Whats bizarre is that I tested the pure water in the van tank with old meter and it was reading 1.4 so I assumed the old meter was working OK
Also checked neighbours water to see if it localised problem to my house - and they had same reading so assumed TDS was accurate
Just checked readings with old and new meter
Under 10 tds both saying about the same but when I add tap water to pure to get a 20 tds reading the old meter showing 60 then when I add more tap water to pure to get real 50 tds - old meter showing 400. Tap water is 300 with new tds and 850 with old meter - so error only shows up over 10 tds.
Anyway, it was a learning experience and all is now sorted
