Pressure Sensor Technologies
Back in the days of mechanical, analog pressure gages, most designs were variations on one basic approach. However with the shift to electronic sensors, a whole new group of measuring technologies emerged. Should you pay attention to the sensor design? Is that instrument magnetic? Capacitive? Stressing a strain gage or crushing a crystal? How much do you need to be concerned?
For most garden-variety applications, any approach will likely do an adequate job. However if you want truly optimal performance, or when more extreme conditions enter into the discussion, some do not perform as well as others.
Evaluating a pressure sensor technology should consider a variety of variables, including:
Accuracy (amount of reading error);
Linearity (signal changes in same proportion to pressure change);
Repeatability (same reading from same pressure);
Hysteresis (does not tend to accept a “set” from a single operating point);
Durability (can survive many high/low pressure cycles);
Stability (stays in calibration);
Electronic interference protection (immunity to local EMI);
Temperature range (can tolerate probable process fluid/ambient temperature extremes);
Physical sensitivity/ruggedness (can withstand pressure spikes, knocks, and vibration);
Practical range limits (suited for its operating pressure);
Failure mode (visibility in reading, potential to release or contaminate process stream);
Safety (certified for hazardous duty or not);
Power consumption (suitable for battery operation or not);
Output signal (what format, analog/digital);
Materials of construction (temperature limits, corrosion resistance);
Some of these are quantifiable and are published in manufacturers’ literature. Others are difficult to specify. For example, virtually any design will have accuracy or temperature limits in the catalog, but stability or ruggedness are far more difficult to put into terms that allow direct comparison. Some of these factors relate to the sensor technology itself. Others are influenced by signal-processing electronics or case design.
Electronic measuring technologies virtually all work on the principal of measuring displacement to a diaphragm caused by the pressure on one side. Differential pressure units either have two diaphragms or one diaphragm that can be displaced in either direction. The configuration and composition of the diaphragm changes from manufacturer to manufacturer, as does the means of measuring displacement.
The main technologies used are capacitive, piezoelectric and strain gage. (See sidebars.) Less common approaches include resonant frequency, electro-mechanical, magnetic, or other approaches. Each of these has its particular advantages and limitations.
How many designs?
Some companies stay with one or two approaches but have developed them extensively. Others offer product lines that incorporate a wider range of offerings and technologies. For example, Endress+Hauser manufactures manyep vacuums at elevated temperatures, abrasive slurries, corrosive chemicals and frequent overpressure (e.g., water hammer).”
Crystal Engineering’s designs all incorporate oil filled silicon crystal piezoresistive technology. Miranda Battenburg, director of marketing says, “Our gauges are digital, hence there are no moving parts that will be affected by vibrations or movement, making them ideal for process plants and refineries. The diaphragm rests against the silicone oil and the sensing element picks up pressure from the oil without contacting the diaphragm directly. The advantage of that is to protect the sensing element from damage from any chemicals that pass through the fitting.”
Yokogawa changed directions and left behind two approaches in favor of one new sensing technology. “In the 1990’s Yokogawa developed a unique digital pressure sensor to replace the differential capacitance and piezo resistive technologies used until then,” says Allen Erwin, transmitters product manager. “Both piezoresistive and capacitance methods date as far back as the 1960’s. The digital sensor we use in our DP harp series of pressure transmitters represents the latest pressure sensing technology. Improvements to performance, features and benefits are the driving forces behind most innovations.”
Using an electronic update to traditional mechanical technology, Ashcroft provides electronic output from an analog direct-reading bourdon tube gage. “The proprietary Xmitr design is based upon an eddy current sensor,” says Mark Zabawa, Ashcroft’s pressure transducer product manager. “With it the mechanical movement of a bourdon tube is translated into an electrical signal. A flag assembly attached to the bourdon tube and moves between two coils providing a signal which corresponds directly to the applied pressure. This gives the user both a local and remote indication of pressure built into one instrument, and can be directly interfaced into existing process control systems.”
