Choosing the right pressure transducer for your application depends on a variety of factors.
Here are 10 important things to consider when selecting pressure transducers:
Accuracy
Pressure type
Gauge, absolute or differential
Pressure range
Signal output
Analog
Digital (query response or continuous)
Ambient and media temperature
Wetted material
Body material
Enclosure rating
Pressure/reference fittings and adapters
Size
There is not a hierarchy of need implied in this list. One attribute or a subset of these may be more or less important to you. Your application will determine which one is most important. For simplicity and with a nod to David Letterman, we'll start at 10 and go down.
Reason: The size of the transducer you choose will have to fit where it is intended to be used. This may not be a problem for an industrial plant application or manufacturing environment, but could be a crucial selection factor for an original equipment manufacturer (OEM) that has limited space within an enclosure.
Action: Measure the space where the transducer will be used.
Reason: The pressure and reference fitting that connects the transducer to the process must be compatible.
Action: Check availability of adapter fittings that will facilitate ease of connection within the existing design.
Reason: The enclosure for the transducer must have a rating that will prevent exposure to harmful external conditions, including ingress of liquids, humidity, dust or gasses.
Action: Check the Ingress Protection Rating (IP) of the transducer to verify its compatibility with the expected external contaminants.
Reason: The body material of the transducer must be suitable for the environment in which it is deployed.
Action: Verify compatibility with any corrosive or weathering elements that may exist in the application environment.
Reason: The pressure medium may be a corrosive liquid or gas, so the wetted material of the transducer must be compatible.
Action: Check the transducer specification for the material(s) that will be in contact with the application medium. Determine if there are suitable isolation components within the transducer that protects the sensing element. If necessary, consider external media isolation components like diaphragm seals or separators.
Reason: The media temperature and the ambient temperature where the transducer will be used should be within the limits prescribed for the transducer. High and low temperatures outside the transducer limits can damage the transducer and affect the accuracy.
Action: Check the temperature specification of the transducer and the proposed environmental conditions and medium temperature for the proposed application.
Reason: The signal output of the transducer must match your existing infrastructure or the communication protocols designed into your application. These outputs could be an analog electrical signal like 4-20 mA, a voltage signal, or a digital signal that is sent on demand or via continuous output. The reading output rate or readings per second should also be compatible with the application to ensure that all data is captured, including intermittent or transient spikes.
Action: Check the communication protocol of the prospective transducer to make sure you have the means to communicate and the rate of transmission is acceptable.
Reason: The pressure range is one of the more important features of the transducer. The minimum and maximum range that will be encountered in the application must be encompassed by the transducer's range. Because the accuracy is often a function of the full scale range, a range just high enough should be considered to achieve the best accuracy.
Action: Check the transducer specification. It will either have a list of set ranges or a customizable range that can be chosen between a minimum and maximum boundary. Range availability will be different for each pressure types (see below).
Reason: The pressure type is defined by the reference pressure of the measurement. Absolute pressure is measured relative to absolute zero pressure, gauge pressure is measured relative to atmospheric pressure, and differential pressure is the difference between one arbitrary pressure and another.
Action: Make sure what kind of pressure you need to measure and check the transducer specification to see if it is available.
Reason: The accuracy may be the most important characteristic. It tells you how close the pressure measurement is to the actual pressure. Depending on the application this may be of utmost importance, or the reading from the transmitter may only be used as a ballpark figure. Either way, it gives some degree of certainty in the measurement being transmitted.
Action: Check the transducer specification. It will give a value for accuracy as a function of the transducer's pressure range. Care should be taken in the interpretation of this value as different manufacturers report accuracy in many different ways.
These are the top ten things that we think are important in choosing the right transmitter. Leave a comment below if you can think of some more.
Related Reading:
Pressure switches are important components in your application and accurate and reliable pressure measurements are key to ensuring a safe and efficient process. An incorrectly specified pressure switch could lead to actuation errors, damaged equipment, worker injury or other dangerous issues.
How do you know if you have the right pressure switch? And what factors should you consider if you need a new one?
We’ve heard this question often here at Ashcroft and I’ve developed some best practices for things to consider while you’re researching. This article will outline the steps I recommend you take when choosing or replacing a pressure switch.
Over 50% of pressure switch applications require the switch to be used to provide safety, whether it be to start or stop a device or to send an alarm signal to the user. Thus, picking the proper pressure switch for your application is critical.
There are many things to consider when selecting the best pressure switch for your application, including the style of switch (diaphragm, piston, bellows or electronic), wetted materials, housing construction, setpoint requirements, application pressures (working and maximum) and whether any approvals are required.
Another important attribute of a pressure switch is the accuracy of the setpoint. Accuracy is also known as the repeatability of the switch, or the ability of the switch to provide the same setpoint time after time.
Different styles of pressure switches have all sorts of different accuracies. Electronic switches can have very good or high accuracies, while some very basic switch designs can have very poor or low accuracy specifications.
Make sure your switch is manufactured by a trustworthy organization that uses quality manufacturing and engineering practices. Your switch should be built and calibrated to industry standards to ensure accuracy and reliability. Also, it is important to remember that pressure switches are calibrated sensors that may require recalibration over time based on your application.
