We all probably have collected a number of different test kits and strips over time. Last year I used a kit purchased at a pool store, the SwimLine 4-in-1, and when it began to give me results which I simply could not believe, I bought a Leslie's Deluxe DPD kit. That one was ok but I found the chlorine values not entirely believable. In October 2007 I ordered a kit from Dave, the TF-100 which has returned accurate and rialiable results every time. My husband is a chemist and did some laboratory magic with known quantities of Clorox dilutions as well as fancy pH meter tests, and his results were always in agreement with the TF-100 measurements. Hence, that's the one I go by.
This morning I spent a little time testing my pool water with the various kits and strips in my collection. The strips and the SwimLine 4-in-1 are now in the trash. Attached is a PDF file with the results. I would have posted them directly but I can't figure out how to make a table here
I'm a little puzzled by the CYA differences. The TF-100 reads at 70 ppm and the Blue Devil reads at 40 ppm. I would tend to agree with the lower value since it was 30 ppm when I closed last year and I haven't added any. It's true that I use trichlor but at a 1/4 setting on the feeder and the pucks having barely shrunk since I put them in a month ago it really can't have gone up by 40 ppm in just one month. More than likely I didn't read the test correctly.
I'm not happy with the low chlorine readings on the Leslie's (Taylor chemicals) kit. I don't believe them. Possibly the chems are old.
The point of this post really is to show the wild variances between tests and the inadequacy of test strips. A good test kit isn't inexpensive but in the long term it's a necessity.