Questions about my RO unit

Back in February we purchased an Apex 100 5 stage RO unit. I plumbed a Tee between the RO membrane output and the storage tank so that I could get water without it passing through the final filter (polishing / taste). When I bought the unit the web site claimed to have a 95% rejection rate. Going to the website now it states 85-95 % rejection rate. Now I could have over looked this fact, but I don’t think I would have. At any rate, I tested the water last night and again this morning and here are the results.



Last night

Tap TDS 58

RO TDS 20

Didn’t check temp

1-(20/58) = 65.5% rejection rate



This morning

Tap TDS 49

RO TDS 17

Temp 70 deg / dropped to 68 deg after running several minutes.

1-(17/49) = 65.3% rejection rate



I’m assuming these differences are due to temp variations. Regardless, the rejection rate seems to be consistent.



We are on well water and the water pressure is 45 on / 60 off.



Could there be something in the tap water that the RO will not remove that the TDS meter will register on? Could the low rejection rate be due to the temp or the water pressure of the tap water? Or is it just a bad RO membrane?