To determine the condition of your membrane, we really need to know what your TDS is after the membrane and BEFORE the DI, but if you're getting 14 after the DI, it's a fairly safe bet that you're due for a new membrane and new DI.
I'd call Spectrapure and get those ordered. While you're at it, order an add on canister for an extra DI stage, and you may as well get the pre filters, too. I order extra pre filters as they burn out faster than the membrane and DI.
With two DI canisters in line, the first one will exhaust first. When that happens, you discard that resin, move the last stage (used resin) forward one, and put new resin in the last stage.
It doesn't have to be Spectrapure, but that's who I use, and I believe they are the best name in pure water there is. Know going in that their shipping is slow, I've waited close to two weeks for a shipment from them.
