Pressure Switches
Pressure Switch is a form of switch that closes an electrical contact when a certain set pressure has been reached on its input. This is used to provide on or off switching from a pneumatic or hydraulic source. The switch may be designed to make contact either on pressure rise or on pressure fall.Pressure switch is an essential component of air mover systems is continual operational reliability. Failure of the system can lead to contamination of optical instruments. To ensure consistent functionality, a pressure switch can be used to indicate air failure when flow drops below a pre-set value. Under undesired flow conditions, an alarm will be activated or protective shutters will go into effect.
Working Principle
Pressure switches are actuated by a change in the pressure of a liquid or gas. They activate electromechanical or solid-state switches upon reaching a specific pressure level. The pressure at which an electric circuit turns on or off is called set point of switch. The most important parameter to consider is the pressure range that is needed to be switched at, or the range over which linear output is needed. A Bourdon tube or bellows can be used to actuate the switch. The pressure is fed inside the bellows which causes the contact of the plate with the circuit and current flows which either on the alarm or send signals to control center.
Pressure switch can be modified so as to make a low pressure contact, In addition to a high pressure contact. It is adjustable. The contacts in a pressure switch may be normally closed when the pressure is below the set point for example; the contact in a normally open switch remains opened until the pressure rises above the set point. Then the sensing element makes the contacts snap to close position. The contacts open again when the pressure falls below the set point. The contacts in a normally closed switch remain closed until the pressure rises above the set point. Then the contacts snap open and remain open until a pressure drops below the set point again. Most switches contain the two sets of contacts, one normally open and other normally closed. Choices for pressure switches measurement ranges include absolute, gauge, vacuum, differential, and sealed. Absolute switches are used where pressures are to be measured independently of the natural fluctuations in atmospheric pressure. The pressure of the media to be measured is compared against a reference pressure of absolute zero in a sealed reference chamber. Vacuum measurement switches measure vacuum pressure . Differential pressure switches give the relative pressure between two points. If both operating pressures are the same, the measuring element cannot move and no pressure will be indicated. A differential pressure is indicated when one pressure is higher or lower. Low differential pressures can be measured directly in cases of high static pressures. Sealed gauge pressure measurement is similar in concept to an absolute Pressure switch, except that the pressure of the media to be measured is compared to standard atmospheric pressure.
Examples
Pneumatic
Uses of pneumatic pressure switches include:
- Switching off an electrically driven gas compressor when a set pressure is achieved in the reservoir
- Switching off a gas compressor, whenever there is no feed in the suction stage.
- In-cell charge control in a battery
- Switching on or off an alarm light in the cockpit of an aircraft if cabin pressure is critically low.
Hydraulic
Hydraulic pressure switches have various uses in automobiles, for example:
- To switch on a warning light if engine oil pressure falls below a safe level
- To switch on brake lights automatically by detecting a rise in pressure in hydraulic brake lines
- In dust control systems , a pressure switch is mounted on the header which will raise an alarm when air pressure in the header is less than necessary to gain or decline energy beyond the set value
- To control automatic transmission torque converter lock-up