TDS 0 then 20ppm?

[Fig]

I have a 5 stage RO/DI unit running a Dow Filmtec 75 membrane with dual DI's. I get 0 tds out of the unit (water from the line before it goes into storage cont). But if i test the actual storage container it reads 20ppm. I've emptied the container,cleaned it to make sure nothing was in it and I keep it covered. What gives. Leaching?? Thanks Tim

 

[bertoni]

There might be a bit of leaching happening, and also carbon dioxide will dissolve into the water over time. Both will increase the TDS. I wouldn't worry at all.