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| Industrial Pressure Switch (Ashcroft) |
Normal status of a switch is the resting state with stimulation. A pressure switch will be in its “normal” status when it senses low or minimum pressure. For a pressure switch, “normal” status is any fluid pressure below the trip threshold of the switch.
One of the earliest and most common designs of pressure switch was the
bourdon tube pressure sensor with mercury switch. When pressure is
applied, the bourdon tube flex's enough to tilt the glass bulb of the
mercury switch so that the mercury flows over the electrical contacts,
thus completing the circuit. the glass bulb tilts far enough to cause
the mercury to fall against a pair of electrodes, thus completing an
electrical circuit. Many of these pressure switches were sold on steam
boilers. While they became a de facto standard, they were sensitive to
vibration and breakage of the mercury bulb.
Pressure switches using micro type electrical switches and force-balanced pressure sensors is another common design. The force provided by the pressure-sensing element against a mechanical spring is balanced until one overcomes the other. The tension on the spring may be adjusted to set the tripping point, thus providing an adjustable setpoint.
One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to re-set the switch to its normal state after it has tripped. The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.
When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).
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| Pressure Switch Symbols |
Pressure switches using micro type electrical switches and force-balanced pressure sensors is another common design. The force provided by the pressure-sensing element against a mechanical spring is balanced until one overcomes the other. The tension on the spring may be adjusted to set the tripping point, thus providing an adjustable setpoint.
One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to re-set the switch to its normal state after it has tripped. The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.
When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).
