Testing RO Rejection Rate - Where to test the tap water TDS

[ElmoC]

I am getting ready to modify my 6 stage RO/DI unit by adding a second dual TDS unit (Got it really cheap). I currently have a dual unit with a tee before the first DI canister (right after the membrane) and another after the second DI. I was thinking of putting a tee between the two DI units so I can tell when that one has been exhausted and was thinking of putting the other tee to measure the tap water TDS. Just wasn't sure if it is better to put it before the prefilters (sediment/carbon block) or after to get the best measurement for RO rejection.

[jedimasterben]

Put the measurement leads before and after the RO membrane, the second set of leads after each DI resin.

 

Still best to just tee off with a ball valve and measure with a handheld meter, they're more accurate.

[ElmoC]

But that is not as much fun as having a control panel of lights and numbers to look at.

[jedimasterben]

I feel ya

[AZDesertRat]

Honestly, I have two dual inline meters on my RO DI and never even look at them since they lack accuracy and are not ATC temperature compensated. Instead I plumbed a tee and ball valve at each location I want to sample and use my handheld meters which are ATC compensated and much more accurate, especially on the treated RO and RO/DI waters.