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Choosing the right pressure sensor

Published: 21 November 2012 - Michelle Winny

With the proliferation of modern sensing technology, choosing the right option can be something of a challenge. Here Jacqueline Leff, Senior Product Marketing Manager at Honeywell offers a simplified guide for designers and product specifiers in choosing the right pressure sensor.

Today’s pressure sensors are called on to work within the harshest of environments - with wide ranging temperatures, hostile and corrosive media – or sometimes to take the simplest of pressure readings.

Whether a particular pressure sensor is suitable for a specific application will depend in great part on the sensor’s attributes, the operating temperature and the environment it’s being specified for, and the job it’s being asked to perform.

So, how do you select sensors that address multiple customer-specific performance requirements? Here is a guide to finding just what you’re looking for based on your performance requirements or design needs.

First, decide what the application for the sensor is. Then, you’ll need to ask some of these basic questions, such as, how extreme are the temperatures or corrosive nature of the media being measured? Is the environment a vibration challenge? What is the pressure range that needs to be measured in psi, bar, inches of water, etc.? How much time are you willing to devote to the integration of the sensor, or must it be ready to implement and forget? What is the pressure type (gage, absolute, differential)? How accurate does the pressure measurement need to be? And, just as important, what are the size and price constraints?

Historically, there have been issues of stability such as drift over time, or how the sensors reacted to temperature and humidity extremes. Until recently, OEMs would have to calibrate their sensors, and end-users did the same. This was a major drain on time and effort, and the end application repeatability and reliability suffered.

Product design sizes continued to shrink, yet sensor miniaturisation often lagged. It was difficult to find modular designs and flexibility in a wide range of package styles and options. Figuring out if a specific sensor actually met accuracy specifications took longer than it should have. Finally, whenever recalibration was necessary, or when the sensors were changed out or upgraded, the process of recalibration started all over again.

Then pressure sensor design began to change. A new breed of silicon pressure sensors was unveiled that would radically affect the experience of design engineers because sensors could come pre-calibrated and temperature compensated. Advances in sensor technology meant that these devices were consistent, reliable and accurate. Of great importance to designers, these sensors also began to be more affordable.

Board mounted pressure sensor varieties

Deciding on the right sensor involves knowing what you need, and not paying for what you don’t. From basic board mounted sensors to heavy-duty pressure transducers, each type addresses specific needs, requirements and cost restrictions.

For example, if you require ease of integration on a PCB, porting and housing options, and small size at an economical price point, a basic board mounted pressure sensor is for you.

These basic pressure sensors are used in applications that require a simple, cost-effective, unamplified, uncompensated, mV output but still need a high quality solution for medical and industrial applications. It’s the smallest package sensor available, designed for customers who don’t need compensation, amplification, calibration, or who want to design in these capabilities themselves.

All basic sensors, however, are not created equal. For example, after soldering, some versions lack stability — and for some, don’t stop drifting for weeks. There are also situations where gel used in some basic sensors can be burned during the soldering process, leading to poorer performance. Finally, based on the plastics used, some devices soak up too much humidity before reflow – and fall apart (popcorning) as a result.

Fortunately, there are new basic sensors on the market that do not have these issues. They meet the latest processing standards including long life after the package is opened. Their ceramic base is thick and is less susceptible to stress. Easy to solder, they stay on the board, stabilising in hours versus weeks. This is a quality sensor for an economical price.

Potential applications include such medical and industrial uses as hospital bed systems, wound therapy, and industrial applications like transmitters.

Continuing to move up the performance-level requirements chain, let’s look for a moment at what design engineers wanted that prompted the recent breakthroughs in pressure sensor technology. For those selecting and implementing pressure sensors into their designs, high on the list of critical features include:

• Very low drift over time or temperature and humidity extremes

• No need to calibrate after mounting to a PCB

• Temperature compensated and calibrated

• Custom calibration availability

• An extremely tight and consistent accuracy specification

• A wide variety of package styles and options with the same performance specifications

• Provides a true measurement of accuracy that could be used to really evaluate, rather than having to test

After basic pressure sensors, the next level of board mounted pressure sensors, are designed for those wishing to leverage a custom algorithm and perform their own compensation, calibration and amplification. What they do provide in comparison to the basic sensor is the ability to retain a high degree of stability, accuracy and repeatability. Ranging from 1psi to 150psi (60mbar to 10bar), these sensors will operate as specified from -40 °C to 85 °C.

These sensors are used when an extremely tight accuracy and miniature form factor are required. Their extremely low power consumption provides extended battery life, and promotes energy efficiency. This type is used for monitoring and control of industrial and medical pneumatic flow and system pressure for precise and efficient performance.

When the highest level of accuracy is needed for applications where stability and repeatability are also required, very high-accuracy compensated, calibrated, and amplified board mounted pressure sensors are the best choice.

Offering ratiometric analogue and digital output for reading pressure over a full-scale pressure span and temperature range, these sensors are fully temperature compensated and calibrated over the temperature range of 0 °C to 50 °C

Of greatest importance for those tasked with selecting pressure sensors, this version first introduced the concept of always indicating total error band—a comprehensive, clear and meaningful measurement that shows the sensor’s true accuracy over a compensated range – versus a variety of different potential errors. This type of specification allows the design engineer to proceed with confidence that all of the potential errors have been considered when implementing this type of sensor into the application.

 Today’s higher-end board mounted pressure sensors should adhere to the moisture sensitivity level 1 requirement to avoid the thermal and mechanical damage during solder reflow processes that lesser rated products often incur. Extremely low power consumption, typically less than 10mW, provides extended battery life. Applications for the compensated, calibrated, and amplified pressure sensors span such medical and industrial uses as ventilators, anesthesia machines, HVAC control, and indoor air quality.

These high-end sensors can often provide the lowest system cost for an end application. It is very important when selecting a sensor to include the costs of all required support components and processes. More ‘basic’ sensors typically require stable, low-noise amplifier circuitry and analogue-to-digital converters to provide a useful signal to the application microprocessor. The additional cost of a microprocessor with an on-board ADC (analogue digital converter) should be taken into account. This is especially true for systems with multiple sensors where external ADCs are needed. Also, consider the production time and expense needed to temperature compensate the sensors in the end application. It is often much more cost-effective to buy a compensated sensor than to test a complex PCB assembly or high-mass device over temperature.

Honeywell sensing and Control



Source: Electronics
Industry Connections: Honeywell Control Systems

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