I should also mention that yesterday morning before I chlorinated, I took a bunch of other measurements using my chemistry kit:
Free Available Chlorine: 0.1 ppm
Combined Chlorine: 0.6 ppm
Total Alkalinity: 35 ppm
Calcium Hardness: 115 ppm
Cyanuric Acid: <30 ppm
I've been reading the booklet that came with the chemistry kit, and I now have a much better understanding of what these numbers mean:
Total Alkalinity measures how much acid it takes to reduce the pH of the spa below a certain threshold. Higher numbers mean that the spa water is more resistant to pH changes, which is good because it means that the presence of bathers will have less effect on the pH.
Calcium Hardness measures how much calcium is dissolved in the water. More calcium makes it more difficult to form soap suds (when using water for cleaning, preventing soap suds is of course bad, but for spas, preventing soap suds is good). It also prevents the water from gradually eating away at the materials that make up the spa. I'm not sure what my target hardness is; the manufacturer didn't recommend testing it, they just recommended adding 5 oz of the "calcium hardness increaser". I'll see if I can find more information.
Cyanuric Acid is a chlorine stabilizer--it prevents sunlight from depleting chlorine. It is present in a lot of chlorine powders (including the chlorine powder I use). The reason for testing it is that it tends to accumulate over time, and if its level gets too high, it starts to reduce the effectiveness of chlorine. I've read a couple of different recommendations for how much CYA should be allowed to accumulate before you change the water, ranging from 100 to 150 ppm.
The booklet also mentioned that to cure an algae problem, you should shock chlorinate to 30 ppm. I'll keep that in mind in case the spa clouds up again.
Anyway, after chlorinating yesterday, the Free Available Chlorine was 8.75 ppm. Unfortunately, with my test kit it's hard to measure a small amount of combined chlorine in the presence of a large amount of free chlorine, so I didn't bother making any other measurements.
Last night I added three more tbsp of "alkalinity up" to the spa.
This morning I measured:
Free Available Chlorine: 0.7 ppm
Combined Chlorine: 0.4 ppm
Ok, I don't know whether the chlorine dissipated so quickly because it had a lot of disinfecting to do or because it always dissipates that quickly. Or maybe because not much chlorine stabilizer has accumulated yet. More measurements will be the only way to tell.
This evening I measured:
Total Alkalinity: 55 ppm.
Then I added 3 more tbsp of "alkalinity up" tonight. It seems that by adding the recommended maximum amount of 3 tbsp each day, I'm only able to raise the total alkalinity by 20 ppm/day. So I have a few days to go yet to reach the desired T.A. of 100-150 ppm. Probably I could be adding more than 3 tbsp/day, because I have a large spa (they give the same information card to all customers), but I'm in no hurry. Total Alkalinity mostly serves to avoid large pH swings, and I'm testing pH daily at this point.
I'm also going to add 1 tsp of chlorinating concentrate this evening, and then have a good soak. Tomorrow I'll re-test the chlorine level.
I never thought I'd say this after being disillusioned by frosh chem, but chemistry is fascinating!