I figured you guys would get a kick out of this. I'm a new pool owner who is just trying to learn how to get consistent and accurate test results. I have what I think is a decent DPD test kit (will buy the Taylor or TF next spring), but I've struggled to make pH and chlorine readings from the ambiguous shades of purple and blue. TA and Total Hardness results seem a bit more trustworthy due to titration method vs comparing shades of purple. Anyway, I've been assuming I could trust the pool store's results, but I decided to do a comparison. I took the same sample to my regular store and also to another nearby store at lunch today. See results below. Wow. It blows my mind that the pool stores could arrive at such different results and subsequently recommend such different courses of action. This is probably common knowledge to most of you.
Pool Store #1 (my "regular" store -- since I've owned the pool the last 2-3 wks)
(not exactly sure of method, but I think they are using reagents to test)
pH: 7.9 (was 7.8 last week, supposedly, and I've added 3 lbs of sodium bisulfate since then???)
CYA: 62 (measured 61, 56, 57 during 3 previous trips to this store)
Pool Store #2
(used a single 6-way or 7-way test strip that was inserted into a machine for color-matching)
CH: 100?? (Actually, it's even worse b/c this is TH at this pool store)
CYA: 110?? (nearly double the other store!)
Given that store #2 used a test strip and given how strange the CYA (and TA) results have been when I've used test strips at home, I'm inclined to disregard the TH/CYA results from store #2. My TH titration test at home consistently indicates about 250ppm, so that would seem to support store #1's CH of 187.
If anyone is interested in a bonus question, why do you think the sodium bisulfate is having such a small effect on the pH? I've steadily added about 9 lbs over the past 2 weeks, and the pH has barely moved (supposedly 8.0 initially). The label indicates it should have moved alot more by now.