Help and Advice

Hi all I have an old hm digital tds pen witch I have just tryed using I have a bucket of ro and I tested it with the pen and pen say 019 srl I have an ro machine and I'm using it for the first time today half the bucket is full so I put the pen in and tested that water and it reads 018 witch is a lower reading then the ro I got from the shop when the ro from the shop was tested with there pen they said it was 002 on there own tds pen here's a pick of the reading it gave me from the shops ro...