I have been using drops to check my pool, but picked up some strips today to check for a couple things the drops don't cover (hardness and stabilizer levels). I was very surprised when the strips said my pH was low (6.8). I had checked the pool a few hours before with the drops, got a pH about 7.8, and added acid based on the titration test.
So I rechecked the same sample with the drops and the strips: strips still said 6.8, drops still said 7.8 (and this was several hours after adding the prescribed amount of acid). WHY IS ONE SAYING pH IS LOW, AND THE OTHER SAYS IT IS HIGH??? Strips and drops are both fresh.
Few possibly pertinent notes: Pool water is green and cloudy. I added 5 liters of 6% bleach about the same time as the acid, which may be why the pH didn't go down (if it didn't...) Until the last week I have been using cal hypo, but thought I would give bleach a try. (Can't get commercial liquid chlorine around here). Strips says the hardness and stabilizer are ok. Total alkalinity is 70 by drops, "low" by strips.
Also, just about every time I have checked pH by drops, it has said it was high. When I add acid, the next time I check it is still high.
Andy