A pressure switch is a common mechanical or electronic device found in various fluid applications, capable of activating when the pressure of a process fluid surpasses a setpoint or threshold. In order to determine that such values have been reached, a pressure switch may rely on various elements such as bourdon tubes, pistons, membranes, or diaphragms that exhibit changes when pressure is exerted. These elements are then attached to the contacts of the switch, allowing for the device to open or close based on its configuration and design. Finding use in gas systems, HVAC equipment, instrumentation, piping, and other settings, it can be highly beneficial to have an understanding of their working principle and parts.
For a standard pressure switch, a piston will be placed within the assembly with one of its sides having contact with fluid pressure. The other side, meanwhile, is faced with atmospheric pressure. With the use of a pre-loaded spring, the force of fluid pressure is counteracted so that the piston remains still until the specific threshold pressure is surpassed. At that point, the spring will then be compressed for the switch to open or close. With a setpoint screw, the spring is pre-compressed, and the screw may be adjusted as needed to change the activation pressure.
Based on the type of element that is used to detect pressure, a pressure switch may be considered to be electromechanical or electronic. Additionally, all pressure switches have two operating points, those of which are known as the “cut-in” and “cut-out” pressure. As an example, a pump and compressor system utilizes a switch that actuates when fluid pressure drops below a set threshold. At this point, the pump or compressor motor will activate, ensuring that fluid pressure returns to standard levels. For efficiency, the switch is designed so that deactivation does not immediately occur when the pressure reaches the threshold, ensuring that sudden tripping does not happen. Instead, the pressure will continue to rise until the higher end of the pressure range is met. At this point, the cut-out pressure is reached and the switch will then deactivate. For cut-in pressures, a similar process will be carried out with a lower pressure end range.
While various types of pressure switches may differ in their design and makeup, there are some components that are fairly common to mechanical pressure switches in particular. With the process or inlet port, the pressure switch assembly is attached to the processing unit. As pressure switches often are connected to the nozzle of a tank or pipe, the inlet port will be threaded, bolted, or welded. The pressure sensing element is the primary part of the assembly, serving to mechanically actuate the switch when exposed to certain pressures. Generally, mechanical pressure switches are classified by their pressure sensing element. With the spring and setpoint adjustment screw, fluid force is counteracted until thresholds are surpassed. If one wishes to widen or narrow the operating pressure range of a switch, the differential is used. Seals are important for preventing leaking, and such components may be made from nitrile, EPDM, FKM, PTFE, and other various materials. Lastly, other assembly components such as the housing, contacts, and terminals are crucial as they ensure the proper protection and functionality of the switch.
Depending on one’s needs, there are a few common types of electromechanical and electronic pressure switches that may be used. For the electromechanical types, common variations include the piston pressure switch, diaphragm pressure switch, bourdon tube pressure switch, and differential pressure switch. For electronic pressure switches, on the other hand, such devices will utilize a pressure transducer such as a strain gauge and other electronic components to conduct operations.
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