Until last week both my pool "AquaChek strips", Taylor "BASIC" test kit" (K-1000) and the R0070 Taylor powder method agreed on the FC levels in my pool.
But last week following an asorbic acid/ metal sequestation procedure to remove light rust stains, the tests I've done don't agree any longer. The rust stain removal procedure required me to reduce the FC chlorine levels to close to zero ( 0.5ppm) and then for a week to keep the FC at under 2.0ppm. That became a problem.
The AquaChek strips and the BASIC test kit kept showing "no chlorine" -- the strip pad remained white and the normally light yellow liquid was clear suggesting I had no chlorine even after I added liquid chlorine at the recommended level. Yet the Taylor "drop method" with the R0070 powder showed FC 1.5 to 2ppm. -- Beginning last evening I took the chlorine levels above 2ppm -- but still only the Taylor drop test shows chlorine now 4ppm. The other two still report no FC. Any idea why one testing method shows plenty of FC while another shows none?
My CC is .5ppm or less. PH (7.3), TA(130), Phosphate levels less than 300. CYA is now 45-50 after a partial drain earlier this season.
Pool is absolutely clear with only minor evidence of dead algae in skimmer sock or on bottom.
Pool Strips are new this season and were working just fine earlier -- as did the Taylor daily test kit.
Any suggestions? Could it be a chlorine lock which effects one testing method but not another? I will try a SuperShock and see if the levels generally hold and/or FC results start to appear again. Can I just go with the Taylor drop method only? Thoughts, suggestions, explanations would be appreciated.