Path: utzoo!attcan!telly!lethe!yunexus!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!cs.utexas.edu!yale!cs.yale.edu!corwin.eng.yale.edu!llw From: llw@corwin.eng.yale.edu (Louis L. Whitcomb) Newsgroups: comp.robotics Subject: Re: dirt cheap immersion sensor? Message-ID: Date: 26 Nov 90 21:20:01 GMT References: <1990Nov24.211134.28944@uunet!unhd> Sender: news@cs.yale.edu Distribution: comp Organization: Yale University, Center for Systems Sciences Lines: 39 Nntp-Posting-Host: corwin.eng.yale.edu In-reply-to: rg@uunet!unhd's message of 24 Nov 90 21:11:34 GMT Greetings: The switches used to control bilge pumps typically are mercury switches which, attached to a float, get tilted when immersed in water. The mercury completes the circuit, and can carry several amps of current no problem. You can get them for $15 to $30 depending on the brand. I use the "rule-a-matic" in my boat. Be warned: they have two problems... 1) they measure water presence indirectly (they actually measure displacement due to buoyancy) and will only function properly if mounted upright... 2) they tend to make intermittent contact ("bounce" in ee terms) when jostled around even when the water level is low (this is because the mercury sloshes around inside the switch.) More fancy bilge pumps use two different techniques: 1) they spin the bilge pump moter occasionally and measure the motor load (back emf and current, I imagine) and shut down when the pump cavitates (runs dry.) 2) they have an ultrasonic sensor to measure the level of the water in the bilge. Water seperating fuel filters have a "water sensor" consisting of two bare metal contacts. A small op-amp circuit can measure the resistance across the two electrodes to detect water. Perhaps you could be more precise in *what* you want to detect about water? By the way, what IS the "Marine Systems Engineering Lab" ? The Best, -Louis. -- ............................................................................ Louis L. Whitcomb llw@corwin.eng.yale.edu ph: (203) 432-4311 Yale Robotics Laboratory fx: (203) 432-7481 Department of Electrical Engineering, 1968 Yale Station, New Haven, CT 06520