I always let my RO/DI run for 5-10 minutes before I start saving any of the water. The water in the canisters is sitting there with everything they have filtered out so it is best to let it run for a bit first.

Do you have a DI unit hooked up as well? You should be getting around 98% reduction in TDS from the RO membrane alone. Given our tap water is around 22 ppm (that's what mine is right now) you should be getting a reading less than 2 or 3 if your RO unit is set up properly. And then the DI should definitely be getting things down to 0.

I had a problem with a home drinking unit that had a "post carbon" canister plumbed in after the RO membrane to add some TDS back in for palatability (taste) for human consumption. I was getting higher TDS readings than I expected. I ended up changing the order of the canisters to have two carbon filters plumbed consecutively before the RO membrane.

What unit are you using exactly? Unless you have a situation like the one I described above (with a "post carbon" canister) then something is amiss with your RO unit.

RO on it's own won't bring to zeo, that's why the di follows.

I just checked my water, 5 after the ro, then 1-2 after the di but then di last changed in July and haven't cal'd my meter so not getting worked up.

Have also noticed a rise in TDS after sitting and seen few other post the same.

So would you say that's normal or harmful?

not ideal since the idea of ro/di is to bring TDS to zero, not the end of the world as there's tanks out there using tap water.

I know I wouldn't be satisfied knowing I paid a couple hundred bucks for an RO/DI unit and I was still getting TDS.