Mercury switch

A Single-Pole, Single-Throw (SPST) mercury switch on millimetre graph paper
Another mercury switch design

A mercury switch is a switch which opens and closes an electrical circuit when a small amount of the liquid metal mercury makes contact with metal electrodes to close the circuit. There are several different basic designs (tilt, displacenent, radial, etc.) but they all share the common design strength of non-eroding switch contacts.

The most common is the "mercury tilt switch". It is in one state (open or closed) when tilted one direction with respect to horizontal, and the other state when tilted the other direction. This is what older style thermostats used to turn a heater or air conditioner on or off.

The "mercury displacement switch" uses a 'plunger' dipping into a pool of mercury and thereby causing the level inside the container to rise to an elevated height where at least one electrode is present. This design is currently used as part of relays in industrial applications when high current loads need to be switched frequently. These relays use electromagnetic coils to pull steel sleeves inside hermetically sealed containers.

Description

Mercury switches have one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. The envelope may also contain air, an inert gas, or a vacuum. Gravity is constantly pulling the drop of mercury to the lowest point in the envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing the electrical circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts, thus breaking that circuit.[1] The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for example, single-pole, double-throw (SPDT) operation.

Advantages

Advantages of the mercury switch over other types are that the contacts are enclosed, so oxidation of the contact points is unlikely.

In hazardous locations, interrupting the circuit will not emit a spark that can ignite flammable gases.

Contacts stay clean, and even if an internal arc is produced, the contact surfaces are renewed on every operation, so the contacts don't wear out.

Even a small drop of mercury has a low resistance, so switches can carry useful amounts of current in a small size.[2]

The sensitivity of the drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic operation.

The switches are quiet, as there are no contacts that abruptly snap together.

The mass of the moving mercury drop can provide an over center effect to avoid chattering as the switch is tilted.

Multiple contacts can be included in the envelope for two or more circuits.

Disadvantages

Mercury switches have a relatively slow operating rate due to the inertia of the mercury drop, so they are not used when many operating cycles are required per second.[3]

Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damaging the envelope.

The mercury drop forms a common electrode, so circuits are not reliably isolated from each other if a multipole switch is used.

Mercury switches are sensitive to gravity so may be unsuitable in portable or mobile devices that can change orientation or that vibrate.

Mercury compounds are highly toxic and accumulate in any food chain, so mercury is not permitted in many new designs.

Uses

Roll sensing

Tilt switches may be used for a rollover or tip over warning for construction equipment and lift vehicles operating in rugged off-highway terrain. There are several non-mercury types, but few are implemented due to sensitivity to shock and vibration, causing false tripping. However, devices resistant to this do exist.

Automotive uses

Mercury switches were used in automobiles for lighting controls (for example, trunk lid lights), ride control, and anti-lock braking systems. Scrapped automobiles can leak mercury to the environment if these switches are not properly removed. These uses have been discontinued in new American-built cars since 2003.[4][5]

Fall alarms

Work performed in confined space (such as a welder inside a tank) has special labor safety requirements. Tilt switches are used to sound an alarm if a worker falls over.

Aircraft attitude indicators/artificial horizons

Electrically driven attitude indicators typically use mercury switches to keep the gyro axis vertical. When the gyro is off vertical, mercury switches trigger torque motors that correct the gyro position back to the correct position. Note that air driven attitude indicators have a different operating principle.

Thermostats

Mercury switches were commonly used in bimetal thermostats. The weight of the movable mercury drop provided some hysteresis by a degree of over-center action. The bimetal spring had to move further to overcome the weight of the mercury tending to hold it in the open or closed position. The mercury also provided a very positive on/off switching action and could withstand millions of cycles without degradation of the contacts.

Doorbells

Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interruptors.

Pressure switches

A pressure switch can be made with a Bourdon tube and a mercury switch; the switch can be reliably operated by the small force generated by the tube.

Vending

Mercury switches are still used in mechanical systems that are controlled electrically where the physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines that have "tilt alarms". When the machine is rocked or tilted in an attempt to gain a product, the mercury switch activates, sounding an alarm.

Bombs

Main article: Tilt fuze

A tilt switch can be used to trigger a bomb.[6][7] Mercury tilt switches can be found in some bomb and landmine fuzes, typically in the form of anti-handling devices, for example, a variant of the VS-50 mine.

Toxicity

Since mercury is a poisonous heavy metal, devices containing mercury switches must be treated as hazardous waste for disposal. Because it is now RoHS restricted, it has been eliminated in most modern applications. A metal ball and contact wires can directly replace it, but a method for reducing switch bounce may be necessary. Low-precision thermostats simply use a bimetal strip and a switch contact; precision thermostats use a thermistor or silicon temperature sensor. Low-cost accelerometers replace the mercury tilt switch in precision applications.

In the United States, the Environmental Protection Agency (EPA) regulates the disposition and release of mercury.[8] Individual states and localities may enact further regulations on the use or disposition of mercury.

See also

References

  1. ↑ Fraden, Jacob (2004). Handbook of Modern Sensors - Physics, Designs and Applications (3rd Edition). Springer - Verlag. pp. 256–257.
  2. ↑ Martin J. Heathcote, The J & P transformer book: a practical technology of the power transformer Newnes, 2007 ISBN 0-7506-8164-0 p.285
  3. ↑ David W. Pessen, Industrial automation: circuit design and components,Wiley-IEEE, 1989 ISBN 0-471-60071-7, page 44
  4. ↑ United States, Mercury study report to Congress, DIANE Publishing, 1997 ISBN 1-4289-0372-0, page 2-11
  5. ↑ Organisation for Economic Co-operation and Development, Instrument mixes for environmental policy OECD Publishing, 2007,ISBN 92-64-01780-1, pg.145
  6. ↑ Vallely, Paul (22 February 2002). "The Airey Neave Files". The Independent. London. Retrieved 12 May 2010.
  7. ↑ "1979: Car bomb kills Airey Neave". BBC News. 30 March 1979. Retrieved 26 March 2010.
  8. ↑ "Mercury: Laws and regulations". United States Environmental Protection Agency. 2008-04-16. Retrieved 2008-05-30.
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