  which are amazing. 

[chem geek]

You can get reasonable replacement refills at TFTestkits.net.  The XL option gets you 10 g of DPD powder and 2 oz of FAS reagent for $15 or you can get the 1 oz R-0871 for $7.50 and the 10 g of R-0870 for $7.65.  Amato Industries is another source for Taylor reagent replacements where you can get 10 g of R-0870 for $5.10 and 0.75 oz of R-0871 for $6.99 or 2 oz or R-0871 for $7.36.

Usually people let the chlorine drop to around 1-2 ppm FC before their soak.  That way they don't smell as many chloramines during their soak.  When you add chlorine after your soak, you kill off anything left though even with no chlorine when you soak it will likely mostly be monochloramine which still (slowly) kills bacteria.  You can certainly soak with higher FC levels if you want and that's something commercial/public spas do and that you might consider if you have strangers in the spa, but most people soaking at home prefer to soak with lower amounts of chlorine.  With family members, person-to-person transmission of disease is less of an issue since you are generally in close contact anyway and as I wrote there is still some disinfection even when starting with lower FC (just not as much).