Pressure switch characteristic
Understanding system dynamics is essential for switch selection. Here is a list of questions to answer when specifying the pressure switch:
How often is the switch activated? Electromechanical switches are prone to fatigue. Bourdon tube or diaphragm switches typically provide 1 million cycles compared to piston or diaphragm sealed piston switches that provide 2 million cycles. Since the solid-state switch does not get tired, it typically runs 100 million cycles. Exceptions may occur when the pressure change in the system is small, 20% or less of the adjustable range. In this case, a Bourdon tube or diaphragm switch can be used for up to 2 million cycles before fatigue.
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What's the cycle speed? Bourdon tube or diaphragm switch metal analog springs, so high-speed cycling should be avoided. When the cycle rate is below 25/min, Bourdon tube or diaphragm switch is a good choice. For cycle rates of 25 to 50 cycles per minute, piston or diaphragm sealed piston switches typically provide 2 million cycles. When the cycle rate exceeds 50 cycles/min, a solid-state switch should be selected because fatigue is not a problem.
How does the switching point relate to the operating pressure range? The relationship between choosing the right switch point and the operating pressure range of the switch affects accuracy and life. The general rules for solid-state switches and electromechanical switches are different. For solid-state switches, the switching point should normally be at the upper 25% of the operating range. For electromechanical switches, the switching point should be in the middle of the working range. Systems that require a switch to start at 140 psi should use a solid state pressure switch with an operating range of 150 psi, or an electromechanical switch with an operating range of 300 psi. Exceptions should be made when the system experiences violent pressure fluctuations or when life or accuracy is the most important issue.
High pressure spikes and surges
Pressure surges and transient pressure spikes can greatly exceed the normal operating pressure of the system. It is not uncommon for the switch to fail because the peak pressure exceeds the validation pressure, which is the maximum pressure the switch can withstand without damage. Bourdon tubes, diaphragms and solid state pressure switches are sensitive to surges and spikes. If the system is expected to be affected by surges, choose a switch with a higher voltage withstand or install a buffer to dissipate spikes without damaging the switch.
How many switch points are needed? When pressure is induced at a certain point, usually only one switching point is needed. However, some systems require two or even four switch points to monitor, control, or alarm. When designing the system, choose a switch for each switch point, or a pressure switch capable of handling up to six separate switch points. Most sensors use a duplex switch, and a few have a built-in three-switch function. Solid-state switches can have up to six or more independent switching points.
Switch housing
Bare wire switches have no housing. They are usually mounted in a panel or multifunctional housing. The closed switch can avoid the danger caused by loose wires in the exposed position. They are typically available in a variety of ratings, with the most widely used industrial switch housings being NEMA 4 and NEMA 4X for aggressive environments. Terminal pressure switch installed and equipped with closed terminal. This eliminates the cost of purchasing and installing external junction boxes. The housing of the explosion-proof pressure switch is designed to meet recognized electrical standards to isolate equipment from hazardous environments.
Set point adjustment
In some applications, set points are permanently fixed, while in others some adjustments are required. Electromechanical switches with factory Settings, coarse adjustment capabilities, or models provided with calibration adjustment knobs. Solid-state switches provide precise keyboard adjustments via digital readings.
Need a tight dead zone or a wide dead zone? The dead zone or drive value of the switch can be adjusted at the factory setting or at a certain percentage of the entire pressure range. Traditionally, security services have used narrow dead zones. Wider dead zones are used in control circuits such as hydraulics. Tight or narrow dead zones often appear on bourdon tubes and diaphragm switches; Piston switches provide a wide dead zone; Solid state switches offer close to 100% full scale dead zones.
Some of the information on diaphragm and Bourdon tube pressure switches was adapted from information published by Delaval's Barksdale Division.
There are several types of pressure switches to choose from. The designer should choose a type that can be expected to give the most satisfactory results for his type of application.
Basic type
At low pressure (compressed air and extremely low pressure hydraulic systems), diaphragm and bellows movements are most often used, and sometimes Bourdon tube movements. At high pressures, piston and bourdon tube movements are most common. With the exception of switches with tilted mercury contacts driven by Bourdon tubes, quick-acting contacts appear to be commonly used.
