Pelagic, I would think that if the membrane is bad you would get a much higher reading than that.
I just replaced all of my filters a couple months back......prior to the change my RO/DI was about 10 ( I didn't have a tds meter). I replaced all filters and new DI resin and it went down to 0. My water from the tap is about 400 give or take.....out of the RO is about 2 and the DI takes it down to 0.
I am guessing that fresh DI resin would drop that 2 to 0.....I have enough to recharge another canister if you need to use it. Just LMK.
Well mine is at 002 after a 3 month change out of all 4 of my filters i run a spectrapure, Pre filter, carbon filter, R/O membrain, DI cartridge, but im on a well, Scott told me at Spectrapure dont worry about it until it's at 010, the next thing im going to do is add a whole house filter straight to it that might help a bit.
PS, mine is 454 going in and 002/005 out, i cant complain.
A TDS meter measures total disolved solids, this is good. The unknown factor is are the solids good or bad? I like to test for PO4 and Nitrates once in a while also to make sure the DI is doing its job.
Like others have said the Membrane is good for 2-3 years, so if your getting a TDS reading it is getting time to change out your resin and possibly the other filters.