I change my resin every 3 mnths ( DI unit ) when the TDS creeps up to 003.
Some however change it at a higher reading, there has never been an
industry set standard yet
Cheapest resin, everyone is about the same £62 + vat + delivery is the
cheapest I have seen.
However I will be the cheapest supplying to Scotland £55 + vat + delivery.
Doug