||Understanding the Pressure Switch
Pressure switches can add safety and functionality to your machine designs. Here's how you can specify the right pressure switch for any application.
Pressure switches fulfill an amazing variety of monitoring and control applications, and they are employed in virtually every industry, from appliances to automobiles to supercomputers.
A basic pressure switch can monitor air flow in a home heating system...control gas pressure in a water heater...deliver oxygen in a hospital...move paper through a high-speed printer...operate engine emission controls...switch on an aircraft location system...or tell a farmer when to change his tractor air filter.
Most often specified in air proving, process monitoring, and other pilot applications, pressure switches are the silent sentinels of products as diverse as ovens, inhalers, copiers, harvesters, and helicopters.
This article will examine some of the things a pressure switch can do, and will review the steps you can follow to specify the right pressure switch for any application.
Finally, when you need vendor input to refine your pressure switch specification, we also have some suggestions on how to evaluate supplier capabilities.
What is a Pressure Switch
A pressure switch is a mechanical device which converts a pressure change into an electrical function. The pressure change might be measured as pressure, vacuum, or differential between two pressure inputs. In every case, the pressure switch will employ a diaphragm, piston, or other pressure-responsive sensor, which has been coupled to the mechanical means of actuating a switch.
Diaphragm-Actuated Pressure Switch
This is a pressure switch with a silicone diaphragm as the pressure-responsive sensor. Because of its large surface area and very flexible diaphragm material, this type of sensor is able to convert a relatively small amount of pressure or vacuum into sufficient mechanical force to actuate a snap-action switch.
Piston-Actuated Pressure Switch
This is a pressure switch with a metal piston as the sensor. Its robust design and stronger materials enable this type of sensor to work at high pressures, or in hostile media.
Flex Circuit Diaphragm
This is a flex circuit, in which a small metal diaphragm, etched from one layer of a circuit board, is able to make contact with another layer, combining sensor and switch. The advantage of this device is that it can open and close at a very high frequency, over a very long duty cycle.
What Task Will the Switch Perform?
Can a pressure switch add safety or functionality to your design? To answer that question, begin by examining the job you want done. The development of your specification will proceed quite naturally from your detailed requirements for guarding or switching a particular electrical circuit.
- When the patient takes a breath, I want to turn on the medication pump.
- I want to turn off the fryer if air flow in the exhaust flue drops below a minimum.
- If the helicopter sinks in water, I want to automatically turn on the locator beacon.
- I want to turn on a panel light when the filter begins to get dirty.
In the following examples, the magic words are "turn on" and "turn off". These sample designers want to monitor their system for a pressure condition, and at a specified pressure, or setpoint, they want to open or close a circuit.
Let's try a few questions about the circuit you want to control:
- How many volts are you switching?
- How much current will be carried?
- Is the current AC or DC?
- Is the load inductive (converted into work) or resistive (converted into heat)?
These circuit fundamentals will begin to determine what type of switch can be used in your application. If you are switching a very low energy circuit (below 20mA) you may be able to use a simple, momentary action leaf switch. Figure 4 shows a typical switch of this type.
Momentary-Action Leaf Switch
The momentary-action leaf switch is very simple in operation. A change in pressure applies force to the diaphragm, which transfers the force to a moving switch contact. As force increases, the moving contact is deflected until, at the set point, it makes a circuit with a second, stationary contact. The second contact rests against a vernier adjustment screw, which permits fine calibration of the set point.
In a pressure switch, positive pressure pushes the diaphragm. In a vacuum switch, negative pressure pulls the diaphragm. In a differential switch, both sides of the switch housing are ported to two pressure sources, and the diaphragm responds to the resulting net force.
In some applications, a momentary switch can be combined with a triac (bidirectional triode thyristor), enabling the device to switch loads of several amps AC.
Figure 5 shows a switch of this type. The built-in heat sink dissipates heat from the triac. The advantage of this switch is higher current capacity while preserving the momentary switch action. However, for most higher-current applications, an entirely different kind of switch action is called for.
When you examine the mechanical action of the momentary switch, you can see that, as pressure rises and falls, the switch turns on and off at almost the same set point. For some, this may be an advantage. However, other designers may need to work with an on-to-off range, usually referred to as differential or "deadband".
