The TFC [Thin Film Composite] is the last filter before the
Di in most setups and last on set ups without
DI.
It is the filter usually in the non clear housing sitting horizontal on top of unit...
It is the most expensive of filters and should last 2-3 years if you care for the filters before it.
The
Di expires the fastest of all media used..
I like the BRS color changing
DI media as just one look tells you where it is at..It starts blue and dies yellow....
TDS will 'migrate' through the TFC when the unit is off..This is called osmosis as the water seeks equilibrium inside the unit without pressure...
My
Tds is350 from source.When I open my
ro the
TDS meter [built in BRS] it goes up to 200 sometimes [quickly] and then returns to 10-20 in under 30 seconds..I have installed a T after the
TDS sensor and discharge the first bit of water until it gets back to low
TDS..
I use a lot of
RO water without
DI for my fresh water breeding efforts..I use the
DI[have BRS dual
DI] for my marine tank.It is always zero or I change one of the
DI units..