RO water is very pure. DI water is ultra pure. Di is considered lab grade. The RO output will still leave a few dissolved solids in the water. DI resin absorbs and locks in any and all particles. DI resin is considered spent when the TDS meter reads 0002PPM. You can easily plumb in a large DI bed into any old RO unit. Check with Buckeye field supply for a canister and refillable cartridge.