For these applications, the designer wants a device that will trip at the set point, and remain tripped until a fixed re-set point is reached, at which point the switch will return to its original operating position.
This requirement is readily fulfilled by the familiar snap-action switch. Many pressure switches are configured simply by coupling a diaphragm to the actuator of a snap switch. Figure 6 shows a typical snap-action pressure switch.
Snap-Action Switch, External Mechanism
In operation, force against the diaphragm is transferred to a guide disk, which depresses the actuator of the snap switch. To depress the actuator, the guide disk must also push against the opposing force of a spring. The compression of the spring can be modified by a vernier adjustment screw, and this permits fine calibration of the switch set point.
Snap-Action Switch, Internal Mechanism
This is another type of snap-acting pressure switch. This switch provides a larger diaphragm area for very low pressure applications, such as air proving in HVAC applications. Rather than coupling a diaphragm to a standard snap-action switch, this pressure switch features a unique internal switch mechanism, which requires very low operating force, but develops greater contact force than a conventional snap-acting switch.
The characteristics of switch contacts can be influenced by a number of design factors, including materials used (copper, bronze, silver, gold, etc.) and by mechanical forces applied when the contacts are in operation.
In some applications, gold contacts are specified, in order to resist corrosion over long use, or to achieve more reliable switching in very low-energy circuits. In high-current applications, snap-action contacts are required. Contacts which are held together with greater contact force are considered more reliable.
Here, the circuit that you must control will again determine the form of the switch. Will the required switch be SPST or SPDT? Is the switch normally open or closed? What type of terminals are required?
Some switches require solder lugs, or a standard .187 or .250 quick-connect tab. Some OEM switches require wire leads, or other specialized termination.
This is a pressure switch designed for mounting on a PC board. Other switches might require an automotive, computer, or other industry-standard connector.
Medium Determines Diaphragm Material
Where will the pressure switch have to do its work? Will the fluid medium be air, water, combustion gases, or a chemical suspension, such as fuel vapor? Even if the medium is inert, such as cooling water, will contaminants present at any time during your process?
These factors will determine your vendor's advice on diaphragm material, and in some cases, the type of switch that can be used. Fuel vapors may require a fluorosilicone diaphragm. Devices which come into contact with food or medical processes may require EPDM. Depending on the medium to be sensed, the diaphragm membrane may be made of silicone, polyurethane, Teflon®, VITON®, BUNA®, or other materials.
In some cases, the fluid medium may be altogether inappropriate for the use of a diaphragm as the pressure sensor. For very hot, dirty, or otherwise hostile media, some pressure switches use a piston, or a spring, rather than a diaphragm. Figure 9 shows a piston-sensor pressure switch.
Piston Sensor Pressure Switch
In this switch, the pressure sensor is exposed to the medium, while the switch mechanism is protected. As pressure reaches the switch set point, a magnetic piston moves closer to the contacts, which are isolated behind a metal (non-magnetic) barrier. At the set point, the piston attracts the contacts, actuating the switch.
What sort of temperatures will your machine be exposed to? What is the typical range of storage temperatures? What are the maximum and minimum operating temperatures? Your switch vendor will factor these environmental criteria into your final specification.
Will your machine require a pressure, vacuum, or differential switch? To answer that question, consider the pressure source the switch will be monitoring. A positive column of air being pushed by a blower will probably require a pressure switch. Likewise, a negative column of air being drawn by an exhaust fan will probably call for a vacuum switch. If you have two pressure references, such as the opposite sides of a filter, then you most likely need a differential switch.
Whether you choose a pressure, vacuum, or differential switch, it's critical to measure, test, and measure again until you have precisely determined your operating pressures.
First, determine your set point, the point at which the pressure switch must switch your circuit. You may also want to specify a proof pressure, the maximum pressure your machine may expect to encounter. Some designs also require that you specify a burst pressure, which is the pressure required to rupture the switch diaphragm.
To measure operating pressures, a high-quality digital manometer will prove a solid investment. A wide range of bench-top and hand-held instruments are available. Figure 10 shows a hand-held digital model.
Hand-Held Digital Manometer
You can use the manometer to detail your ambient and switch source pressures, making each of these a part of your specification. You can also use the manometer to confirm that incoming samples meet your exact specification, and to troubleshoot your machine when a switch appears to be malfunctioning.
