Machine Design

Selecting a Pressure Switch, without the pressure

Sorting through pressure switch strengths and weaknesses makes specifying the correct one a snap.

Charles Isaac
Product Manager, Pressure Switches
Barksdale Inc.
Los Angeles, Calif.

Edited by Miles Budimir

The Barksdale PS-4 is an example of a solid-state pressure switch in a compact unit. The switch, enclosed in an ABEX housing, consists of a pressure transducer, digital display, four fully adjustable switches, adjustable deadband, and analog output with optional serial interface.

Switches are commonly grouped together based on the type of sensor technology employed. The characteristics of the sensor tend to define the relative accuracy and life expectancy of the switch. Below are the typical switch categories and their attributes

Pressure switches act as safety devices, alarms, or control elements within a system. In its simplest form, a pressure switch detects a pressure change, and at a predetermined level or set point, opens or closes a contact.

Although there are many different types of pressure switches, they can generally be classified as either electromechanical or solid state. Traditionally, pressure switches have been electromechanical devices. However, solid-state devices are becoming more common.

The most common electromechanical pressure switches are composed of a sensing element and an electrical snap-action switch. A number of different types of sensing elements are used with one thing in common; they move in response to changes in the system pressure. The movement opens and closes the switch contacts.

Differential pressure switches have two pressure ports, a low pressure and a high-pressure side. The low pressure and high-pressure fluids act on the same sensing diaphragm. Since the surface areas are equal on both sides, the switch is in equilibrium when the pressures are equal. As pressure on the high-pressure side increases, a piston moves up, and at a preset differential pressure, activates the snap switch.

Solid-state pressure switches come with one to four or more switch points, digital displays, analog and digital outputs, and full programmability. In addition to opening or closing the pressure switch circuits, they provide a proportional 4-20 mA analog signal or digital output. The analog signal can interface with PLCs, distributed control systems, and computers. Adding an optional RS-232 or RS-485 serial port permits a digital signal to be transmitted to the same devices.

Solid-state switches provide a number of advantages over electromechanical ones, the major advantage being longer cycle life. Solid-state switches routinely have an operational life of 100 million cycles. Other advantages include improved accuracy, high resistance to shock and vibration, the ability to handle a wide range of system pressures, a broad frequency response, and long-term stability.

Some systems require more than a pressure switch, such as a local gauge and remote signal. The traditional approach is to use individual devices for each parameter. However, a microprocessor-based, solid-state pressure switch can eliminate individual components by placing the features into one package. Most solid-state switches have four or more independent switches, digital readouts, and analog or digital outputs.

One concern with solid-state switches used in industrial and process settings has been electromagnetic interference which can corrupt signal data. But most solid-state switches are designed to be unaffected by EMI and RFI interference.

However, the price increase of solid-state over electromechanical switches prevents many from upgrading. In designing a new system, selecting a solid-state switch may, in fact, be the most cost-effective solution. When a system requires multiple switch points, local gauges and transmitters, using solid-state pressure switches can significantly reduce the installed cost.

When selecting a solid-state pressure switch, choose a switch point in the upper 25% of the pressure range. However, since electromechanical switches have sensing elements (diaphragms, tubes, and pistons) that are constantly being stressed, the location of the switch point versus the operating range is critical to both accuracy and life. An electromechanical switch's life cycle is longest when operated in the lowest 25% of the operating range. On the other hand, accuracy will be the greatest when operating the switch at the upper end of the range. The best compromise is to operate the electromechanical pressure switch in the middle of its operating range.

Pressure switch characteristics
Understanding the system dynamics is crucial to switch selection. The following is a list of questions that need to be answered when specifying a pressure switch:

How often will the switch be activated? Electromechanical switches are subject to fatigue. A Bourdon tube or diaphragm switch typically provides one million cycles, compared to a piston or diaphragm-sealed piston switch that provides two million cycles. Because a solid-state switch is not subject to fatigue, it typically runs 100 million cycles. An exception may be made when pressure changes in the system are slight, 20% or less of the adjustable range. Under such conditions, a Bourdon tube or diaphragm switch can be used up to two million cycles before fatiguing.

What is the cycle speed? The metal of a Bourdon tube or diaphragm switch emulates a spring, so high-speed cycles should be avoided. When cycle rates are less than 25/min, a Bourdon tube or diaphragm switch is a good choice. For cycle rates between 25 and 50 cycles/min, a piston or diaphragm-sealed piston switch normally provides two million cycles. A solid-state switch should be selected when the cycle rate exceeds 50 cycles/min because fatigue is not an issue.

How does the switch point relate to the operating pressure range? Selecting the proper relationship between the switch point and the operating pressure range of a switch affects accuracy and life. The general rule differs between solid-state and electromechanical switches. For a solid-state switch, the switch point should normally be in the upper 25% of the operating range. For an electromechanical switch, the switch point should be in the middle of the operating range. A system which requires a switch to activate 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 undergoes dramatic pressure surges or when either life or accuracy is an over-riding concern.

High-pressure spikes and surges
Pressure surges and transient pressure spikes can greatly exceed the normal operating pressure of a system. It is not unusual for a switch to fail because the pressure spike exceeds the proof pressure, which is the maximum pressure that the switch can withstand without damage. Bourdon tube, diaphragm, and solid-state pressure switches are all sensitive to surges and spikes. If it is anticipated that the system is subject to surges, then one should select a switch with a higher proof pressure or install a snubber which dissipates spikes without damaging the switch.

How many switch points are needed? When sensing pressure at one point, only one switch point usually is required. However some systems require two or even four switch points to be monitored, controlled, or alarmed. In designing a system, select a single switch for each switch point, or a single pressure switch capable of handling as many as six separate switch points. Most sensors use duplex switches and a few have built-in triple switch functions. Solid-state switches can have as many as six or more independent switch points.

Switch housing
Stripped switches do not have enclosures. They are normally installed inside a panel or multifunction en-closure. Housed switches avoid possible hazards from loose wires in exposed locations. They are normally available in a variety of ratings with the most widely used industrial switch housings being NEMA 4 and NEMA 4X for corrosive environments. Terminal block pressure switches are housed and equipped with enclosed terminal blocks. This eliminates the expense of buying and installing external junction boxes. Explosion-proof pressure switches are designed with housings built to conform to accepted electrical standards in isolating the units from hazardous environments.

Setpoint adjustment
In some applications, the setpoint is permanently fixed, while in others some adjustment is required. Electromechanical switches have models that are factory set, have gross adjustment capability, or offer calibrated adjustment knobs. Solid-state switches offer precise keypad adjustments with digital readout.

Is a tight or broad deadband needed? The deadband or actuation value of a switch can be factory set or adjustable over a percentage of the full pressure range. Traditionally, a narrow deadband is used in safety services. A wider deadband is used on control circuits like hydraulic units. Tight or narrow deadbands tend to be found on Bourdon tube and diaphragm switches; wide deadbands are available in piston-type switches; while solid-state switches offer near 100% of full-scale deadband.

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