Pressure Switch

Overview of the pressure switch

The pressure switch has a range of applications that make it an important part of industry.  It also has a number of specialist applications that include switches designed to operate effectively in the harsh environments found in some areas of the manufacturing and marine sectors.

At a fundamental level the pressure switch shuts an electrical contact at the point at which a preset pressure has been reached or achieved. In order to make sure the switch works in both 'directions', some are configured to make an electrical contact either when pressure increases or diminishes. Despite this, pressure switches are not exclusively designed to respond to changes in gaseous or water pressure for example – they can also respond to the action of a mechanical force such as a weight placed on an area designed to move and thus close the electrical contact. An example of the application of this technology is the garage door, where the switch can 'force'  the door into an upcycle if it detects minute changes in pressure. Commercial buildings with mats that force building doors to open when pressure is applied to them also contain pressure switches.

Typically, the pressure switch allows the pressure-generating activity within equipment to be monitored, and will often sound some form of alarm when the level of pressure starts to exceed its safe range.

Designs vary depending on the type of action required. If manual action is needed (for example, to speedily activate a venting process or a machine shut-down), the toggle switch design will frequently be used for the responsible pressure switch, chiefly because it is simple to operate very quickly in the event of an alarm sounding. Some pressure switches are designed to work in conjunction for use with computer technology, in which case they typically take the form of micro switches. If a computer program determines that the next logical sequence in an equipment system is a safety shutdown or pressure release, it sends a command to the pressure micro switch which rapidly actuates the required action.

Pressure switches have also provided genuine safety gains for engineers and others working in close proximity to equipment regulated by them. By shutting down or venting systems before overload or explosion point has been reached, operatives have a reduced chance of suffering serious injury. Virtually all machinery that incorporates compressors, for example, will possess pressure switches at key phases to ensure safe operation. Automated pressure switches have grown in popularity in recent years, although manual switches are also widely installed as a back-up safety tier that can actuate should some form of an electrical failure occur.

Despite the enormous variety of pressure switches, all can be classified under two broad categories: pneumatic and hydraulic.

 

History

The French scientist Lucien Vidie invented devised and constructed the world’s first aneroid barometer for measuring atmospheric pressure in 1843. Using Vidie’s idea of an indicator system based on the amplification of the movement of a spring extension under pressure, Eugene Bourdon invented the “Bourdon tube” in 1849, a device which is still in use in some of today’s pressure switches.

By 1930 the mechanical movement of springs and diaphragms in pressure measurement devices was being harnessed in electrical transducers and transduction mechanisms. Today, as described earlier, the variety of switches incorporating pressure measurement technologies and electrical circuit actuation in response to pre-set pressures has proliferated throughout the domestic, commercial and manufacturing environments across the world.

 

Technical aspects

Fluid pressure switches incorporate a moving element such as a bellows, capsule, Bourdon tube, piston or diaphragm that bends or moves in proportion to the pressure being exerted. This element’s movement is either amplified or transmitted directly to an electrical contact. Pressure may rise or fall slowly, but when the pre-set level is reached, the switch must be capable of opening or closing rapidly. This is typically achieved by the use of an over-centre or tipping point mechanism such as a small snap-action switch, although a particularly sensitive variety employs mercury switches incorporated into a Bourdon tube. The weight of the mercury provides a sensitive over-centre characteristic as it shifts in response to pressure changes.

Some pressure switches are designed to be adjustable, usually by changing the tension in a counterbalance spring or moving the contacts. Calibrated scales with pointer needles are frequently found on industrial pressure switches to clearly indicate the set point, although typically there is always a differential range around it. This range, which can be adjusted in some switches, prevents small fluctuations in pressure from altering the state of the contacts.

It’s also possible in some pressure switches to configure them it to respond to the difference between two pressures. Responding to such differences can be of crucial importance in some scenarios, such as detecting when a water supply system is being affected by a clogged filter. However, these switches must be capable of responding only to the designated pressure difference and not to operate falsely in response to normal pressure fluctuations.

Typically, pressure switch contacts are rated a few tenths of an amp to approximately 15 amperes, but more sensitive switches usually have finer ratings. Pressure switches are often used to actuate a relay or some other form of control mechanism but some are used to control small electrical motors directly.

The materials used in the construction of fluid pressure switches are of key importance. Their internal parts will come into direct contact with the fluids being pressure-monitored, so they must be capable of contact with the process fluid without degrading or reacting as well as having a reasonable life expectancy. Rubber diaphragms, for instance, would be ideal if water is the process fluid but would begin to disintegrate very rapidly in contact with mineral oil.

A crucial safety feature required on switches used in danger zones where, say, flammable gases are present, is an enclosure protecting against the danger of arcing at the contacts. If there were no such enclosure, any arcing would likely ignite the gas and cause an explosion. These enclosures are also required on pressure switches designed to be submersible, weather-proof or corrosion-resistant.

Where the pressure switch is used in manufacturing

Pneumatic pressure switches are ubiquitously used to shut down electrically powered gas compressors when pressure in the reservoir reaches the set point, or when there is no feed during suction. They’re also used to regulate the rate of charge in rechargeable batteries, protecting the battery from receiving excessive current. Crucially, they’re widely used in aircraft cockpits to trigger alarm lights when altitude-based cabin pressure falls to potentially dangerous levels. Petrol stations also frequently use pneumatic pressure switches in air-filled hoses in order to count traffic (the switches are activated each time a vehicle drives over the hose).

Hydraulic pressure switches are extensively used in automobiles, most commonly to activate warning lights when engine oil pressure drops below safety level. They’re also used widely in air conditioning and filtration systems, activating or shutting down according to set-point pressures. Many heated swimming pools incorporate hydraulic pressure switches, which help regulate the temperature of the water.

How the pressure switch differs from other switches

Electronic pressure switches employ a transducer of some kind, such as a capacitive element or a strain gauge, along with an internal circuit, which ‘collaborate’ to compare the pressure being measured to the set-point. These devices are generally measurably more accurate, precise and more capable of rapid repetition than a mechanical switch.