Test Kits - Back up

[BeakerBob]

I can see the logic of maintaining a backup test kit, but have to wonder how you use it.

Do you use the 2nd kit because you don't trust the result from the first kit? If so, then which results do you go with....the first or second test?

Why don't you use a test standard (sample with known concentration) to check the kit out for accuracy before testing the tank water?