I've used Taylor's K-2006 kit for about a year and a half to measure the chemistry in our indoor pool taking care to store the kit away from direct sunlight. Recently, the kit required a massive adjustment to ph (10 drops of acid demand), following a temperature increase by about 15F. I followed the adjustment but became suspicious and grabbed the K-1004 from a local store, just to double-check the K-2006. The ph readings I just got were radically different. After 1 1/2 gallons of muriatic acid acid, which should have balanced the ph, the K-2006 still shows ph around 8 while the K-1004 shows it to be around 7. I trust the K-1004, because its reagent is fresher and because it makes sense.
I did not see any of the signs mentioned in this forum about the reagents getting old (staining, non-dissolving, etc.). My question is, what's a good practice for double-checking the chemistry in order to catch reagents going bad early? Right now, I don't know for how many months I've had potentially substantial imbalances in chemistry while I was trusting the K-2006.