A pressure switch is a mechanical or electronic device activated by the pressure of fluids, air, or gas when the fluids, air, or gasses reach a threshold or setpoint. The designs of pressure switches include bourdon tubes, pistons, diaphragms, or membranes that move or deform with the amount of pressure exerted by the system.The components of a pressure switch are connected to one or more contacts in the switch. With enough force, a contact closes or opens the switch depending on its configuration. Although pressure switches have a variety of methods used to detect pressure, they can be primarily categorized as electromechanical or electronic.
Benefits of Pressure Switch
Easily Readable Display
Pressure switch such as usually have an integrated LCD/LED display in a prominent, forward-facing position. This makes taking readings at the machinery side, quick and easy for engineers. Current pressure within the system can, therefore, be obtained efficiently and without requiring the use of any additional instrumentation, with signals converted to readable output within the single unit.
Simple Operation
Pressure switch are blessed with simple operation and functionality. With its small number of buttons (often two), and a display giving visual feedback of all changes, configuration is easily completed locally. A simple combination of button presses enables switch points to be adjusted, saving time and effort devoted to such tasks by engineers.
Improved Reliability
The lack of moving components in pressure switch makes them more reliable and permits them a longer, trouble-free service life. Fewer small parts within the electronic switches makes them less susceptible to the negative effects of vibration. Contact wear-out is also less of a concern than with traditional, mechanical switches.
Higher Accuracy And Repeatability
Pressure switch are able to handle thousands of repeated pressure cycles without issue. This ability is inherently due to the high levels of accuracy that they provide and their superior reliability, particularly in applications where vibration is a consideration. The set points are maintained throughout the sensors' lifetime with any deviation from the parameters not the issue that it is with mechanical switches.
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Pressure switches rely on the pressure generated by the hydraulic fluid to operate. It’s this pressure that will trigger the switch to turn the electrical circuit on or off, therefore activating or deactivating the hydraulic pump. The change in pressure in the hydraulic system moves up through the diaphragm in the pressure switch. The diaphragm presses against a piston and spring, which open or close the contacts within the switch when the pressure is applied. Open contacts close when the pressure drops, completing the circuit and activating the pump. When the pressure threshold is reached, the contacts will open again – shutting off the circuit and turning off the pump.
Mechanical pressure switch
Simple and robust, these switches are commonly used for less complex tasks. Mechanical pressure switches are triggered using a spring and a piston to control the pressure at which they are activated. The spring is the force opposing the inlet pressure, and its tension is adjusted via a set screw or knob. In turn, the spring pretension is directly related to the pressure at which an electric contact occurs. In the event of a drop in pressure, the switch resets to its original state. The mechanical pressure switch is better suited to handling high voltages and amperages compared to an electronic pressure switch. You can use them to increase or decrease pressure through a contact change.
Electronic pressure switch
Electronic pressure switches contain a pressure transducer, typically a strain gauge, and additional electronics that convert signals into readable output. An electronic pressure switch offers many advantages over a mechanical pressure switch. Some of the advantages include greater accuracy, less contact wear, excellent long-term stability, simple operation, and the ability to perform thousands of switching cycles.
Pressure switches maintain safety by turning off or alerting people of a pressure change. For example, a furnace pressure switch can detect negative pressure during its start-up which then shuts the furnace down if there is low pressure. This could be due to a leak, and it should be investigated before the pressure is returned to working levels.
Air compressors are another example of why a pressure switch is an essential safety device. Using an electronic pressure switch, the motor is switched off on an air compressor when the desired threshold has been met. If this switch wasn’t in place, the tank could exceed a safe pressure level which can result in damaged components, or worse, an exploding tank.
In the case of a common lift in offices, shopping malls or residential flats, pressure switches constantly monitor the pressure levels in the hydraulic cylinder. This ensures that if the pressure drops too low, the lift’s power is cut and it won’t be operating at an unsafe pressure level which otherwise might cause it to hurtle towards the ground.
The pressure switch is mainly composed of power or pressure sensitive elements (feeling external pressure), mechanical linkage (transmitting pressure), microswitch (executed by normally open and normally closed contacts) and other parts.
Switch Protection: Bottom spring retaining pads limit the travel of the action lever. Even if there is strong pressure, it will not break the power steering pressure switch.
Pressure receiving part: The low pressure part adopts diaphragm and bellows type, and the high pressure adopts piston type.
Diaphragm type: The durability of the diaphragm is considered.
Piston: O-ring seal.
Bellows: Good adaptability to pressure media. Low temperature, high temperature resistance, good temperature characteristics. The change with time is small. If the medium is not corrosive, the change of time to the pressure switch can be ignored.
Pressure switch adjustment: The minimum pressure value that makes the pressure switch act is called the rated value of the pressure switch, and its size can be adjusted by screws according to the actual use of the controlled object to meet the needs of different requirements and occasions.
9 Considerations When Selecting Your Pressure Switch
● Identify the standard operating pressure along with the maximum pressure of your application that the switch could see.
Why is it important to identify these pressures? You must make sure the pressure switch can be safely used in your application. Choose a switch that is ranged to handle not only the normal operating pressure of your application but also any pressure spikes you encounter in your system.
● Determine if your required setpoint can be achieved by the switch.
Regardless of whether you choose a mechanical or electronic pressure switch, there are limitations as to what setpoints can be achieved that are directly tied to the specific range of the switch. For example, with mechanical pressure switches, the setpoint range of the switch is often limited by the speed and travel of the actuator assembly, meaning the switch cannot provide setpoints in the lower 10% to 15% of the range. Electronic switches can provide setpoints almost anywhere in the range of the switch. You should also determine if you need single or dual setpoints. Having two setpoints can be useful for having two separate alarms (high and higher, low and lower, or high and low).