Other companies produce a wider range of technologies, working to create the right tool for the right job. “Honeywell Sensing & Control (S&C) has one of the broadest pressure sensing portfolios in the sensor industry,” says Lamar F. Ricks, global engineering leader, pressure measurement. “Technologies utilized in S&C pressure products include bonded strain gage, bonded foil gage, silicon piezoresistive, oil-filled silicon piezoresistive, thin film, advanced thick film (ATF) and surface acoustic wave.”
With all those options available, how do you decide what to recommend to a customer? “Oil-filled silicon piezoresistive technology is perhaps the most widely utilized pressure sensing technology in pressure transmitter products,” Ricks adds. “It is a highly reliable and stable technology with a proven track record, offering a media isolated pressure sensing solution specifically developed for sensing hostile media in heavy duty or harsh environments and compatible with 316 stainless steel. Advanced thick film (ATF) strain gage technology is a more recent development but is also proven and has some very unique benefits and performance advantages for certain applications such as hydraulics. ATF technology based solutions can offer superior reliability and mean time between failure (MTBF) performance in dynamic pressure environments.”
How to choose
Most end-user companies have internal preferences for specific instrumentation manufacturers which may guide a new selection. But if you’re starting with a clean sheet of paper, what’s the best way to choose a technology and know that you are getting the right tool for the right job?
“Like most process instrumentation, the user must work with the manufacturers to identify the best technology for the application,” says Kevin Lavelle, product manager Ametek PMT pressure sensors. “Factors such as pressure range, performance, environmental requirements, and price/volume all play a significant factor in choosing the right technology. The user must evaluate the spectrum of pressure technologies and weigh the various decision criteria before selecting the best approach for the application.”
Making a choice can also be based on knowing when not to choose a specific approach. Manufacturers are usually clear about applications where a design should be avoided. “Turck’s sensor technology employs a ceramic diaphragm, adapter, housing and an O-ring as part of the sensing package,” says Roger Saba, senior instrumentation product manager. “In this design, the media comes in contact with the ceramic diaphragm, O-ring and adapter. These sensor types can only be used in benign environments, such as air, clean water and sweet natural gas that require no hermetic seal and medium pressures up to 1000 psi. These sensors are not suitable for operation in ammonia, hydrogen, oil & gas production, oxygen service and many other mild to harsh applications.”
Technology or performance
If a given device has the necessary capabilities for your process conditions, including accuracy, range, operating temperature, etc., should you care about the sensing technology? “These days it’s more practical to select the total instrument that will match a given application, says Allen Hood, product manager with ABB instrumentation. “Today’s pressure instrumentation affords many more specialized choices depending on what your main criteria may be, whether it’s accuracy, materials of construction, or critical features and functions.”
Kevin Coffey, president of HBM, deals with extreme applications daily, and knows the importance of those specialized choices. He suggests durability gains importance as pressures rise: “Strain gage based pressure sensors like those supplied by HBM are different from other classes of sensors because they are capable of delivering high accuracy and temperature stability over a very high number of life pressure cycles at very high pressures.”
Other manufacturers who deal with more modest process requirements would likely still say sensor choice is important, but relatively few actually discuss sensor technology to any great extent in their literature. Most users and manufacturers understand that each approach has its advantages and downsides. A company that is intimately familiar with the characteristics of a given technology will know how to mitigate problems either through mechanical design or corrective electronics. Advanced signal processing has become very sophisticated in current design devices and can smooth out the performance of virtually any sensor.
Scott Nelson, vice president, pressure products for Emerson’s Rosemount division, has witnessed many changes during his involvement with instrumentation for 20 years. “I think of what we used to do with the training of our salespeople and our demos: the sensor was key and our tools were very sensor-centric,” he says. “But with the advent of microprocessors and the ability to correct a lot of errors in the electronics, they’re much more reliable today. Rosemount’s transmitters and a lot of our competitors’ are far better than they were 20 years ago. I can’t remember the last time when we’ve been talking to Mobil or Dow or Exxon where we’ve discussed pressure sensor technology, although that’s still common when discussing flow or level measurements.”
