I was looking for another supplier as the current RO water I was buying was a rip-off. So, to cut the story short I bought a Hanna TDS1 that has been factory calibrated which includes a certificate etc etc.
The expensive RO water gave a reading of 003 on the TDS meter, if this is correct then I'm guessing this is really good quality water. Now to my surprise the cheaper water gave a reading of 001
I even dipped the tester in and out a few times but it always remained on 001. My tap water kept going up from 80 up to 109 before I stopped putting it back in the water.