What is a pressure switch? Where do we use pressure switches?
Karin Weller
Posted On July 22, 2019
A Pressure Switch is a type of switch that closes an electrical connection when a certain set fluid pressure has been attained on its input. The switch is specifically designed to make work with a rise or fall in pressure. Pressure Switches are widely used to automatically supervise and control systems that involve pressurized fluids. In layman terms, a pressure switch is simply used to measure pressure in any set environment.
A pressure switch comprises either one or two ports. In a pressure switch with a single port, the pressure will enter through the port and direct hit on an internal diaphragm. The contact switch will break or make the circuit; if the force entering the port is higher than that the force exerted by the pre-loaded set spring. An alarm or signal will be generated to inform the user about the change. On the other hand, a pressure switch with a double switch has two ports. The pressure entering the switch from both the ports is compared. If there’s any difference between the pressures obtained from both the ports, then the pressure switch will alert the user with its change.
| S.No. | Tips for a swift and hassle-free application |
|---|---|
| 1 | To reduce the effect of surges, the switch should be mounted as close to an accumulator and as far from pumps. |
| 2 | To avoid excessive vibration, practice remote mounting of the switch. |
| 3 | Pressure Switches should not be applied straight on steam exceeding 15 psig. |
Pressure Switch Mechanism and Design
How does it work? A sectional view of a pressure switch shows a micro switch, insulated trip button, operating pin, trip setting nut, range spring, operating piston, and a switch case or housing. A typical pressure switch contains a capsule, bellows, bourdon tube; diaphragm that deflates proportionally to the applied pressure. The resulting motion is applied to a set switch contact through amplifying levers. However, an over-center mechanism like miniature snap-action switch is used to ensure quick operation contacts since pressure may be changing slowly but contacts should operate quickly.
Some pressure switches like industrial pressure switch may have a calibrated scale and pointers to demonstration the set point of the switch. Switch’s Deadband or Hysteresis is a different range around its set point. The pressure sensing element of a pressure switch may be arranged to respond to the variance in two pressures. These kind o switches are generally used to detect a clogged filter in a water supply system. Some switches can directly control small electric devices or other minor loads.
How to maintain the life expectancy of a pressure switch? Since the inner parts of a pressure switch are exposed to process fluid, they must be taken care of to maintain the strength, quality and life expectancy. The solution to this is that rubber diaphragms are commonly used in contact with water. However, the usage of rubber diaphragms is refrained from using in mineral oil since it would degrade the inner parts of the pressure switch.
Different types of pressure switches
Pressure switches fall under 3 basic categories- Electromechanical, Solid-state and Electronic. Conventionally, these switches have been electromechanical devices. With the advancement of technology, their place is being quickly occupied by the Solid-state and Electronic pressure switches.
1. Electromechanical Pressure Switch
These are composed of sensing elements and an electrical snap action switch. Several different types of sensing elements can be used but they have 1 thing in common which is, they move in response to changes in system pressure. With their movement, they directly act on the opening and closing of the snap action switch’s contacts.
2. Solid-State Pressure Switch
The first solid-state pressure switch was introduced in 1980. It was a simple device which combined a triac switch and bonded gage sensor. Nowadays, there is a wide range of solid-state pressures switches available in the industry. They consist of 1-4 switch points, digital display, and analog. The biggest benefit of a Solid-State switch is its cycle life. Other advantages are better accuracy, high resistance to shock and vibration, broad frequency response, and long term stability.
3. Electronic Pressure Switch
A few years later after solid-state switches, electronic pressure switches were introduced. They incorporate some variety of pressure transducer (like strain gauge or capacitive element) and an internal circuit to compare the measured pressure to a given point. Such a device may provide amended repeatability, accuracy, and precision over a mechanical switch.
The single-phase pressure switch (insert link) is a common type found on many brands of the compressor, but quality matters the most. Airtools WA single pressure switch comes with a thicker plastic cover that doesn’t break easily. The Air Tools pressure switch is a pressure operated electric switch which can be used in regulating tank pressure between 2 preset values on electrically driven air compressors. It is available with an un-loader valve, which inhibits compressors from starting under load. Apart from this, a four-port manifold style is available which delivers a means for easy mounting of valves and gauges.
Areas of Application for pressure switches
Pressure switches are generally used in applications where electrical contacts open or close in response to the system. The usage of the pressure switch is spread across many industries. Defense, Marine, Medical, Motorsport, and Agriculture are just a few to name. Some specific application for pressure switches includes-
- DNA sampling machine
- Monitoring oxygen pressure level on medical devices
- Filtration mechanism in aquariums
- Coffee machines and drink dispensing equipment
- Agriculture machinery like crop-dusting equipment
Selecting a pressure switch according to the required application
Different types of pressure switches are available. The designer should choose a type which can be expected to provide the most satisfactory result.
- Basic Types
At low pressures, diaphragm and bellows movements are most often used. At high pressures, piston and bourdon tube movements are used. Snap action contacts seem to be widely used except in switches with tilting mercury contacts.
- Expected service life
The expected service life is usually limited by the type of pressure sensing mechanism. The snap-action contacts are supposed to have a greater life than the sensing mechanism.
- Speed of cycling
The speed of cycling must be considered while selecting a pressure switch. A piston is specified if a switch is expected to cycle more than once every three hours.
- Accuracy
Bourdon and Diaphragm switches have greater accuracy than piston switches. Wherever accuracy is required, these kinds of pressure switches are demanded highly.
- Set points
How many set points do you need in your application? For 1 set point, usage of a differential pressure switch is suggested. For 2 set points, it is advisable to use an adjustable differential pressure switch.
- Pressure range
This is a critical factor of a pressure switch. Experts say that the pressure applied during a normal cycle should never exceed the maximum range value mentioned in the switch. Pressure surges should be less than the maximum permissible pressure listed for the pressure switch.
Piston Actuates Pressure Switch v/s Diaphragm Actuated Pressure Switch
The maximum allowable pressure is comparatively high in piston actuated pressure switches than diaphragm actuated pressure switches. Surges are constant in piston and minimal in diaphragm. Piston supports hydraulic system while diaphragm supports pneumatic system.
How to select the adjustable range of a pressure switch
The adjustable range of a pressure switch is also called as the working range of the switch. It is the pressure range a switch may notice under working conditions. In the process of adjusting a pressure switch, accuracy and the service life could be bargained. Efforts should, therefore, be made when regulating a pressure switch to ensure that the desired accuracy can be achieved while at the same time ensuring that the switch lasts long by choosing the right working range of the switch. For the greatest accuracy, the set point of the pressure switch should be adjusted such that the set point falls under the upper 65% of the adjustable range. For a long service life, the switch set point should be adjusted to fall in the lower 65% of the range.
Summing up, Pressure Switches are widely used in varied industries. Designed to work especially for measuring and performing with fluid pressure, these automatically supervised switches are an essential part for smooth factory working.
