To properly select a mechanical pressure switch, you need to know eight essential specifications. Miss one, and the switch you receive may not be best suited for your intended application. This checklist walks through each specification and how it relates to the pressure switch options you select when putting together a model number. Gather your requirements and specifications, then request a quote.
Quick Reference Checklist
- Adjustable Range, Set Point & Dead Band
- Design & Operating Pressure Ranges
- Housing: Enclosure Rating, Agency Listings & Conduit Connection
- Switching Element: Electrical Rating, Contact Configuration, Dead Band Multiplier
- Wetted Parts Materials: Diaphragm, O-Ring, Pressure Port
- Process Connection: Size, Type & Orientation
- Location Classification & Agency Approvals (UL, CSA, FM, ATEX, SIL)
- Process & Ambient Temperature Ranges
1. Adjustable Range, Set Point & Dead Band
For your set point you will need to know not only the pressure value but must also specify whether the switch will actuate with increasing pressure or decreasing pressure. You will also need to know the pressure value at which the switch will deactuate. These specifications correspond to the adjustable range and dead band, accordingly, and are dictated by the pressure switch’s piston-spring combination.
In the SOR Measurement and Control Pressure Switch Catalogs, each piston-spring combination has a corresponding adjustable range and typical dead band. Your set point must fall within the adjustable range – for best performance, you should select a piston-spring which puts your set point as close to the middle of the adjustable range as possible.
Additionally, the typical dead band values shown in the literature are for a midrange set point. By choosing an adjustable range which places the set point as close as possible to midrange, the dead band will be as close as possible to the published value. That said, the final dead band value also depends on the switching element, which is covered in step 4.
2. Design & Operating Pressure Ranges
A pressure switch must be able to withstand the pressure it experiences during normal operation, as well as the maximum pressure possible for the application. These two specifications correspond to the overrange pressure rating and the proof pressure rating, which are also dictated by which the piston-spring combination. Ensure the piston-spring option you select has an overrange pressure rating that covers your max operating pressure and that the proof pressure rating is suitable for your application’s max design pressure.
3. Housing: Enclosure Rating, Agency Listings & Conduit Connections
SOR Measurement and Control pressure switch housings fall into three categories based on their enclosure rating. General purpose housings are intended for clean, safe applications such as in a laboratory and carry a NEMA 1 rating. Weatherproof housings cover general outdoor applications and carry NEMA 4, 4X and IP66 ratings. Explosion-proof housings carry NEMA 4, 4X, 7, 9 and IP66 ratings and should be chosen when the application requires certification for use in hazardous locations. See step 7 for more details on agency listings.
Most housings are constructed of aluminum or carbon steel. For applications with corrosive environments such as offshore platforms, external surface treatments like epoxy coating and fungicidal varnish can be added for supplemental protection and are available as accessory options.
The housing selection will also determine the number of conduit connections, their size and thread style, and where they are located on the housing.
4. Switching Element: Electrical Rating, Contact Configuration, Dead Band Multiplier
Start by defining your electrical load: is it AC or DC? What are the max voltage and max current? Next, choose whether the contact configuration will be SPDT (single pole double throw) or DPDT (double pull double throw). Lastly, for the selected switching element, you will need the dead band multiplier; ensure you use the multiplier that matches your selected contact configuration.
Taking the typical dead band value determined in step 1, multiply it by the value of the dead band multiplier. The resulting value will be the dead band, or pressure change from the set point required for the pressure switch to reset. This reset point will be in the opposite direction of the set point – for example, a set point of 5 psi increasing with a 1 psi dead band will reset at 4 psi decreasing.
Depending on your application, you may want a narrow dead band for tighter control or a wide dead band to prevent the switch contacts from chattering when the process pressure fluctuates. If the dead band value you calculate is too large or too small, select a different switching element with a dead band multiplier that gets it closest to your desired value.