Finally, when you discuss switch set points with a prospective supplier, check to make sure that you are working with the same units of pressure measurement. This sounds obvious, but units such as inches of mercury are easily confused with inches of water.
More Application Details
Will the pressure inputs be steady or will they pulsate from fan or pump action? Many low pressure switches are sensitive enough to detect each turn of a fan blade or pump impeller, so test to make sure that pressure pulses don't cause false actuation.
How will your pressure switch be mounted? When you are using a very low pressure switch, the setpoint can be slightly changed by moving the orientation of the diaphragm from vertical to horizontal. To stay accurate, you need to specify which way the switch will be mounted in your machine design.
If your switch supplier asks about barometric pressure, bear in mind that they are referring to atmospheric pressure in a specific location, and at a specific temperature. Standard barometric pressure is 29.22 inches of mercury, at sea level, and 70 degrees F.
You should note whether your set point is reached on rising or falling pressure. Most pressure switches are set for actuation, which means that, as pressure (or vacuum) increases to the setpoint, the switch trips. Switches are also sometimes specified for deactuation, meaning that as a pressure decreases to the setpoint, the switch trips.
To make the foregoing just a little more confusing, a snap-action pressure switch has an actuation (or deactuation) point, and also has a reset point, which is the point at which the switching mechanism returns to its normal operating position.
A momentary pressure switch which has been calibrated to close at 0.12"wc of rising pressure will also open at 0.12"wc on falling pressure. However, a snap-action switch which has been calibrated to close at 0.12"wc of rising pressure will remain closed until rising pressure reaches its reset point. For this example, let's say that this occurs at 0.18"wc. Every snap-action switch has a characteristic mechanical travel which produces a trip-to-reset differential, sometimes referred to as "deadband" or "hysteresis".
Engineers often use mechanical switch differential to enhance the safety or function of their designs. For example: In a typical gas-fired furnace application, a pressure switch is set to deactuate at a specific drop in the column of induced air, guarding the furnace against various conditions of impaired flow.
Using the mechanical differential of the switch, the engineer also specifies a desired reset point, such point being the minimum flow that must be reached in order to permit system turn-on.
The mechanical differential in a snap switch also prevents nuisance tripping, which, in our example application, could be caused by transient forces on the air column, such as venturi effect in strong winds.
In general, it can be said that a momentary-type switch is slightly more "efficient", turning a circuit on or off at virtually the same setpoint. However, the price of such sensitivity is low contact force in the switch, thus limiting the switch to low-current applications. Conversely, the snap-action switch provides the greater reliability of higher contact forces, but the price is greater mechanical differential.
Set Point Range
The range of a pressure switch is determined by the available variation in its sensor and means of calibration.
In a diaphragm switch, the resilience and free movement of the diaphragm, and also the torsion of the calibration spring are two important factors in how much range can be controlled in the set point.
Should your design require a custom range, the switch manufacturer can adjust this by selecting different types or sizes of diaphragm material, switch contacts, or calibration springs.
Although a custom set point range can offer flexibility, more often the OEM equipment designer wants very specific actuation points, with little or no adjustment. In other cases, differences between equipment models or installation techniques may require alternate settings, or final adjustments at installation.
Field Adjustable vs. Tamper Resistant
Sometimes, an equipment designer specifies a switch that must be calibrated in the field.
For example, a swimming pool heater which detects water pressure, and is typically installed at water level, could require a different setting if the heater were installed higher (or lower) than water level. In such an application, an installer might make final calibration adjustments to a pressure switch in the field.
However, in most cases, designers prefer that pressure switches are tamper-resistant, as this is the only way to maintain quality assurance from the switch vendor through assembly by the OEM.
Calibration Adjustment Screw
Once factory settings are calibrated and checked by the vendor, switches can be rendered tamper-resistant by sealing the threads of the adjustment screw, or sealing the openings which were accessed for factory calibration.
Ports, Fittings, Brackets and More
Ports are available in a wide variety of configurations. Your specification should be guided by your pressure source, the medium, and the means available for conveying pressure to the switch.
Many applications are fulfilled by a simple straight port, which receives a tube from the pressure source. Straight ports are available with barbed or multi-barbed designs which can grip the inside diameter of 3/16", 1/4", or 5/16" tubing.
