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Pressure switches are devices that are configured to sense a change in pressure and respond in a specified manner.

A pressure switch turns an electric circuit ‘ON’ or ‘OFF’ at a preset pressure.

The pressure is the set point of the switch

Pressure is use in some form of control

The pressure switch is usually a micro switch

or mercury switch.

The pressure is fed to the inside of a bellows which

carries a contact plate

The contact plate touches contact point.

The pressure switch can be modified so as to make a low pressure contact in addition to high pressure contact.

Most switches contain two sets of contacts, one normally open and the other normally closed.

Pressure switch is uses as limiting the pressure

The pressure switch is used to operate a safety valve which vents steam when the pressure exceeds the upper limit.

Another important uses of the pressure switch is in the computer panel.

In the computer panel, blowers’ are used for cooling purposes.

Whenever the blower fails due to any reason, a pressure switch is actuated which cuts off the power supply of the panel.

The computer panel components are protected from the high temperature whichcan occur due to failure of the blower.

Pressure switches in their simple form detect a pressure change, and at a predetermined level or set point, open or close a contact. They act as.

1. Safety devices2. Alarms3. Control elements within a system4. Steam power plant5. Electric motors6. Gas compressor7. Automobiles

8. Pumps9. Boilers10. Steam turbines

Types of Pressure Switch

Electromechanical pressure switch

Solid state pressure switch

The most common electromechanical pressure switches are composed of a

◦ Sensing element ◦ Electrical snap-action switch.

Solid-state pressure switches contains following characteristics:One to four or more switch points, Digital displays, Analog and digital outputs, Full programmability. Provide a proportional 4-20 mA analog signal or digital output.

Electromechanical Pressure Switches Solid-State Pressure Switches

Lesser life cycle. Longer life cycle about 100 million cycles.

Low accuracy. High accuracy.

Lesser frequency response. A broad frequency response.

Short-term stability Long-term stability

Low resistance to shock and vibration. High resistance to shock and vibration.

Ability to handle a Lesser range of system pressures. Ability to handle a wide range of system pressures.

The calibration of the process ids to adjusting the instruments output signal to match a known range of pressure.

All instrument tend to drift from their last setting. Because the springs stretch, electronic component undergo slight changes on the atomic level, working part also loses their elasticity.

For low pressure, up to 3 inch of water gauge, a micrometer or an inclined water gauge may be uses.

For pressure up to 144 in, water gauge or 12 in. Water manometer gauge may be use.

For higher pressure range from72 in mercury manometer gauge

may be used.

For still higher pressures, the dead- weight piston tester must be used.

1. Turn on the transmitter and allow the internal component to reach normal operating temperature.

2. Uses the weight tester or a portable regulated air supply to apply 25 psig pressures to the instrument.

3. Adjust the zero to set exactly 4mA output, indicated on precession miliammeter.

4. Supply 125 psig pressures to the instrument.5. Adjust the span to get a reading of exactly 20mA6. Apply 75 psig pressures (half-scale input pressure).7. Adjust the linearity to bring the output signal within the specified

tolerance of 12mA (half-scale output current)

Usually the periodic maintenance of an instrument are given in themanual supplied by the manufacturers

Individual plants or industries also prepare their guideline for periodicmaintenance. (Steps that should be taken for pressure measuringinstrument maintenance and repair).

a)Visual inspection -any leak or damage in piping and wiring of theinstrument.b)Blow down and venting -clogging of lines in major cause ofinstrument failure.c)Cleaning and lubrication -instrument with mechanical linkagesundergo wear and misalignment.