Expected service life
The expected service life is usually limited by the type of pressure sensing mechanism - Bourdon tube, diaphragm, piston, dish spring, etc. It is assumed that the quick-acting contact has a longer service life than the sensing mechanism.
If the service life (the number of cycles expected for switch operation) is less than one million, it indicates a Bourdon tube or diaphragm type. If there are more than a million cycles, the piston type should be used. An exception to this rule is when the pressure in the system changes very little (20% or less of the adjustable range). In this case, Bourdon tube or diaphragm switches can be used for up to 2.5 million cycles before metal fatigue or contact failure.
Riding speed
In addition to service life, riding speed must also be considered. If the switch is expected to cycle more than once every 3 seconds, the piston switch should be specified. The metal of any Bourdon tube or diaphragm switch acts as a spring, which heats up and tires during extremely fast cycle operation, thereby shortening the service life of the switch.
precision
Diaphragm and Bourdon tube pressure switches generally have higher accuracy than piston switches, and in cases where accuracy is important, they will be preferred if they meet service life and cycle speed requirements.
A switch with a dish spring (quick action) seems to offer the greatest repeatability, but the manufacturer should be contacted for the life expectancy of the spring.
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Figure 1. On the switch with diaphragm and Bourdon tube movement, the highest accuracy is 65% above,
The optimal life factor is in the lower 65%, and the optimal combination is usually in the middle 30% of its operating range (zone A).
Adjustable range
The term "operating range" defines the range of pressures that a switch may see under normal operating conditions. This is usually an adjustable range.
For maximum accuracy, the set point should fall within the upper 65% of the adjustable range. But for the most favorable life factor, the set point should be in the lower 65% of the adjustable range. Therefore, the optimal combination of accuracy and life coefficient is located in the middle 30% of the adjustable range, as shown in the figure. This general rule applies to diaphragm and Bourdon tube pressure switches. The piston switch has closer to consistent accuracy and life coefficient in its adjustable range.
Type of switch action
Standard pressure switches sense a single pressure source and open or close a set of contacts.
The differential pressure switch has two connections that can sense the differential pressure of the entire circuit.
The double switch senses the upper and lower limits of the same pressure source and activates two sets of electrical contacts. A wide range of dual pressure sensing can be achieved using two standard pressure switches. See the picture below.
Fluid medium
The compatibility of the fluid with the structural material must be considered. Consult the switch manufacturer directory.
Withstand voltage
Voltage resistance is the highest pressure that a switch can withstand without permanent deformation, usually defined as 1.5 times the maximum operating range. Although the pressure gauge in the system may show a constant operating pressure, surges suppressed by holes in the gauge may occur, which can damage the diaphragm and bourdon tube elements in the pressure switch. Therefore, the working range should be much higher than the actual working point.
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Figure 2. Infinite difference between cut and cut pressure.
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Differential pressure sensing While some pressure switches can regulate differential pressure (the difference in pressure required to open and close switch contacts), this differential pressure may not be wide enough for some applications.
The circuit in Figure 2 can be adjusted for almost infinite switching differences. It uses two standard pressure switches and a hold relay.
One pressure switch is used for the high pressure cut-off point and the other pressure switch is used for the low pressure cut-off point. The relay CR has a set of normally open holding contacts CR1 and a set of load switching contacts CR2, which can be normally open or normally closed according to circuit requirements. The circuit acts as follows: starting from zero system pressure, with the pressure, as the pressure rises, the "low" pressure switch will close, but this has no effect on the switching circuit. As the pressure rises further, the "high" pressure switch will close. "The pressure switch will close and the coil of the relay CR will be energized. The relay is electrically locked by contact CR1 and "low" pressure switch. Relay contact CR2 will disconnect the switching circuit. As the pressure drops, the "high" switch will disconnect, but this has no effect on the switching circuit. As the pressure drops further, the "low" switch will open, unlocking the hold relay. Contact CR2 closes the switching circuit.
