Fun at the pool store -- test results compared

[PhillipH]

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)
FC: 3.6
TC: 3.6
pH: 7.9 (was 7.8 last week, supposedly, and I've added 3 lbs of sodium bisulfate since then???)
CH: 187
TA: 120
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)
FC: 7.7
TC: 7.7
pH: 7.8
CH: 100?? (Actually, it's even worse b/c this is TH at this pool store)
TA: 120
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.

 

[waste]

I'd say don't trust any of the results :!: The sodium bisulfate is supposed to lower the pH, NOT raise it!

SOP is kicking in here -- get a good and accurate drop based test kit

We'll be here to help you, but ... we need good chem test results

 

[duraleigh]

This is probably common knowledge to most of you.

Yep.......that's why you will always be encouraged to do your own testing with an accurate kit....there's no substitute.

 

[PhillipH]

Any ideas regarding my sodium bisulfate question? 9 lbs over the past 2 weeks (1 lb here, 2 lbs there, very steadily) is way more than the label or the Pool Calc say is required. Even though I'm not totally confident in my reading of my test kit's pH result, I can still tell that it's 7.8 or higher. The color change from 7.8 to 7.6 is pretty obvious, so I think I'll know when I finally get in the right range, but it's not there yet. My kit has the acid demand titration test, and I'm down to 1-2 drops to force the color to change the the 7.4-7.6 range. Maybe I'm getting there, but it just seems like I'm having to add way more dry acid than expected.

Here's another question that might be relevant. My chlorine level is pretty high, and I can't even say exactly how high (see the pool store test discrepancies above, and the best I can tell on my kit is perhaps "5-10ppm" -- I don't have FAS titration yet). Should I add more than 1-2 drops of the chlorine neutralizer when I'm doing the pH test?

Thanks,
Phillip

 

[PaulR]

An extra drop of the neutralizer might help. The Taylor (and therefore TF100) pH reagents include neutralizer that will work up to about FC 10 or so.
--paulr