RO/DI flow restrictor question

[AZDesertRat]

You are at 93% rejection rate. you want to be at 98% or better.
Your DI is definitely shot so is nothing more than a wide spot in the pipe at this point.
I would always use DI, there are things that RO bys itself are not particularly good at removing like silicates, nitrates and phosphates among others. Its cheap insurance.

With a new membrane such as Spectrapures hand tested Select series you should be able to get your RO only TDS down to 2 or 3 or less but DI is still worth the $18-$25 or so and it will last almost forever. My Phoenix tap TDS is 630, my Select membrane gets that to between 2 and 3 TDS using a handheld COM-100 TDS meter and my DI lasts over 1000 gallons per cartridge.

You need to know your waste ratio also, when you buy a new Spectrapure membrane they suggest you get a new capillary tube flow restrictor which comes with directions on how to calculate your waste ratio and trim the capillary tube for your exact measured conditions. I check mine every so often to make sure things are not changing.

Something interesting I forgot to mention is, for every 2% you increase your RO rejection rate, you double the life of your DI resin. Imagine how long DI will last if you improve your rejection frpom 93% to 99+%, thats a 6% improvement which means the DI will last probably up to 7x longer than without the changes. More than pays for a RO membrane every several years in DI savings alone.