To test the rejection rate of a RO membrane you need to test the tap water TDS and the RO only TDS. Let the system run several minutes to clear the TDS creep the get your readings. I much prefer a good handheld meter as the dual inlines are ballpark accurate since they are not truly temperature compensated, plsu they are limited to two places unlike the handheld that can test anywhere.
Take the tap TDS, subtract the RO only TDS, divide that number by the tap TDS then multiply by 100. Example: Say you have a tap TDS of 250 and RO only TDS of 10. 250-10=240, 240/250=.96, .96x100= 96% rejection rate or removal efficiency. 96% is borderline acceptable depending on your TDS since for every 2% you can increase the rejection rate you DOUBLE the life of your DI resin. Spectrapure membranes are better than 98% rejection due to the treatment they receive then the quality control testing they go through unlike normal off the shelf dry untested membranes.
If you have more than one dual inline TDS meter then you want the IN probe of the first meter either on the incoming tap water or after the carbon block, it really does not matter since the sediment and carbon have little to no effect on TDS, they remove suspended solids and chlorine only. The OUT probe then should be after the RO membrane so you can do the above test. The second inline meter should have the IN probe after the MaxCap DI and the OUT probe after the SilicaBuster DI so you can monitor both DI cartridges conditions seperately. You should go through about three MaxCap cartridges before you start to see any evidence of TDS after the SilicaBuster DI and replace it the first time.
What I have done on my MaxCap UHE system is place a John Guest tee and ball valve with a short piece of 1/4" tubing after the carbon block, after the membrane and after the MaxCap cartridge so I can draw samples and test the TDS with my handheld TDS meter for greater accuracy. It never agrees with the inline meters so I rarely even turn them on anymore except as a quick litmus paper type ballpark test.
If you are replacing filters you will want the 0.5 micron absolute rated sediment filter, the 0.5 micron carbon block, a 90 GPD SpectraSelect series hand tested membrane with matched capillary tube flow restrictor and if you need DI possibly a MaxCap and a SilicaBuster or the new Super series replacements for a couple $$ more. I would not order DI replacements if you do not expect to need them soon as they have a shelf life like all DI does.
The 90 GPD starts life as a Dow Filmtec 75 but they treat it with a special proprietary process to improve both its GPD and its rejection rate then package it wet in a special mylar foil bag. The regular 90 is then batch tested for quality control but the Select series for only $5 more is individually 100% hand tested, the results are written on the membrane as well as the testers initials and it is guaranteed in writing to be better than 98% rejection rate. My personal Select membrane is still producing 99.43% rejection rate after 4 years of usage and thats with a tap TDS over 600.
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