Caveat: Cross-posted on Pool & Spa Forum - no disrespect.
I have a relatively new spa and am struggling with correctly measuring pH. Simply put, I don't know which test to trust. With the ranges I am seeing below, I am concerned my true PH may be too high and I will have problems with scaling. I badly want to believe in my Taylor test because of the precision relative to other tests, but I think I may be seeing some type of interference.
280 gal Emerald spa
SpaFrog (BCDMH bromine and Silver/Limestone mineral cartridges)
Bromine 4.5 (high today, been targeting 3-4 ppm)
pH: 7.5 - 8.1+, depending on what you believe
1) Taylor K2106 Test Kit dated 12/07: light/bright purple, >= pH 8.0, 2 drops acid demand to get to pH 7.6. This has been consistent over a range of bromine levels, 0.5ppm to >5 ppm. Initially I was adding a lot of pH-, eating up my TA, adding TA+, etc. but eventually I lost confidence in the Taylor test.
2) Taylor K2106 Test 2 (assuming Bromine interference): Added 1 drop Thiosulfate first. Light/bright purple, > 8.0, 2 drops acid demand to get to pH 7.6. I've done tests in the past with 2 drops Thiosulfate, same result. I have bromine tested a water sample treated with Thiosolfate and read 0 ppm bromine.
3) Aquacheck Red Test Strips: pH 7.2-7.8, estimating 7.5 in color gradient. This is the test I have been believing and managing my pH against.
4) Nature2 Test (bought to try to get another read on things): pH 7.9. The first time I tried it the color seemed purple like the Taylor test.
5) TetraTest pH (sodium hydroxide / phenolphtalein / ethyl alcohol; yellow = 5.0, light blue = 7.5, purple = 10.0): pH ~8.1. I wanted to use a different totally test so I bought a non-strip pond test kit that handles a wide pH range.
6) I took a water sample to a dealer on 4/21 for a test, whu used an (AquaCheck?) test strip and an electronic reader. Results: TA 80, pH 7.4, Hardness 140, Sanitizer 2.8. This is part of what made me stop believing in the Taylor test.
I read about Iron interference on the Taylor literature - could the silver or something else be affecting the test? I've considered using a metal sequestering agent, but I think this would eliminate one of the benefits of the SpaFrog mineral cartridge.
I'd really appreciate your thoughts on this. I'm happy to provide more info if required.
PS> I read TA: What is it really and got to thinking that maybe the Taylor test is right and my TA increasing, pumping/aeration, and mineral limestone cartridge are contributing to a bona fide high pH. But, if I am wrong and I fix what isn't broken, I risk corrosion of my equipment. Hmmmm....