Thought I would share this as I've discovered it after having problems when producing my own RODI water and keeping the tds down.
I have a vyair pumped 4 stage which was struggling to keep the tds to 0, often was around 3 - 5. Tried changing resin and replacing filters, adding another carbon and final resin stage but nothing helped to get to 0 tds.
When changing the resin recently i discovered that the resin holder in the canister was loose. After examining the water flow it appeared water was coming into the resin chamber, flowing over the resin canister, not through it, and out.
I examined the seals at the top of the resin canister and there was a massive gap, so after using some more thick washers from some other expired cartridges and a thin wide plumbing washer i managed to get a good seal at the top of the resin cartridge when the canister was screwed closed.
Bingo! Tds went down to 0 after a few minutes and stays that way for ages, id hope so too as it's now a 7 stage system.
This might already be common knowledge but i thought i would share.
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