[cummings66]

Unless something bad happens mixes don't change over time once they're mixed up. Measure today and it will be the same 3 months from now.

As to needing one, yes I believe if you dive it you need it. I've been to shops where theirs quit and I had to use mine or not get a mix. I've seen mixes that didn't come out right. For some reason when I got the tank back the mix ended up way too strong for the dive I was going to do, even though they analyzed it.

I believe it's a good idea to second check a mix.