different liquid detection sensor
i am building a biodiesel production system. at different points in the process, a tank will have different liquids in it (sometimes more than 1 type, separated out). for instance, there is a point where glycerol sinks to the bottom, while biodiesel floats on top.
what i want to do is drain the glycerol from the bottom of the tank (via electronically controlled solenoid valve), then when a sensor detects a change in material (biodiesel), shuts off that valve and opens another (final product tank). for the life of me, i cannot find a sensor that will do this for less than $100. preferably, i'd like to spend less than $30.
i can work with a resistive type or voltage type sensor. i just need to be able to make that distinction of material change... any ideas?