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PRESSURE SWITCH

A pneumatic switch is a device that activates or deactivates an electrical signal in a system based on the level of air pressure detected by the component.

Now let’s look at the various types of pneumatic switches

Pneumatics Pressure Switch

A pressure switch opens and closes the electrical contact point when the pressure of compressed air reaches the designated value. It consists of the pressure receiving part, the electrical contact point that generates electrical signals, and the pressure setting part.

A pressure switch monitors the air pressure, and when it goes below or above the preset value, the monitor operates a micro switch to open and close the contact point to generate an electrical signal. Pressure switches are available in four types: diaphragm, bellows, bourdon-tube, and piston.

A pneumatics pressure switch is used for the following purposes:

  • It monitors the pressure of the pneumatic power source and generates alarm signals when the air pressure drops below the preset value, the pressure stops for safety purposes.
  • It detects the pneumatic pressure of the driver apparatus to generate a signal.
  • It detects the operation of pneumatic pressure. For example, it can be used to count the number of operations of a direction-control valve.

The pressure switch is mostly applied to control machinery that uses both pneumatic circuits and electrical controls as shown in Figure 1. Pressure switches have both differential and pressure characteristics. For instance, when you set a pressure switch’s air pressure to a certain value and the pressure exceeds that value, the air pressure will reduce back to that value. When differentia is too small, the precision of control increases, but the switch is turned on and off too often, the stability and controllability of the system is jeopardized. It is important to find the best value of differentia based on the conditions of control.

Currently, pressure switches in the market are classified as 3 percent or 0.5 percent of maximum control pressure. When selecting a pressure switch, choose the right one by considering the range of operating pressure, the range of differential control, and the frequency of operation.

Direct Type Pressure Switch

A diaphragm type pressure switch, as shown in Figure 2, uses a diaphragm to receive the pressure. When air pressure reaches the diaphragm through the inlet, the top of the diaphragm is pushed up to create a force that is greater than the compression of the spring. Then, the diaphragm elevates to operate the microswitch to open and close the electrical contact point. When air pressure drops, both the diaphragm and the microcontact point returns to their original positions.

Diaphragm type pressure switch

A diaphragm-type pressure switch sees low friction with the driving apparatus, so it has low hysteresis (an output of a system that depends on input and history of input). In some cases, the diaphragm’s displacement is used to open and close electrical contact points to ensure highly stable performance.

Bellows Type Pressure Switch

The Bellows Type Pressure Switch shown in Figures 3 and 4 has a pressure-receiving part that consists of a bellows. When the input signal pressure enters the bellows, the lever moves counterclockwise until the compression of the main spring matches the force of the bellows and the microswitch is operated by the lever.

Bellows type pressure switch (left)
TPC Bellows type pressure switch (right)

The bellows is either made from phosphor bronze, or beryllium bronze, stainless steel, and it is generally used for pressures between 0 and 20 cm sq. A double-bellows structure is also above 40 cm sq. Note that the bellows-type pneumatics switch has a greater hysteresis value.

Bourdon-Tube Pressure Switch

A bourdon-tube pressure switch uses the bourdon tube of manometer (see Figure 5). It has high accuracy and durability, but the bourdon tube might deflect under high pressure.

Components on a Bourdon-tube Pressure Switch

Piston-Type Pressure Switch

A piston-type pressure switch shown in Figure. 6 combines a piston and resistant spring in the pressure-receiving part and allows a wide range of displacement. However, it requires a seal to prevent any leakage from the piston. The seal increases friction and lowers accuracy. The range of available pressure is about 10-100 cm2 and is mostly suitable for special high-pressure applications.

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