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| Glossary of Terms Accuracy - The limit of deviation from the set point of the switch. It is normally defined in either points 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 between the lowest or highest point, within which the switch can be set. It is normally expressed in PSI, inches of mercury or inches of water column. 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 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. Hysteresis - The difference in pressure or vacuum switch response to increasing and decreasing pressure or vacuum. 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. 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 or mercury or 408 inches or water, above absolute "0" ("0" PSIA). Pressure, Barometric - Actual atmospheric pressure in a specific location and altitude. The standard is 29.22 inches of mercury at sea level at 70°F. 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 or the pressure switch that moves with a change in system fluid pressure. Pressure Switch - An instrument that converts a pressure change to an electrical function. 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 returns to its original position. Reset Point Range - 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 - 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 inches of water column. 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 inches of water column. Switching Current, Maximum - The maximum amperage load that the electrical switch will carry. Temperature, Ambient - 21° C or 70° F. Also know as "room temperature". Temperature, Shift - A change in switch set point due to changes in surrounding temperature. Vacuum - Gauge pressure less than ambient pressure using ambient pressure as a reference. |
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