This is interesting as a number of years ago our club wanted to know if all pH kits gave the same reading. We handed out film canisters and asked all to attend the next meeting with samples of water, from tank(s) and tap plus any pH tests they had - strips, liquid, whatever brand. We also had a scientifically calibrated commercial pH probe.
The kits ranged from brand new to many years old (and I mean around the 10 year or more mark for a few) and all, except the test strips, gave accurate readings that matched the probe.