● Consider the compatibility of process fittings and/or other wetted materials such as diaphragms and pistons/O-rings.
Be sure to check the compatibility between the process media and the wetted materials of the switch (i.e., process fittings, pistons, O-rings and diaphragms). Incompatibility can cause corrosion issues, safety concerns, leaching into the process media, etc. Ashcroft offers a Compatibility Guide for your reference.
● Recognize any possible high temperatures and compare them to the capability or specifications of the switch.
The datasheet typically lists the temperature specifications that your switch can withstand. As mentioned, using a switch beyond its stated temperature specifications can lead to setpoint drift, component issues and possible safety concerns.
● Select a microswitch based upon your application’s electrical requirements.
The electrical ratings of the microswitch are guidelines of the voltages and currents that the switch can be used with to ensure the maximum cycle life of the switch. The listed ratings provided by microswitch suppliers are the voltages and currents tested by third-party independent test labs for the required cycle life. However, keep in mind that microswitches are mechanical pass-through devices, that send the supplied voltage and current to your load. Meaning that the microswitch can be used with many different voltages and currents but may see a reduction in the cycle life.
● Identify if the application requires hazardous area approvals or industry type approvals.
This includes hazardous approvals such as explosion-proof, intrinsically safe, non-incendive/increased safety as well as industry type approvals such as boiler and steam limit control approvals. Determining if an application requires hazardous area approval and general-purpose/safety approvals dictate which switch can be used for these types of applications. These approvals are provided by independent and nationally recognized test labs known as NTRLs. Agencies include FM, ATEX, CSA, IEC and UKCA.
● Determine if your switch needs additional options.
Do you require a factory setpoint, tags, oxygen cleaning, special materials for housing/enclosure, wall/pipe mounting brackets or specific certifications? One of the most common variations is to have the setpoint of the switch calibrated at the factory. This is known as Factory Set (XFS). Using this variation ensures the accuracy of the switch when received by the customer while adding to the ease of installation because the switch is ready to install. Other options include material choices, special enclosures, NACE certification, metric labeling, pilot lights and much more.
● Fixed or adjustable deadband.
The term deadband in pressure switches is the difference between the pressure at which the switch activates (the setpoint) and the pressure at which the switch deactivates (the reset point). Switches can have two different kinds of deadbands, fixed or adjustable. Switches that have fixed deadbands have a deadband value that is determined by the mechanical properties of the switch. Items such as diaphragm material, the switching mechanism and the pressure range of the switch all influence this fixed value. While adjustable deadband switches have deadbands that can be adjusted or selected within a specific range to meet a customer's application requirements.
● Using a switch indoors or outdoors.
The environment in which you use your pressure switch can impact its functionality. There are many challenges when using a switch outdoors, including temperature and weather effects (rain, snow, sleet, etc.). You must also consider what IP/NEMA ratings you need to satisfy the location of your pressure switch.
Application of Pressure Switch
Water Pumping pressure switch Systems: This may be the most common use of pressure switches. Pressure switches are used in water pumps to cut-in power into the motor which drives the pump in case of low level or low line pressure. Well water pressure switch upon reaching the set pressure, power is cut-out.
Compressed Air pressure switch Systems: This is similar to transmission fluid pressure switch. Pressure switches are used to cut-in power to the compressor motor when low pressure is detected. This maintains the pressure of the compressed air system. If for negative pressure, it should used vacuum pressure switch.
Pneumatic and Hydraulic pressure switch Systems: These are control systems that use pneumatic and hydraulic actuators. Pumps and compressors maintain reservoir pressure and level through HVAC pressure switch or oil pressure switches
Air Conditioning and Refrigeration pressure switch system: In a refrigeration system, the thermostat provides the controlling feedback signal. However, in case there is a problem in the system, the thermostat will only sense the temperature in the cooled space but not the state of the equipment. A pressure switch serves as a safeguard that trips the compressor motor in case of overpressure. Another use of a pneumatic pressure switch in a refrigeration system is protection on the low-pressure side which indicates a possible refrigerant leak.
Furnace and Boiler Systems: The pressure switch in a furnace or boiler serves as a safety interlock to prevent the igniter from operating in case there is a problem with the draft system. This prevents the combustion chamber from operating which can result in incomplete combustion.
Filtering and Screening Equipment: A differential pressure switch is used to measure or monitor the pressure drop across filters and screens. The pressure switch triggers an alarm or notification to indicate that the filter is blocked or clogged and is due for maintenance, cleaning, or replacement.
A common misconception about pressure switch and pressure transmitters is that they perform the same roles within any given application. However, there are major differences between the two components.
A pressure transmitter converts pressure into an electrical signal and transfers that to a PLC (Programmable Logic Controller). A pressure switch, however, only triggers at a specific preset pressure, and, depending on the set pressure, a circuit can be engaged or disengaged.
Both devices measure pressure, but transmitters provide continuous feedback that is normally connected to a controller that monitors pressure. In comparison, a pressure switch doesn't provide constant output and feedback and is much more similar to an on/off switch. A pressure switch is designed to open or close a circuit when pressure rises or falls, not transmit messages.
A pressure switch directly controls a fluid system and can operate without a power supply, but pressure transmitters just indicate pressure level with a continuous signal. They do not directly control a circuit and are used for more sophisticated applications like monitoring, predictive analysis, or process control.
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