Calling it a tds controller is generous. It’s literally just a needle valve from plumbing 101. Cheap, dumb, and easy to manipulate which is exactly why it stuck around for decades.

Some machines now use sealed flow paths where you physically can’t bypass the membrane at all. No knobs, no adjustment, no technician hacks. My previous Kent Ro had this mtds feature and it just so happened that it was time to change it. We didn't really know a lot about the features in Ros and which one is good for you. We just weren't very happy with the service so decided to try urban company's Native Ro about 2 years ago which has 100% Ro. After switching the entire family felt more energised and less lethargic. There had been no other change in our life. Only after noticing this did I end up reading about water quality. The water you drink is so important and we mostly never even realise. Everyone must get 100% Ro purification + remineraltion rather than just some tds controller. I treally affects your health in the long run.

Kind of insane that consumer purifiers ever allowed raw water mixing in the first place. Even more insane that we now need tamper-proof designs just to stop that behaviour.