It is to provide an insight in different types of pressure switches available and how to select the one appropriate to your application.
What are Pressure Switches?
Pressure switches are one of the most commonly used fluid control components used at industries as well as home. Some applications of pressure switches are refrigerators, dish washer, washing machines, air compressors and pump sets. Pressure switch in a its simplest form is a device capable of detecting a pressure change, and at a predetermined level, opening or closing an electrical contact. Pressure switches are broadly of two types: Electromechanical and Solid State.
1. Electromechanical Pressure Switches
The most common electromechanical pressure switches are composed of a sensing element and an electrical snap-action switch. Sensing elements move in response to changes in the system pressure and directly act on the opening and closing of snap-action switch’s contacts. These switches are commonly grouped based on the sensor technology employed. Electromechanical Pressure Switches and their attributes are as follows:
A. Diaphragm Switches
These switches use a weld-sealed metal diaphragm which directly acts on a snap-action switch.
Operating Characteristics
B. Bourdon Tube Switches
A weld sealed bourdon tube is used to actuate the snap-action switch.
Operating Characteristics
C. Diaphragm Piston Switches (Dia-Seal Piston)
An elastomeric diaphragm acts on a piston which in turn actuates the snap-action switch.
Operating Characteristics
D. Piston Switches
An O-ring sealed piston acts directly on the snap-action switch.
Operating Characteristics
2. Solid-State Pressure Switches
In its simplest form it is a device consist of bonded strain gage sensor and triac switch. In many cases they cross the line from being simply a switch to an open-loop controller. In addition to opening or closing the pressure switch circuits, they provide a proportional analog 4-20mA signal or digital o/p. The analog signal can interface with PLCs, DSCs and computers.
Solid-State pressure switches provide a number of advantages over their electromechanical counter parts:
Things to consider for selecting a Pressure Switch
1. Switch Activation
Below 2-1/2 million cycles Electromechanical switch be selected and above that Solid-state is a right choice
2. Cycle Speed
When the cycle rate is 25 per min., a Bourdon tube or Diaphragm switch is a good choice. For cycle rates to 50 cycles per minute, a diaphragm piston or piston switch could be used and for greater then that rate a Solid-State switch is a right choice.
3. Switch Point and Operating Pressure range
When a solid-state pressure switch is selected, the switch point should 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. For ex., 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.
4. 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. If it is anticipated that the system is subject to surges, then one should select a switch with higher proof pressure or install a snubber which allow fast spikes to move by the switch without damage.
5. Accuracy
For a pressure switch accuracy is the ability of a switch to operate repetitively at its setpoint. Typically, if the switch is used to alarm, 2% accuracy is sufficient. If one is controlling a process where error of various devices is additive, then 0.25% accuracy is absolutely necessary.
6. Switch Housing
Stripped switches don’t have their own housing. They are normally installed inside a panel or multi-function enclosure. Due to cost and space consideration this is favoured choice of OEMs.
Housed switches avoid possible hazards from loose wires in exposed locations. They are normally available in a variety of ratings with the most popular industrial switch housing being NEMA 4 and NEMA 4X for corrosive environments.
Terminal block pressure switches are housed and in addition equipped with terminal blocks. This eliminates the expense of buying and installing external junction boxes.
Explosion proof pressure switches are designed with heavy housings built to conform to accepted ATEX, UL and NEMA standards for containing explosions in hazardous environments.
7. Set point Adjustment
In some applications one doesn’t want anyone changing the set point. In other situations, adjustment is required based on system dynamics. Electromechanical switches have models that are factory set, have blind adjustment capability, or offer calibrated adjustment knobs. Solid-state switches offer precise keypad adjustments with digital readout.
8. Deadband
Deadband is the difference between actuation point and re-actuation in a pressure actuation switch. For example: if a pressure switch is set to operate at 100 psi on increasing pressure, the switch will close when pressure rises to that point. As pressure drops to 95 psi the switch opens (this is the re-actuation point). The deadpoint of this switch is 5 psi, the difference between the set point of 100 psi and re-actuation point of 95 psi. Traditionally, a narrow deadband is used in safety services. A wider deadband is used on control circuits like hydraulic units. Tight or narrow deadbands are found on bourdon tube or diaphragm switches, wide deadbands are available in piston type switches, while solid-state switches offer a fully adjustable deadband to 100% of full scale.