I bought a FAS/DPD refill for my TF-100 kit. I took chlorine readings with the old bottle of drops and got a reading of 3FC. Took it with the new drops and got a reading of 5FC. Re-ran the tests again with no change. Which one is right?

Go with the new stuff. If new reagents are different, obviously the old stuff is bad.

Which is weird... since there was a recent report about old reagents testing 2-3 times high. I'd think old/bad reagents would test low like you're seeing vs testing very high personally.

I don't know what the answer is, but I would think old reagents would give you a false high since common logic would say they are weaker....therefore taking more drops. Guess it depends on what happens to them as they break down, and this is probably a bad place for common logic to apply. Whatever the answer, I'd go with the new reagent result. Besides...what are you going to be adjusting by from here on anyhow?

I would be curious too though and just happen to have some old and new. I'll be comparing this weekend to see what I get. In the end, two PPM isn't that big a deal if it's a 10 mL test to me.

Could the new stuff have been sitting on the shelf for a long time? Maybe one possibility. I might order another bottle and see if they are the same. Or another possibility is that the old bottle has a defective dropper size?

The R-0871 is light sensitive which is why it's in the opaque bottle. I'm not sure if that's helpful, though, because I don't know what happens after it has been exposed to light.

Virtually none of the chemistry we have sits for more than 30 days and it has a promised shelf life of one year from Taylor. My instinct says go with the newer but I don't think this issue has been reported before so I am not sure of the cause.

Well, I store it in a kitchen cabinet under the counter, so light should not be the issue.