R/o water tds???

Hi there, quick question, i bought a second hand deltec r/o unit. I also bought a brand new sediment filter, carbon block filter and membraine for it.
I tested the tds of the water as i made it up to put in my new set up, and it showed a reading of 002. i ran some r/o water out today to use as top up and before i added it to the tank i tested it and it showed 005?

Am i doing something wrong? i would ideally like it to show 0!
also any tips on how to calibrate my electronic tds tester?

Thanks for any help and anvice you can give.

 

you'll need to add a DI unit to it to get the 0 reading. but 2 to 5 is acceptable most times.

 

Yeah, single digit TDS after the RO is prefectly normal. RO membranes typically do something in the high 90%'s of TDS removal and the DI canister takes the rest out. It may fluctuate from day to day depending on the water coming in's TDS levels depending on the temperature of the water and stuff like that.

 

Even with DI, you will sometimes see a TDS of 1-2ppm. Just keep an eye out for algae blooms too. My filter has 2 stages of DI, that way when one canister exhausts, I divert the water to the other DI and get 0 TDS. I just have to refill both the DI stages when I replace the filter media.