I have to say I was wrong the other year when I'd said the resin calculators were way off. I was bored and checked some records i'd kept and 18000 litres of pure from 11 litre di and still good to use now (Imo) at tds of 3. just now going up to 4. Im only part time so its great really. Bear in mind this is from a nightmare tap tds of 530, the daqua 4021 getting that down to 11 before the di. Not bad at all ?