Well, nothing like getting a fresh start, I guess. When I mentioned to pmb a couple weeks ago that I was kind of baffled by the water chemistry, he recommended I blog about it. Seems like a good idea to me. So here goes. Any chemists out there, feel free to correct any of my faulty reasoning.
The spa has three sanitizing/oxidizing systems: chlorine that I'm supposed to replenish each week, an ozone generator that operates 24/7, and a powder called "spa shock" (Potassium peroxymonosulfate) that I'm supposed to add to the water each time the spa is used. There's also a silver ion cartridge--I don't know what that's for.
The chlorine level is the most confusing part to me, because I have a lot of conflicting information about what I'm supposed to do about it. The laminated information card from the spa dealer says I should add chlorine each week and then test immediately afterward to make sure it's at least 10 ppm, but the test kit they provided can't test more than 3 ppm, and it indicates 1 ppm is "ideal". The card also has a "weekly chlorine" table that says my spa needs 4 tsp of chlorinating powder (sodium dichloro-s-triazinetrione dihydrate) each week. The inside lid of the filter compartment says to add 1.5 tsp/250gal of chlorinating powder each week, which works out to about 2.5 tsp. It also says I need to maintain 3-5 ppm FAC ("free available chlorine").
Enter the internets. From several web pages, I've learned that FAC is the form of chlorine before it's done any sanitizing work. When chlorine kills cells, it generally turns into good ol' chloride ions (Cl-). When chlorine reacts with ammonia, it forms chloramines (mostly NH2Cl), also known as "combined chlorine". Chloramines are a skin and eye irritant, and they are responsible for what most people call "chlorine smell".
I suspect that part of the reason I'm having trouble finding consistent information about how much chlorine to add is that the amount you add depends how much gets used up, and that in turn depends on how much use the spa gets, and how much dirt and germs people bring into the spa while using it.
Fun fact about combined chlorine: if the amount of free chlorine is more than 10x higher than the amount of combined chlorine, a phenomenon called "breakpoint chlorination" occurs, and all the combined chlorine gets broken down into N2 and Cl-, resulting in clean-smelling, non-irritating water. "Superchlorinating" a spa or pool means dumping in enough chlorine to reach breakpoint chlorination. Most of the web sites I've found recommend testing free and combined chlorine separately, and superchlorinating only when necessary. (I only recently bought a test kit that's capable of distinguishing the two types of chlorine). Some of the web sites go on to say that when the spa manufacturer tells you to superchlorinate every week, that's just because they want to make more money selling you chemicals.
The last time I superchlorinated was Tuesday the 25th, in the morning--I added 4 tsp of chlorinating powder and then measured a FAC level of 12 ppm. I used the spa Tuesday evening. Emerson and Claudia used it Wednesday evening. Thursday morning I tested the FAC and it was 0.6 ppm. I was a little concerned at how quickly the FAC had dropped, but I thought to myself, "let's not obsess over this--I'll retest on Tuesday and decide what to do". I used the spa again Thursday evening and Saturday evening. This evening when I found the spa cloudy, I tested the FAC again and it was nearly zero.
My theory: the free chlorine dropped so quickly because the spa got a lot more use than usual last week. When I measured it at 0.6 ppm, I should have added just enough chlorine to get back into the good range (which, I gather from this web site is about 1-2 ppm). Since I didn't add chlorine and continued using the spa, the free chlorine got used up, allowing algae (or something) to grow.
What I think I'm going to try this time is: test the chlorine as frequently as necessary in order to be confident that the FAC is always at least 1 ppm. For starters I think I'll test it after each use, and then ease up as I start to get a feel for how quickly the chlorine depletes. For a while, I'll continue superchlorinating weekly, and track the free vs combined chlorine to see whether that seems like too often.
The spa is now nearly full, so I'll document the chemistry involved in the start-up process. Most of what follows is just me blithely following the directions that came with the spa:
My tap water measures as 20 ppm total alkalinity (about what I expected after the previous fill). I'm supposed to raise it to between 100 and 150 ppm. According to the table, that should require adding between 4 and 6.5 tbsp of "alkalinity up" (sodium bicarbonate). But I'm not supposed to add more than 3 tbsp/day, so I'll add 3 tbsp as soon as the spa is full and it's safe to turn the jets on.
Ok, spa is full, jets are on. Added 5 tbsp "stain & scale control", 3 tbsp "alkalinity up", and about 5 oz "calcium hardness increaser".
Now I'm supposed to let it heat overnight and chlorinate it in the morning. Good timing, because it's late and I'm tired.
[Update in the morning: The water is still nice and clear. I added 4 tsp chlorine and ran the jets for 20 minutes. After that I tested the FAC and it was 8.75 ppm. I know I was supposed to get to 10 ppm, but I'm out of time and have to go to work now. Also, I think I'm ok because the combined chlorine was only 0.6 ppm last night, which means 8.75 ppm should be sufficient for breakpoint chlorination. Next time I superchlorinate I'll try adding slightly more chlorine.]