Hey guys I just got a used RO/DI system (6 stage dual DI). I also purchased an inline TDS meter. I hooked it up so the the in line is hooked to the tube that comes from the membrane and the out line is hooked up to the tube comming out of the 2nd DI filter. However the in reading (comming out of the membrane) reads 67 but the out reading (comes out of the 2nd DI filter) is reading 0. is that accurate? Is it time for a new membrane? The lady i bought it from said it had only been used for a year. Thanks for your help in advance!