Absolute Pressure:
Pressure scale based on PSIA "0". The sum of atmospheric pressure and gauge pressure.
Accuracy:
Comparison of actual output of a sensing device to the actual value of the input pressure. Errors may be attributed to hysteresis, repeatability, switch differential, and temperature shift. Errors are usually expressed as percent of full scale output (FSO).
Actuation:
Refers to the action of operating (or releasing) a switch mechanism.
Actuation Set Point:
When a pressure sensing element is used to actuate and deactuate a snap-acting electrical switch: the point at which the electrical switch is operated on increasing pressure (see Deactuation Set Point).
Actuation Value:
Difference between actuation and deactuation set points.
Actuator:
The mechanical link that moves switch contacts together or apart.
Adjustable Range:
Difference between lowest and highest adjustable actuation or deactutation set points (see Set Point Range).
Ambient Pressure:
Pressure of environment immediately surrounding a pressure sensing element.
Ambient Temperature:
Temperature of environment immediately surrounding a pressure sensing element.
Atmospheric Pressure:
Pressure caused by the weight of the atmosphere. At sea level, equal to 14.7 psi, 30" Hg, or 408" water above absolute zero PSIA. Equal to "0" PSIG.
Barometric Pressure:
Atmospheric pressure in a specific location and altitude. Standard is 29.92" Hg, at sea level, and at 70 degrees F.
BFSL:
Best Fit Straight Line: a measurement of sensor accuracy, showing average deviation from full scale output (see FSO).
Break Distance:
The minimum distance between separated mating contacts in their fully open position.
Burst Pressure:
Pressure sufficient to rupture the pressure sensing element, but not the sensor case.
Calibration:
Modifying sensor response to alter sensor range or improve sensor accuracy.
Contact Erosion:
Loss of material from one or both surfaces of a pair of mating contacts, as a result of switching an electrical load.
Contact Force:
The force holding closed contacts together.
Contact Resistance:
Electrical resistance of the pair of closed contacts only. This term is often used erroneously, when referring to the total resistance of a switch (see Switch Resistance).
Contacts:
The elements of a switch which move together or apart to mechanically complete or interrupt an electrical circuit.
Cross-point Contacts:
Contacts employing prism shapes which close by touching at right angles, rather than by touching flat parallel surfaces. Cross-points increase reliability for low voltage circuits, and can extend reliable switch duty to millions of operations. Cross-points also optimize contact pressure, and are less susceptible to closure interference by foreign matter.
Deactuation Set Point:
When a pressure sensing element is used to actuate and deactuate a snap-acting electrical switch: the point at which the electrical switch is operated on decreasing pressure (see Actuation Set Point).
Deadband:
Area from slightly above "0" gauge pressure to slightly below, within which sensor travel is not sufficient to reliably actuate a mechanical switch.
Dead Break:
Imperfect action of a snap switch, in which the normally closed circuit opens before the plunger reaches the operating point, or the normally open circuit opens before the release point.
Dead Make:
Imperfect action of a snap switch, in which the circuit fails to close when the plunger reaches the operating or release point.
Deadweight Scale:
A calibration instrument, employing a weighted scale, which uses gravity to adjust the accuracy of a pressure gauge.
Diaphragm:
The membrane of flexible material [polyurethane, silicone, Viton(R), Buna, fluorosilicone, EPDM, or Teflon(R)] which is deflected by input pressure, comprising the sensing element of a pressure, vacuum, or differential switch.
Differential Error:
Variance in switch response due to mechanical motion lost within a switching element during actuation, or during reversal to normal position. Usually greater in high-amperage switches.
Differential Pressure:
Input from two independent pressure sources. Output is proportional to the pressure difference between the two input sources.
Differential Travel:
Reference to a snap-action switch: the distance from the operating point to the release point.
DPDT:
Double pole, double throw: A switch having two independent circuits operated by a single actuator. DPDT switches typically contain two common, two normally open, and two normally closed terminals.
Drift:
Undesirable change in output over time which is not a function of any change in input pressure.
Dry Circuit:
An imprecise reference to almost any low energy circuit. There is no general agreement as to what current or voltage values constitute a dry circuit.
Electrical Interference:
Undesirable voltage or current fluctuations that are conducted to or radiated to a sensor or control.
Factory Calibrated:
A sensor which is not designed for field adjustment of set points, or on which calibration adjustments have been disabled, or made resistant to tampering.
Field Adjustable:
A sensor which has been designed to permit adjustment or calibration of set points in field application.