5. Wetted Parts Materials: Diaphragm, O-Ring, Pressure Port
On an SOR Measurement and Control pressure switch, there are three components that will be in contact with the process media. Before reviewing the wetted part materials, identify which materials are chemically compatible with your process media. The diaphragm and o-ring materials are selected together – diaphragms can be constructed of metal or polymer while the o-ring is always a polymer. Alternatively, welded metal diaphragms are available for extremely corrosive, hot or harsh processes where o-rings are unsuitable. The final wetted part is the pressure port. It is normally made of metal but can be specially constructed of non-metal materials when necessary.
6. Process Connection: Size, Type & Orientation
While the material of the pressure port was selected during the previous step, you still need to specify the process connection. Determine the required connection size and type, such as ¼” NPT(F). Other thread types, such as BSP(F), or alternative connection styles, like flanges, are available as special options.
Finally, note the mounting orientation and any space constraints for where the pressure switch will be installed. It should be mounted to a bulkhead, panel rack or pipe stanchion using bolts – line mounting by either the process connection or conduit connection is not recommended. While the device can be mounted in any position, SOR Measurement and Control recommends installing it with the conduit connection at 6 o’clock (or facing down) to prevent condensate from collecting inside the housing.
7. Location Classification & Agency Approvals
SOR Measurement and Control pressure switches satisfy the requirements of approval agencies worldwide. Often, a common design is capable of multiple approvals. For example, in the Pressure and Vacuum Switches Catalog (Form 216), one model number can carry the following approvals:
- CSA – Class I, Groups B, C, & D; Class II, Groups E, F & G; Divisions 1 & 2
- UL – Class I, Groups B, C, & D; Class II, Groups E, F & G; Divisions 1 & 2
- ATEX/IECEx or INMETRO – Ex db IIC T5/T6 Gb
Available approvals and the corresponding required options can be found in the pressure switch catalogs. Review the Agency Listings section to ensure that both the agency approval is suitable for your area classification and that the model number you’ve selected complies with the approval’s required options.
8. Ambient & Process Temperature Ranges
The final specifications you need are the ambient and process temperature ranges. The ambient temperature range comes into play with the switching element that was selected in step 4. Ensure the temperature rating of the switching element covers the ambient temperature conditions it will be subjected to.
Next, compare your process temperature range against the temperature ratings of the diaphragm and o-ring materials you selected in step 5 and ensure they are compatible. If the standard material options are not compatible with the process media and temperature, special materials are also available. Contact your SOR Customer Service Representative if you need assistance finding suitable materials for your application.
Frequently Asked Questions
Q. Which adjustable range is best for my set point?
A. For best performance, you should select the adjustable range where your set point falls close to the middle of the range. Additionally, by choosing an adjustable range which places the set point as close as possible to midrange, the dead band will be as close as possible to the value published in the SOR product literature.
Q. What is dead band on a pressure switch?
A. Dead band, also called differential, is the difference in pressure between the pressure that actuates the switch (the set point) and the pressure that deactuates it (the re-set point). A narrow dead band gives tight on/off control. A wide dead band stops the switch from chattering in a process with pressure swings.
Q. Does a mechanical pressure switch need power?
A. No. A mechanical pressure switch uses process pressure alone to move the sensing element and actuate/deactuate the switching mechanism. It provides independent, hardwired protection that keeps working without external power or a control system, which is why it often runs alongside an electronic transmitter instead of replacing it.
Q. Can one pressure switch cover both a high and a low set point?
A. Yes. The SOR Measurement and Control Dual Hi-Lo Pressure Switches have two independent set points, so you get a high alarm and a low alarm without installing two separate switches. It can also be calibrated with two high set points or two low set points, making it suitable for a wide range of applications. See the Dual Hi-Lo Pressure Switches Catalog (Form 217)(PDF) for more information.
Q. What applications use mechanical pressure switches?
A. Mechanical pressure switches are specified for compressor shutdown, pipeline pig launcher monitoring, rupture disc supervision, pulp and paper mill process control, and tire pressure applications, among others.
Getting Your Switch Built
Write down your answers for the eight specifications and submit them using the form on our website to request a quote. You can also find your local SOR representative to work through your specifications directly. Browse the SOR pressure and vacuum switch literature to learn more about our offerings for each of the eight options we reviewed.
