New meter for RO water quality

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.