andy said:
Doesn't look good? What should we try next.
Well, the fact that the FC went down so fast does mean the tanning bed is putting out UV rays in the range that at least is absorbed by chlorine, but the CC wasn't affected. This isn't a bad sign -- if the CCs are that "stable", then they may not be as hazardous either since they are less reactive (this rule doesn't always hold, of course, since compounds like mercury are very stable but also very toxic, but for the typical compounds in the pool the less reactive then generally the less likely to cause problems).
OK, next up is non-chlorine shock, potassium monopersulfate (MPS). The normal recommended dose for pools is 1 pound per 10,000 gallons once or twice a week while for spas it's a higher dose of 1-2 ounces per 250 gallons after each use. Let's go with a higher dose just to see if it will do anything. Take a new bucket of pool water (the others you can either put back into the pool or dump their contents -- there's nothing wrong with them) and add one-tenth of a teaspoon per gallon. So 1/4 teaspoon in 4 gallons. It's not very much, but I want a realistic test since we're not going to put too much into your pool. Just keep the bucket uncovered and stir up a little to mix the MPS after you add it.
Now the trick is measuring the CC since any residual MPS will actually measure as CC itself. After one day with the MPS in the bucket, I want you to measure the CC as you normally would and then repeat the test, but add 2 drops of Sodium Thiosulfate solution (the stuff you add at the start of your Total Alkalinity test) to the test sample and then proceed with the CC test. You will certainly find that the FC goes away in the second test, but the CC that remains might be real and will certainly not be MPS. To confirm that, test a regular pool sample both ways as well -- first without the thiosulfate and then again with the thiosulfate added (this makes sure the thiosulfate doesn't get rid of the CC which, unfortunately, it might during the CC portion of the test that "releases" the combined chlorine to become "free").
Another option to the thiosulfate method is just to wait and test after 2 days, 3 days, etc. as the MPS will slowly go away as it finds things to oxidize (it's similar to chlorine in that way), but I don't think this is particularly reliable.
The other option you have for a pure CC test is yet another test kit, the Taylor K-2041
here which removes the MPS interference from the CC test. I hate to have you spend more money and get another test which is why I offered the other possible alternative about, but what I described above may not work. The Taylor K-2041 test will certainly work and I've used that myself.
Anyway, if the MPS doesn't get rid of the CCs and if no one else comes up with any idea, then you'll probably just live with it and slowly dilute it away over time with partial drain/refill of water. At least with the CYA in the water I think you are going to be building up CCs more slowly than you did before. I suppose one other thing to try in a bucket test would be a phosphate remover just to see if that changes the results of the CC test in case of some strange interference and another thing you could try is some Metal Magic in the bucket just in case the iron has come back and is interfering with the CC test.
Richard