A simple pressure instrument may have more sensing capabilities than meet the eye. “All of our pressure products are internally temperature compensated,” says Ametek’s Lavelle. “Some models offer that on-board temperature measurement as an alternate output. Ametek PMT pressure sensors have seen growth in demand for multivariable devices that can provide outputs for static pressure, differential pressure, and temperature within one transducer. This is useful because a user can now make a differential pressure measurement, compensate for both static pressure and temperature effects all from the outputs of one device. Our customers are decreasing the sensor ‘footprint’ which is very important for OEMs, minimizing the number of penetrations into their piping, optimizing their power management, as well as reducing their inventory of spare parts.”
Nelson says those capabilities are a good start, but engineers today want even more. He adds, “We were talking to customers yesterday and the dialog was, ‘Yes, we need to know how reliably your instrument generates the process variable (PV), but we want to know how your device can help us improve the safety and efficiency of our plant, and asset utilization.’ Then the discussion turns to advanced diagnostics where we’re monitoring the process signature of what’s flowing through the pipe or sitting in the vessel. At 22 times per second we’re giving them the mean of the process or the standard deviation of the changes in the process and other types of information above and beyond the basic PV measurement. The intelligence of the devices and what they’re able to do is incredible. One pressure transmitter can give you super-accurate differential pressure, gage or absolute pressure, process temperature, device temperature, calculate mass flow in real time, while providing the mean, standard deviation and other annunciation when there are changes in these variables against user selected limits. And it does all this without being any larger or consuming any more power than our unit from 20 years ago.
“These process diagnostics are having an increasing effect on our customers. If you have to have thousands of pressure transmitters in your plant, they can be like stethoscopes against the pipe, using something that’s already out there to monitor process changes. If you have a liquid flow in a pipe across an orifice plate or an annubar, you have a nice, stable, consistent smooth signal. If there’s a plant disruption where there is a change in the material going through, like air bubbles or gas infusion into the flow or solid particulates, you’re going to change the noise signature dramatically, and the intelligence and speed of response of the transmitter allows us to provide diagnostics that apply to safety and other changes that are happening in the process. The sooner I know about it, the quicker I can react. There are more enabling capabilities for all sorts of things.”
What matters, ultimately?
When looking at that blank sheet of paper and trying to decide which vendors to call, the best thing you can do is find as many answers about the process as you can. Return to the list at the beginning of this article and see how many of those you can answer for the process. How much accuracy do you need? How dynamic are pressure changes? What is the operating temperature range? Having that data in hand when you talk to a prospective supplier is far more important than pondering the subtleties of sensor technology.
|Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .|
This approach is simple mechanically and very adaptable to differential, gage and absolute configurations. Internal electronic conversion functions are straightforward with an excellent signal to noise ratio, however there is enough temperature sensitivity to require internal correction of the output. Capacitive sensors are among the most accurate, but relative to other technologies, they have a narrow operating temperature range and low practical maximum pressure range, although both are adequate for most process applications. The diaphragm is effectively a charged plate and placed opposite one or more other stationary plates across a gap. As the diaphragm deflects, the capacitance between the plates changes. Diaphragms can be metallic or ceramic.
Capacitive designs use relatively little current which makes them suitable for battery powered wireless applications. Their internal temperature sensing capability can often be accessed as a secondary process variable.
They also are sensitive to temperature and typically have an internal temperature sensing capability to correct the main measurement. Often this can be accessed as a secondary process variable.
Strain gage sensors
Strain gages are particularly sensitive to temperature, so internal correction is standard. Often this can be accessed as a secondary process variable.