For applications with higher pressures, ports can be specified with metal barbs, or with threaded compression or NPT fittings.
Specialized Pressure Ports
Figure 12 shows two examples of specialized ports which are available on both momentary-leaf and snap-action style pressure switches.
For applications where pressure spikes or pulses must be dampened, ports are also available with sintered metal dampening or snubbing material.
In a custom application, a pressure switches can be configured to couple directly into the OEM designer's system. Figure 13 shows a pressure switch designed for an tractor air cleaner.
Engine Air Cleaner Switch
This switch is ready for integration into the OEM manufacturer's assembly, with a pressure port configured for the body of the tractor air cleaner, and a weatherproof electrical connector which is appropriate for the manufacturer's engine wiring harness.
While on the subject of ports and fittings, this is also a good time to consider your means of mounting, as a bracket will be the other part of a switch that must physically fit up to your systems.
The configuration of brackets or other means of mounting are sometimes dictated by factors such as existing tooling for mounting holes, available space, or predetermined location of pressure taps.
In most cases, a capable pressure switch supplier will be able to accommodate your existing requirements, or create something new to fulfill your objectives for function or cost.
Pressure switches often require leak-resistant sealing of the cavity between the pressure sensor and the switch contacts. Sometimes, a specification will call for a "hermetic seal" when this is not actually needed.
A true hermetic seal is capable of excluding ten to the minus 9th cubic centimeters per second of helium. More often, in applications such as those in the medical equipment industry, switches might be leakproof to within a few tenths (or, in some cases, hundreths) of a cubic centimeter over a 48-hour period. Pressure switches for these applications may require extra gaskets or o-rings, as well as special leak testing.
Component, Environmental and Other Approval Testing
As you refine your specification, be sure to outline any component, environmental, or endurance testing that may be required. Potential pressure switch vendors may have appropriate data already on hand. Most will work with you to design an appropriate test at their facility or, lacking the right existing data, will provide you with samples for your own testing.
Switch materials can differ widely between similar models, so ask your potential vendors for basic material data on the properties of their housings, diaphragms, and other components, and make sure that these are compatible with your application.
Check to see that potential vendors have submitted switches for general approvals such as UL and CSA, as well as industry-specific recognition (American Gas Association, etc), as required.
You should also ask your potential vendors for data on environment or endurance testing. Experienced suppliers will have worked with equipment designers in many different industries, and they may have already conducted appropriate tests on the bench and in the field.
What to Look For in a Pressure Switch Supplier
Look for experience... Pressure switches are not a commodity. Most applications require a significant degree of vendor support, during both design and implementation, so you want to work with someone who can enhance your specification with the depth and breadth of their knowledge.
Look for vendors who are responsive on the application engineering side of the company. Once you penetrate the company beyond the field salespeople, look for a level of technical assistance that has answers to your questions, and is accessible when you call.
Look for vertical integration in the supplier company's core cababilities. Do they fabricate and build pressure switches from raw materials which they control...or do they simply package components which were fabricated elsewhere?
Look for design expertise. How many of their employees are engineers? Is the potential vendor the designer, with in-house capability to thoroughly understand pressure switches and, if necessary, to create a custom solution for you? Remember that the best vendors are those who can put their engineers to work for you.
Look for state-of-the-art engineering and manufacturing tools. How well is the potential vendor equipped to support you? Do they use 3-D design environments? Rapid prototyping? Automated assembly methods? Can they exchange data or drawings with you on-line?
Look for cost factors. Does the potential vendor design and make much of his own tooling, or is he tied to outside services? Does the potential vendor fabricate more components than he buys, or vice versa?
Look for a track record. Every experienced pressure switch supplier has a track record. Ask for the names of some design engineers, preferably in your own industry, who are working with the potential supplier or are using the specific product that you are considering.
Ask about supplier performance ratings. Major OEM customers closely track and rate their suppliers, month-by-month, on the quality of their product and their overall performance. Ask every potential vendor to tell you about recent reports from large customers.
Look for a relationship. In general, you should evaluate every potential pressure switch supplier, not just on their product strengths, but on the overall qualities that will help or hinder when it comes to maximizing a relationship with you. Today, the very best pressure switch suppliers are capable of serving you as a co-designer, cost-cutter, stock-reducer, value-adder, idea-generator, and all-around strategic partner.