Jobo Process Timer, Jobo Processor and "sensor"

There is a Jobo CPP setup for sale (maybe the person is on here? ) http://sfbay.craigslist.org/nby/pho/4345154002.html

The description includes this sentence "The JOBO Process Timer 16 is programmed for B&W, color negative (C-41) and transparency, i.e. slide (E-6) processing and includes a sensor that detects each step for accurate processing."

So... What would be the "sensor" and how would it detect each step?

Probably detects raising and lowering of the lift.

The process timer 16 came with an optional accessory, which was glued to the lift on the rear, and a small magnet was glued to the lift arm. Every time the lift arm is lowered and raised the sensor notices it and you can program the steps according to your needs. (in the photo of the timer in that listing the sensor module is in a little plastic baggie).

Its a nifty little gadget. Just watch out for those times you need to drain the tank twice....

I need a sensor that can tell when I'm about to grab the wrong chemical...

Thats called an ATL