CYA dissipation rates

[TdiDave]

I know I have read that CYA does dissipate slowly so it would be necessary to switch back to diChlor  periodically. What does everyone see as dissipation rate?  40ppm per month seem high?
 

[chem geek]

We normally see around 5 ppm per month CYA loss.  So yes, 40 ppm loss would be extraordinarily high.  Can you give us more specifics?  What is the usual FC level and what was the CYA level you started with?  Do you have an ozonator?  How are you making these measurements -- using a Taylor K-2006 test kit?  Test strips are pretty much useless for testing CYA levels.

[TdiDave]

Normally keep the free cl around 2 before (measured before soak). Try to soak every day but miss a day hear and there and will still have some cl2 left.  The tub has the "clear-ray" UV system. Testing with the Taylor kit.  Fresh fill just over a month ago, raised cya to about 40 with dichlor then switched to bleach. Noticed I cl levels getting lower before each soak so checked the cya and found I had none.

Think this UV system is chewing up cya faster than normal?  I am back on dichlor now to get the cya up.  Then I will check it once per week to get a better idea on the loss rate.

[chem geek]

CYA does absorb UV, but it's not supposed to break down from it.  If there is a way for you to turn off the UV system, then you can see if the CYA holds and then turn it back on to see if it declines.  That would be a definitive test and something good to know.

CYA does get oxidized by chlorine, but that's slow and accounts for the roughly 5 ppm per month that is more typical.

[TdiDave]

3/22 CYA of 35 to 40
4/4 CYA of 0

Seems as if Clear-Ray is eating CYA

Guess this could mean a light user could stick with DiChlor and not have a problem with elevated CYA levels?

[chem geek]

Wow, that's great information and thank you so much for doing that experiment.  Is there any way for you to turn off the ClearRay and see if the CYA doesn't drop over the same sort of time period?  That would be definitive confirmation.

We know that CYA absorbs the UV in sunlight and doesn't seem to break down from it, but the UV in a UV system is a lot lower wavelength (higher frequency) and therefore could break down some kinds of organic molecules such as CYA.  I just never heard about that happening before -- not even in pools with a UV system.  According to this link, the CLEARRAY® Water Purification System uses UV-C.  I wonder if it's a single-wavelength low-pressure UV or a broader spectrum medium-pressure UV.  Is there anything in your documentation that mentions that?

So this is good news and bad news.  The good news is that yes, you could use more Dichlor or perhaps even only Dichlor depending on your spa usage.  In that case you may want the TA to be somewhat higher to compensate for the net acidity of Dichlor.  The bad news is that if UV is breaking down CYA, then it is likely breaking the CYA ring turning CYA into a chemical such as Biuret and that would get more rapidly oxidized by chlorine so would increase chlorine demand.  When chlorine alone oxidizes CYA, 1 ppm CYA takes 2.5 ppm FC so are you seeing an unusually high chlorine demand as well?  Specifically, what is your daily chlorine demand (% of FC that is lost) during periods when you aren't using the spa?  If it's much more than 25%, then this could be what is going on.  40 ppm CYA over 13 days could use up to 100 ppm FC so 7.7 ppm FC per day -- that sounds extraordinarily high so I doubt that is happening.  The more usual 5 ppm CYA per month translates into 5*2.5/30 = 0.4 ppm FC per day chlorine usage.

[TdiDave]

I have only had one good extended (more than 48 hours) no use test. And that CD was Very low, (less than 10%) seems a bit suspect but not sure how I could have screwed up the test. I have done several 24hour test and 20% - 30% CD has been the norm. I do tend to keep the free Cl level higher than some as it seldom is below 2 before a soak (I'm in municipal water treatment as a profession, and know the nasty stuff that can grown in water). I have yet to get anything above 0.4 CC
I am going away for a few days tomorrow so I will check my levels in the morning then I can compare when I return on Monday. This test will have very little CYA, I know my last extender CD test did have CYA so not sure if that will screw the results.

I haven't really looked to much under the hood, but I would assume the uv plugs into the controller.  So I think after I get my CYA back up this time i will try and unplug it. For 2 weeks and do some CYA test.

Lots of variables in all this but it does seem the CYA dissipation rate is higher than normal.

Makes you wonder if those Jacuzzi engineers knew this could be a advantage over ozone when they switched.  But then someone would have to admit high CYA levels are a problem and I haven't seen much of that.

[chem geek]

Yeah, having little to no CYA will screw up the results since chlorine will outgas faster and generally react with anything it can faster.  So chlorine demand will probably be higher with no CYA.

So yes, when you get back if you can unplug the UV for another test, that would be great.  I'd like to get to the bottom of this, not only to understand and possibly help your situation, but this could be useful to others generally if indeed there were a way to break down CYA without side effects.

Yeah, I doubt that the Jacuzzi engineers knew anything about the CYA issues with Dichlor-only usage.  They were probably just adding a UV system to compete with the HotSpring® ACE® Salt Water Sanitizing System which uses boron-doped diamond electrodes to produce hydroxyl radicals in addition to chlorine.  It's a premium product beyond standard ozonators so I'm sure Jacuzzi was looking for something competitive.

As for CC, the UV will likely keep that pretty low.  Ozonators tend to do that as well.  Just so you know, when CYA is present so that the active chlorine is moderated in its strength, the CC can be higher but not a problem.  This is because the CC will mostly be composed of chlorourea which is not volatile or irritating.  If you've got CC but you don't have any bad smell, then that's OK.  Usually this would only be under conditions of somewhat higher bather-load where the chlorine isn't quite able to keep up, especially as the water gets older (more buildup of slow-to-oxidize organics).  No problem for you starting with a higher FC for your soak; most people don't do that because they don't like the smell of chlorine/monochloramine during their soak.  Yes, there is greater risk for person-to-person transmission of disease with chlorine running out during the soak, but in practice the monochloramine will prevent bacterial growth and such bacteria can't grow very much anyway in the hour or so of a soak (generation rate for bacteria is 15-60 minutes best case under ideal conditions).  8 hours with no chlorine would be a problem, but 1 would not.

(Legal note: "It is a violation of Federal law to use this product in a manner inconsistent with its labeling."  Just a reminder since what I wrote above may be inconsistent with the labeling on products you are using.  Of course, I don't see the feds going door-to-door inspecting residential spas.)

[TdiDave]

Friday 5th 9:00am - FC 7.0  CYA 0
Sunday 7th 8:00pm - FC 3.0  CYA 0

CD rate of 20% with no CYA.  That seems pretty good?

As soon as I get the CYA back up I will do the "draw down" of CyA without the UV to compare.

[chem geek]

Yeah, that's very good to have the chlorine drop be so low with no CYA in the water.  I wonder if there's something wrong with the CYA test.  I suppose we'll find out when you increase the CYA again.

By the way, I get 1 - (3/7)^(24/59) = 0.29 so 29% 24-hour chlorine demand.  That's still pretty good given no CYA.