
The 'inlet' probe is behind the di vessel so it can't be seen. It on the yellow hose that is the pure outlet from the r/o. It registers the rejection rate of the r/o.
The second probe is after the di vessel and can be clearly seen. The probe is fitted exactly the same way on the hose before the di. The hose goes off into my IBC tank.
The TDS meter has an on/off button and then two buttons to switch between inlet and outlet. It stays on for a time (not sure how long) and then will switch off, so saving batteries.