Fluid:
In engineering terms, a liquid or gas which tends to conform to the shape of its container, and which alters its shape in response to applied force.
Free Position:
The position at which the plunger of a precision snap-acting switch is completely released, with its common contact at rest against its normally-closed contact (see Pretravel).
Gauge Pressure:
A form of differential pressure measurement which uses atmospheric pressure (14.7psia) as the zero reference.
Gold Contacts:
Gold switching elements provide high corrosion resistance and high reliability when switching low voltage circuits.
Gravity Unit:
Unit of measurement of acceleration, abbreviated by the letter "g", equal to 32.2 feet per second per second..
Hermetic Seal:
Often misused in reference to any pressure seal, specification of a hermetic seal actually refers to a seal capable of excluding ten to the minus 9th cubic centimeters per second of helium.
Hysteresis:
The maximum difference in output observed, at any measured value within a specified range, when the value is approached with increasing pressure, and then decreasing pressure. Hysteresis is usually expressed as a percent of full scale output (FSO).
Impedance:
In a circuit, the opposition to flow of an alternating current, consisting of ohmic resistance, inductive reactance, and capacitive reactance.
Inductive Load:
Load from electrical devices which are made of wound or coiled wire. Current passing through the windings creates a magnetic field which produces mechanical work. When an inductive circuit is switched open, energy stored in the coil can reverse flow, sparking at the switch contact surfaces.
Manometer:
A device used to measure pressure in force per unit area (pounds per square inch, etc.).
Maximum System Pressure:
Rated pressure above the normal system pressure, including surges or spikes.
Maximum Switching Current:
The maximum current load or amperage an electrical switch is capable of carrying.
Normally Closed:
Current flows through the switch until pressure or vacuum Switching Element actuates the switch, opening the circuit.
Normally Open:
No current flows through the switch until pressure or vacuum actuates the switch, closing the circuit.
Null:
Condition when pressure on both sides of pressure or vacuum sensing element are equal.
Operating Force:
In reference to a snap-action switch: the force which must be applied to cause the moving contact to snap from the normal contact position to the operated contact position.
Pressure Range:
Minimum and maximum pressure for which a sensor has been calibrated or specified.
Pressure Switch:
A device which converts pressure change into an electrical function.
Pretravel:
In reference to a snap-action switch: the distance of button or actuator movement from free position to the point of switch operation.
Proof Pressure:
Maximum pressure which can be applied to a switch without harming its operating characteristics. Usually 150% of maximum system pressure.
PSIA:
Pounds per square inch (absolute): a measure of pressure in psi that is referenced to zero absolute pressure.
PSIG:
Pounds per square inch (gauge): a measure of pressure in psi that is referenced to ambient pressure.
Repeatability:
The ability of a switch to repeatedly actuate at the desired set point within sensor tolerance.
Reset Point:
The point at which an electrical switching element returns to its normal operating position
Reset Point Range:
Difference between set point and reset point. Range is determined by hysteresis of the sensing element, and differential of the electrical switch.
Resistive Load:
Load from devices which use electrical resistance to produce heat or light. Resistive loads cause current to flow in only one direction through a circuit.
Response Time:
Time elapsed between a change in system pressure and a corresponding change in switch position.
Sensing Element:
The portion of a pressure switch which directly responds to changes in input pressure (see Diaphragm).
Set Point:
The point at which motion of the pressure or vacuum sensing element causes the switch to function.
Set Point Range:
Range within which a switch can be set from lowest set point to highest set point (see Adjustable Range).
Snap-Action Switch:
A mechanically operated electric switch which, once its actuator has reached its operating (or release) point, immediately transfers to its opposite position without further travel of the actuator..
SPDT:
Single pole, double throw. SPDT switch contains one common, normally open and normally closed terminal.
Switching Element:
Opens or closes an electrical circuit in response to movment of the pressure or vacuum sensing element.
Switch Resistance:
The total resistance of the conducting path between the wiring terminals of a switch.
System Pressure:
Normal system pressure, without surges or spikes.
Temperature Shift:
A change in set point due to temperature change.
Transducer:
A device which generates an electrical signal from changes in physical measurments.
Triac:
Bidirectional triode thyristor: a solid state gate switch which enables low current controls to switch high current. Closing the gate causes the triac to conduct current. Opening the gate causes the triac to block current.
Typical:
Expected value of a given parameter. Error can be expected to be no more than 1 standard deviation.
Vacuum:
Pressure less than "0" PSIG.
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