Hi Steve,
i found one on ebay quite a reasonable price as i have looked around. Total Desolved Solids. a tds meter will read how hard the water is well sort of. Mine is around 275 from the tap. put it through a R.O. system like the merlin and it should come out zero or very close. You can then pass this through a D.I. resin to finally bring it down to zero. Some parts of the country do not need R.O. but just pass the water straight through D.I. resin because it is so soft.My waistage is about 25% or so pure the rest waste. Quite slow too but I have a small capacity R.O.
Merlin is generally excellent, needs around 50 psi to see it at its best. if psi lower then waste will creep up.
Type in merlin on search box and you can read past posts some excellent information.
Good luck
regards
paul