Price can also be a consideration when choosing a switch. You can find low-cost switches that use simpler designs and low-cost materials (i.e., ABS plastic) as well as switches that will be a higher cost but can provide better accuracy, reliability and a higher level of safety.
It is important to understand all the requirements of your application to pick a switch that best suits your application. Here are more considerations when choosing a pressure switch:
Identify the standard operating pressure along with the maximum pressure of your application that the switch could see.
Why is it important to identify these pressures? You must make sure the pressure switch can be safely used in your application. Choose a switch that is ranged to handle not only the normal operating pressure of your application but also any pressure spikes you encounter in your system.
Determine if your required setpoint can be achieved by the switch.
Regardless of whether you choose a mechanical or electronic pressure switch, there are limitations as to what setpoints can be achieved that are directly tied to the specific range of the switch. For example, with mechanical pressure switches, the setpoint range of the switch is often limited by the speed and travel of the actuator assembly, meaning the switch cannot provide setpoints in the lower 10% to 15% of the range. Electronic switches can provide setpoints almost anywhere in the range of the switch. You should also determine if you need single or dual setpoints. Having two setpoints can be useful for having two separate alarms (high and higher, low and lower, or high and low).
for your reference.
Be sure to check the compatibility between the process media and the wetted materials of the switch (i.e., process fittings, pistons, O-rings and diaphragms). Incompatibility can cause corrosion issues, safety concerns, leaching into the process media, etc. Ashcroft offers a Compatibility Guide
Recognize any possible high temperatures and compare them to the capability or specifications of the switch.
The datasheet typically lists the temperature specifications that your switch can withstand. As mentioned, using a switch beyond its stated temperature specifications can lead to setpoint drift, component issues and possible safety concerns.
Select a microswitch based upon your application’s electrical requirements.
The electrical ratings of the microswitch are guidelines of the voltages and currents that the switch can be used with to ensure the maximum cycle life of the switch. The listed ratings provided by microswitch suppliers are the voltages and currents tested by third-party independent test labs for the required cycle life. However, keep in mind that microswitches are mechanical pass-through devices, that send the supplied voltage and current to your load. Meaning that the microswitch can be used with many different voltages and currents but may see a reduction in the cycle life.
Identify if the application requires hazardous area approvals or industry type approvals.
This includes hazardous approvals such as explosion-proof, intrinsically safe, non-incendive/increased safety as well as industry type approvals such as boiler and steam limit control approvals. Determining if an application requires hazardous area approval and general-purpose/safety approvals dictate which switch can be used for these types of applications. These approvals are provided by independent and nationally recognized test labs known as NTRLs. Agencies include FM, ATEX, CSA, IEC and UKCA.
Determine if your switch needs additional options.
Do you require a factory setpoint, tags, oxygen cleaning, special materials for housing/enclosure, wall/pipe mounting brackets or specific certifications? One of the most common variations is to have the setpoint of the switch calibrated at the factory. This is known as Factory Set (XFS). Using this variation ensures the accuracy of the switch when received by the customer while adding to the ease of installation because the switch is ready to install. Other options include material choices, special enclosures, NACE certification, metric labeling, pilot lights and much more.
Fixed or adjustable deadband.
The term deadband in pressure switches is the difference between the pressure at which the switch activates (the setpoint) and the pressure at which the switch deactivates (the reset point). Switches can have two different kinds of deadbands, fixed or adjustable. Switches that have fixed deadbands have a deadband value that is determined by the mechanical properties of the switch. Items such as diaphragm material, the switching mechanism and the pressure range of the switch all influence this fixed value. While adjustable deadband switches have deadbands that can be adjusted or selected within a specific range to meet a customer's application requirements.
Using a switch indoors or outdoors.
The environment in which you use your pressure switch can impact its functionality. There are many challenges when using a switch outdoors, including temperature and weather effects (rain, snow, sleet, etc.). You must also consider what IP/NEMA ratings you need to satisfy the location of your pressure switch.
Ashcroft pressure switches come in a variety of configurations to meet most installation requirements in nearly every industry. Our products include differential pressure switches, explosion-proof pressure switches, NEMA 4- and NEMA 7-rated pressure switches and electronic pressure switches.
Our A Series switches are a good solution for OEM system integrators, while our B Series switches are designed for industrial applications. We also offer the T Series of temperature switches and the L and G Series of dual setpoint switches.
There are many options available, so make sure you select the one that is best suited to your specific process.
Now that you know the steps to choose the best pressure switch for your needs, your search for instrumentation should be easier. And with the right switch, you will be able to ensure that your pressure instrumentation is reliable and efficient, and your application is accurate.
If you want to learn more about pressure switches, we have a video available on the basic calibration instruments for Ashcroft pressure switches.
You can also read my other switch article: When Should You Use an Electronic Pressure Switch?
The experts at Ashcroft can help you find the right pressure switch for your process. Contact us today to talk to one of our industry experts and get your questions answered.
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