Sounds like you are talking about a TDS meter. I did this a while back, and
don't remember exactly how I figured it out......

Test the tap water. Get a reading. Mine was 277
Test the RO water. Get a reading. Mine was 7
Test the DI water. Get a reading. Mine was 0

Take the RO reading and divide it by the tap water. In my case, that is 0.025.
Taking that number from 100, I get 97.5% rejection rate.

Or maybe that is just Marc's fuzzy math. grin

Marc, waiting to be corrected again....

Timothy Tom wrote:

I have a couple of RO units for my aquarium (automated top-off unit,
and unit in garage for water changes). One of the problems that I
have had is knowing when to change the RO membrane. I just got off
the phone with Spectrapure, and they just started to sell a Dual probe
meter which helps to quantify the efficiency of the RO unit (I was
told that the DI portion of water purification systems require a
resistivity meter which this meter does not measure). I was told it
is a digital battery-powered meter which measures contaminants in PPM
in the supply water and the outflow water (after purification).
Supposedly you can then easily calculate the rejection fraction which
is a direct indicator of the efficiency of the RO membrane. Anyway I
ordered two of these which cost 27 dollars each. These units are not
currently listed on their online catalog so you will need to ask for
them.

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