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What Is A Swichgage®?



murphy pressure swichgage

Swichgage® is the registered trademark on FW Murphy's best known line of instrumentation. As the name implies, a patented visible and adjustable wiping electrical contact, or switch, is added which combines monitoring, control and automation all in one simple instrument. This page shows images of several of Frank Murphy's early designs to illustrate the original Swichgage® limit contact concept.


The switch is set to trip when temperatures, pressures, or other variables measure above or below acceptable levels. When the switch trips it activates an alarm and/or a shutdown device, thereby helping you protect your engines and equipment from damage. These electromechanical control devices offer easily adjustable, single-pole contacts or snap-acting SPDT switches.

murphy pressure swichgage

murphy pressure swichgage

In 1940 Frank W. (Pat) Murphy started building and distributing the "Guardian Safety Switch", which was originally designed to protect oil pipeline pumps. In the early 1960's the "Safety Switch" was officially renamed the "SWICHGAGE®". Over the years, many minor adjustments were made to original design in order to accommodate different applications and markets.


On a contemporary Swichgage®, such as Murphy's 20P-50 pressure gage, the gage indicating pointer acts as a pressure indicator and as one switch pole which completes a circuit when it touches the adjustable limit switch contact. The contacts are grounded through the Swichgage® case and have self-cleaning motion to ensure electrical continuity.

murphy pressure swichgage

murphy pressure swichgage

Engines have life signs, such as pressures, temperatures, levels, and overspeed conditions. These vital signs can be monitored by a Swichgage® instrument which constantly monitors pressures, levels, vibration, temperatures, and overspeed conditions and activates alarm and/or shutdown when preset limits are reached. These protective instruments override the normal operation, shutdown the engine, and lock-out further automatic operation should failures occur in any of the protected functions.


The Swichgage® is a simple design. It's easy to understand - and it works!