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Glossary of Switches

ACCURACY - The limit of deviation from the set point of a pressure or vacuum switch. It is usually defined in either pounds per square inch, or percentage of full scale.
ACTUATION POINT - See SET POINT.
ACTUATION VALUE - The difference between the set point and the reset point.
ADJUSTABLE RANGE - The range within which the switch can be set from the lowest to the highest point, usually expressed in PSI, inches of mercury or PSIA.
BUOYANT FORCE - A body submerged in fluid is pushed or buoyed up by a force equal in magnitude to the weight of the displaced fluid. Buoyancy is dependent on both weight and shape of the float.
CRUSH PRESSURE - The maximum pressure to which the floats may be exposed without deformation. Tanks containing liquids are frequently pressurized. Crush pressure varies with the materials of construction, wall thickness, shape and desired density.
DEADBAND - The difference between the increasing and decreasing readings when the switch is operated between set point and reset point.
DIFFERENTIAL - The mechanical motion lost within the electrical switch element while it reverses itself. This is usually greater in high amperage switches than in low amperage switches.
ELECTRICAL RATINGS - The reed switches are specified as VA (Volt Amps) or Watts. See Table A.
ELECTRICAL SWITCHING ELEMENT - Opens or closes an electrical circuit in response to movement from the pressure or vacuum sensing element. Single pole, double throw (SPDT) snap action switches are standard, may be used as single pole, single throw (SPST). NO/NC circuitry is selectable, but it must be specified at order time.
FLOAT - The liquid level sensor, the portion of the level switch that rises and falls with the changes in the level of a liquid. The float contains the magnet used to operate the reed switch. It is made of various materials and densities to achieve a material compatibility and to be able to float in liquids with various specific gravities.
FORM A SWITCH - A single pole single throw electrical switch - the preferred electrical switch for liquid level devices.
FORM C SWITCH - A single pole double throw electrical switch.
HYSTERESIS - The difference in pressure or vacuum switch response to increasing or decreasing pressure or vacuum.
INTERFACE - The surface between two liquids that have different Specific Gravities, e.g. oil floating on water.
INTERFACED - A float whose buoyancy has been adjusted to float at the interface of the two liquids that have different Specific Gravities.
LIQUID LEVEL SWITCH - An electromechanical device that opens or closes an electrical circuit in response to a change in the level of a liquid.
LIQUID LEVEL SWITCH OPERATION - A float containing a permanent magnet riding on the surface of a liquid. The motion of the float is guided by a stem. The stem contain a reed switch that is actuated by the magnet in the float.
MOISTURE PROTECTION - Our liquid level switches are sealed with potting compound. The only path for liquid to the electrical switch would be through the wires. If the wires are terminated in an appropriate manner (e.g. - NEMA VI connectors), the level switch will meet or exceed NEMA VI.
NEMA VI - A device suitable for submersion.
NORMALLY CLOSED SWITCHING ELEMENT - Current flows through the switch until it is broken by a pressure or vacuum change.
NORMALLY OPEN SWITCHING ELEMENT - No current flows through the switch until contact is made by a pressure or vacuum change.
PRESSURE, ABSOLUTE - A pressure scale based on PSIA "0" or a perfect vacuum.
PRESSURE, AMBIENT - The pressure immediately surrounding a pressure switch. It is usually, but not necessarily, atmospheric gauge pressure.
PRESSURE, ATMOSPHERIC - The pressure caused by the actual weight of the earth's atmosphere. At sea level, atmospheric pressure equals 14.7 psi, 30 inches of mercury, or 408 inches of water, above absolute "0" ("0" PSIA).

PRESSURE, BAROMETRIC - Actual atmospheric pressure in a specified location and altitude. The standard is 29.22 inches of mercury at sea level at 70єF.
PRESSURE, DIFFERENTIAL - The difference between a reference pressure and a variable pressure.
PRESSURE, GAUGE - Uses atmospheric pressure as a zero reference point so there is no compensation for changes in barometric pressure.
PRESSURE, MAXIMUM SYSTEM - System pressure, including surges or spikes.
PRESSURE, PROOF - The maximum pressure which can be applied to a pressure switch without causing irreparable damage. It is usually 150% of the pressure sensing element's rated maximum system pressure.
PRESSURE, SYSTEM - Normal system pressure level not including surges or spikes.
PRESSURE SENSING ELEMENT - The portion of the pressure switch that moves with a change in system fluid pressure. Whitman Controls pressure switches employ capsule, diaphragm, and piston sensing elements.
PRESSURE SWITCH - An instrument that converts a pressure change to an electrical function.
REED SWITCH - A hermetically sealed magnetically operated electrical switch.
REPEATABILITY - The ability of the switch to actuate repeatedly at the desired set point within sensor tolerance.
RESET POINT - After the pressure has reached set point and operated the electrical switch, it must return to the reset point before the electrical switch can return to its original position.
RESET DIFFERENTIAL - The difference between the set point and the reset point. It is caused by the hysteresis of the pressure or vacuum sensing element and the differential of the electrical switch. This is a fixed function of the switch and is not adjustable.
RESPONSE TIME (REACTION TIME) - The amount of time taken between a change in the pressure of the system, and the change in the electrical signal.
SET POINT - The exact point at which the electrical switching element functions. This is generally expressed in PSI, inches of mercury, or PSIA.
SET POINT RANGE - The range within which the switch can be set from the lowest to the highest point, usually expressed in PSI, inches of mercury, or PSIA.
SPECIFIC GRAVITY - The ratio of the mass of the liquid to the mass of an equal volume of water at a standard temperature, typically 60єF. Liquids with a Specific Gravity less than one will float on water. Those with a Specific Gravity greater than one will sink in water.
WORKING PROOF - Maximum pressure which can be applied to a presure switch without causing set point shift.


Table One - Typical Specific Gravities
Acetone .791
Alcohol, ethyl .789
Alcohol, methyl .796
Antifreeze 1.05 - 1.13
Benzene .899
Hydraulic Oil .900
Gasoline .66 - .69
Glycerine 1.26
Kerosene .82
Mineral Oils .88 - .94
Muriatic Acid 1.20
Nitric Acid 1.50
Sea Water 1.02 - 1.03
Sulphuric Acid 1.80
Vegetable Oils .91 - .94
Water 1.00

Note: Specific Gravities can change significantly with temperature.

STEM - The portion of a liquid level device about which the float slides. It contains the reed switch, and provides the exit from the tank for the electrical wires. They are made of various materials to achieve material compatibility.
SWITCHING CURRENT, MAXIMUM - The maximum load (amperage) that the electrical switch will carry.
TEMPERATURE, AMBIENT - The temperature immediately surrounding a pressure switch.
TEMPERATURE SHIFT - A change in switch set point due to changes in ambient temperature.
VACUUM - Gauge pressure less than ambient pressure using ambient pressure as a reference.

Whitman Controls Corporation 201 Dolphin Road, Bristol CT 06020 Phone: 800-233-4401, Fax: 860-583-5293