I just looked at the membrane in my RO/DI unit and realized it is a 100 GPD membrane (Filmtec TW30-1812-100). I got this unit used about a year ago and have had TDS reading in the 14 - 18 range from day one. I was never too concerned since the RO water I was purchasing from the local grocery store had similar readings. However, I'm now ready to get down to 0 TDS.

I think the root of the TDS problem may be the flow restrictor... I replaced the old one prior to running the unit with a 50 GPD model (the quick connect type) because that was what was on the unit and I made the assumption that it had a 60 GPD membrane (it is a 60 GPD spectrapure unit).

So, my questions are as follows:
1) Would a 50 GPD restrictor on a 100 GPD membrane be why I am getting TDS above 0?
2) Is the current membrane shot due to the mismatched flow restrictor?
3) Is a spectrapure 90 GPD capillary restrictor appropriate even thought the membrane is 100 GPD?
4) Can someone point me to the thread @AZDesertRat posted about how to install a capillary flow restrictor?

Thanks for the help,
Tom