for Restaurant Applications
http://www.merl.com/papers/docs/TR2002-21.pdf
capacitance type
http://www.discovercircuits.com/PDF-FILES/capgage.pdf
http://www.discovercircuits.com/PDF-FILES/h2oalm.pdf
with a 1 GHz Transmission Line Probe
http://www.forestprod.org/drying02_ahmet.pdf
The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The
http://www.discovercircuits.com/PDF-FILES/dust2.pdf
When the dust reaches a particular level sufficient light is reflected into the phototransistor to change the logic state of the circuit
http://www.imagineeringezine.com/PDF-FILES/dust2.pdf