Mercury-free switches

June 2, 2005
Mercury switches have long served as tilt sensors.

Their operating principle Tilting the device past a threshold causes the conductive mercury to bridge between two conductors, thus making a connection.

An international push to eliminate heavy metals from the waste stream broadened the use of devices that behave like mercury switches without employing mercury. Such components have been available since 1970s but until recently were confined to uses that couldn't tolerate mercury leakage.

usually simulate the action of mercury through use of a metal ball, a damping fluid, and two or more contacts. The usual technique has a metal ball either making or breaking a connection between two metal conductors when the switch tilts. The ball generally rolls off the case and onto the contacts to make the connection. A viscous dielectric fluid sealed in the case is specified so ball movement mimics the switching action of an equivalent mercury device.

The dielectric fluid is generally a silicone compound. The exact formulation depends on the expected operating environment. For example, switches targeting outdoor applications may contain a fluid with less viscosity than those used indoors to ensure proper operation in cold temperatures.

The silicone fluid mainly serves to minimize chatter that arises naturally when the metal ball touches the metal contacts of the switch. Ordinary mercury switches experience no chatter because the mercury provides a wetting effect.

The rolling action of the ball gives mercury free devices a switching with hysteresis. The result is typically a 5° difference in tilt angle for opening closing. But sophisticated versions of these switches may incorporate contacts and special vectoring of the ball path to compensate. The hysteresis of about 1°. One use for such switches is in buoys containing that must transmit only when the antenna is pointing up and out of the water.

Typical specifications for mercury-free tilt switches allow for currents of about 50 mA and contact resistance on the order of 10 . Operating life is long enough to be essentially infinite under normal operating conditions.

Aerodyne Controls Inc. (aerodyne-controls.com) provided information for this article.

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