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  Float Switch Containing Mercury

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Author Topic:   Float Switch Containing Mercury
sternorama posted 10-12-2009 05:33 PM ET (US)   Profile for sternorama   Send Email to sternorama  
I guess [manufacture of] float switches [which contain Mercury] has been discontinued, but I found more than one on a local shelf. I bought one for myself, and I suppose, if anyone else wants one, I could see if [more] are still there. In the context of a general discussion of the various float switch technologies out there. is there anyone who really preferred the original types containing Mercury?

By the way, the one I replaced had a rolling metal ball inside the floating arm. You could hear it move, and the switch tested okay on an ohmmeter, but wouldn't allow enough current to pass through to run the motor of the bilge pump. Turns out testing a switch for just continuity and resistance doesn't clear it for service. Anyone else been down this road?

sternorama posted 10-19-2009 03:35 PM ET (US)     Profile for sternorama  Send Email to sternorama     
cricket... cricket...
jimh posted 10-20-2009 01:45 AM ET (US)     Profile for jimh  Send Email to jimh     
Switches or relays using Mercury were common. Use of Mercury to wet the contacts was generally intended to help decrease contact resistance, which can be particularly important in low voltage circuits. A common example is a thermostat control where the operating voltage is generated by a thermocouple and pilot flame of a furnace. The circuit voltage is very low and no voltage drop in the contacts can be tolerated. Use in a thermostat may also have been intended to add hysteresis to the control circuit by shifting the weight of the Mercury in relation to the bi-metal actuator of the thermostat. This mechanical technique causes the on-circuit and off-circuit temperature points to move apart, permitting better control of the furnace.

A float switch requires a similar hysteresis for it to be effective in controlling the pump. The rolling ball bearing replaced the moving Mercury as a method to change the weighting of the float switch, resulting in the pump-on and pump-off levels to be different. This keeps the pump from chattering on and off.

jimh posted 10-20-2009 01:50 AM ET (US)     Profile for jimh  Send Email to jimh     
Also see

Designing Automatic Pump Control Systems
http://continuouswave.com/ubb/Forum6/HTML/001963.html

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