Adding salt: why did my level go too high?

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I'm using Aquachek to test the salt. Like you my yellow bar never really turns brown, but my white bar is white and pretty easy to read. The reading I took before adding any salt was 1170. I needed some dichlor so I had the pool store test my water when I went in. Their reading was 1200 ppm, and with the exception of chlorine (which was off by 1ppm) all their other readings matched mine. So I feel pretty comfortable that my starting level was around 1200.

It's possible that my 'after' reading could be off, but since my first reading was confirmed I know the strips are good. I'll run another test, but we had some storms last night that added about 4" (so far) to the pool, so I'm sure that will have some impact.

Ethany's chart was pretty close to what happened in your pool. Doing the math it's pretty close to what happened in mine, too. 400 lbs raises 20K gallons 3500 ppm. So 40lb raises 20K gallons 350 ppm. 350ppm x 4 bags= 1400ppm. 1400 ppm + 1200 ppm start value= 2600 ppm in 20,000 gallons. Since I only have 17,500 gallons ending up with the higher value of 2830 is pretty close.
 
Terry said:
400 lbs raises 20K gallons 3500 ppm.
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No, 400 lbs of salt will raise the salt level of 20,000 gallons by just under 2400.

The Zodiac manuals I have seen, for example this one (page 18) recommend using 584 lbs of salt to raise salt from 500 to 4,000 in 20,000 gallons. All of the other SWG companies have similar tables, all of which agree with the Pool Calculator results to within about 2% or better (some of them round quantities off by more than others, thus the small differences).
 
Most pool stores use an electronic meter to test salt levels. The electronic meters tend to drift fairly badly and so need to be calibrated to a standard regularly. Some stores do the calibration, but many ignore it and thus have wildly wrong results. The electronic meters are also subject to interference from a couple of other things that can read as if they were salt, though that is not usually a problem.
 
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