Originally Published MPMN March 2008

PRODUCT UPDATE

Under Pressure: Evaluating Different Sensing Technologies

Pressure sensors and transducers can incorporate a range of sensing technologies; however, each one brings distinct pros and cons

Shana Leonard

Pressure sensors and transducers are critical components in many medical applications. They monitor the pressure of gas or fluids and provide feedback to ensure that everything is normal, or provide alerts to indicate otherwise. Owing to the components’ important role in a finished product, pressure sensor selection should not be taken lightly. When choosing a pressure sensor or transducer for a medical application, OEMs must carefully consider a variety of factors that include accuracy, media, package size, and environment, among others. Taking these into account, OEMs must then determine which sensing technology is best for their application—a debate among sensor suppliers that centers heavily on metal- versus silicon-based processes.

Metal Strain-Gauge Technology

Consisting of mounting metal elements onto a metal diaphragm using an insulator or epoxy, metal foil strain-gauge technology is typically utilized in sensors operating in high pressure ranges. Critics are quick to point out that such sensors have a tendency to drift over time, while supporters of the technology tout its media compatibility and relative stability.

“Foil strain gauge is a proven technology that has been around for more than 70 years now,” says Javad Mokbery, president of Futek (Irvine, CA; www.futek.com), a sensor supplier specializing in the technology. “It is far more stable temperature-wise and [enables] better control than silicon MEMS technology.”

Metal-based sensing technology also offers greater design flexibility as compared with silicon, Mokbery says. “With MEMS, you are limited in terms of pressure range. With [metal] technology, you can go as high as 100,000 psi if you wish, and as low as a fraction of an inch.”

Building on its experience with load cells and force sensors for medical devices, Futek has introduced the foil strain-gauge-based PMP410 pressure sensor to the market. Suited for medical applications subjected to mechanical shock, vibration, and EMI, the product can be employed in filtration systems, some dialysis equipment, and any device that needs to monitor pressure in fluid lines.

The sensor has stainless-steel construction for media compatibility and is designed without soft internal sealing materials that could react to the media or deteriorate over time. Additional features include accuracy to within 0.5% and standard signal output of 5 V dc, which enables integration with existing applications. A piezoresistive measuring cell is employed for applications with pressure ranges up to 300 psi, while thin-film technology is used when higher pressure ranges are required.

Silicon MEMS Technology

MEMS-based sensing technology features implanted piezoresistors on a thin silicon membrane that changes in response to applied pressure rather than strain, as is the case with strain-gauge designs. Drawbacks of silicon include its poor performance in extreme temperatures and its tendency to experience warm-up drift during start-up mode. Because of the high accuracy demanded of many medical applications, warm-up drift can be a particular danger since it can cause pressure sensor feedback to vary until the equipment achieves operating temperature.

Among the benefits of silicon-based pressure sensors, on the other hand, are that they can be mass-produced easily, are effective for low-pressure applications, and offer repeatability. Furthermore, MEMS technology enables extremely small package sizes, which has proven especially valuable in the minimally invasive market. But while MEMS-enabled miniaturization is a clear advantage of silicon, it also has limitations. “In the case of a pressure sensor, we cannot shrink below certain limits because it is an electromechanical device,” says Bala Kashi, product manager for medical devices at Measurement Specialties Inc. (Hampton, VA; www.meas-spec.com). “Because it is also a mechanical device, it has to have a minimum size; that’s where a lot of limitations are.” Kashi cites some invasive and implantable probes as especially challenging because of the extremely small pressure sensor size required.

Measurement Specialties focuses its efforts on silicon sensing technology, although it does offer foil strain-gauge products as well. The company’s MSP300-series pressure transducer is designed for external medical equipment and also can be used in cryogenic angioplasty procedures to monitor the liquid-nitrogen pressure applied as well as the dangers of bursting the balloon.

The product features the company’s proprietary Microfused technology and features a compact vibration-resistant package. Engineered without seals in the pressure port, the transducer features a port and diaphragm machined from stainless steel. The sum of all errors—including linearity, hysteresis, and temperature—is ±1% full scale. Customization includes snubbers for hydraulic hammer protection.

Thin-Film Technology

Thin-film technology combines elements from both MEMS and foil strain-gauge methods. Like foil-strain gauge technology, thin-film processes employ metal rather than silicon and are better suited to high-pressure applications. But unlike with foil strain-gauge sensors, thin-film-based-units pressure do not feature an epoxy, which many experts speculate contributes to greater stability.

Gems Sensors & Controls Inc. (Plainville, CT; www.gemssensors.com), a supplier of sensors and fluidic systems, draws from silicon MEMS processes for its thin-film technology. The company has semiconductor fabs where it builds sensing elements in a wafer format on which it deposits resistors and circuitry, treating the metal ‘wafer’ as if it were silicon, according to Kris Lafko, product marketing manager. However, the company does not micromachine the metal, as would be the procedure for a MEMS wafer.

“What this [process] brings is incredibly long-term stability that is far, far better than silicon,” Lafko explains. “Because it’s metal-to-metal, it’s extremely good in critical environments, high pressure ranges, and wide temperature ranges.”

Furthermore, thin-film technology is suited for pure oxygen applications in anesthesia equipment, whereas oil-filled stainless-steel MEMS sensors pose a risk in such applications because the oil can become toxic or flammable if the stainless-steel diaphragm ruptures or the weld fails over time, Lasko says. Applicable for such uses, the Gems Sensors 3100- and 3200-series pressure transducers feature stainless-steel configurations, measure less than 1 in. in diam, weigh less than 32 g, and demonstrate long-term stability. The products can measure pressure ranges from 0–100 to 0–30,000 psi. Accuracy is 0.25% and long-term drift is limited to 0.1% over the full scale per year, according to the company.

Kavlico Corp. (Moorpark, CA; www.kavlico.com), a sensor and transducer manufacturer, is involved with numerous kinds of sensing technologies ranging from ceramic capacitive to several MEMS-based methods. The company also provides pressure sensors based on a thin-film process. Benefits of such sensors include its hermetically sealed configurations, insensitivity to most media types, and lower cost in high- volume applications as compared with some competing technologies, according to Chris Dixon, director of sales and marketing.

But, as with all technologies, there are some drawbacks. “All thin films are not created equal,” cautions Dixon. “A disadvantage of thin film is the relatively small signal that responds to the pressure. Kavlico uses a proprietary film that produces an output of 4–5 mv/V, where competing technologies can be 3 mv/V or lower. That has implications in the ultimate accuracy and temperature performance of the sensor, as a lower signal is more prone to signal or noise issues that will ultimately limit accuracy.”

The company’s most recent product, however, incorporates piezoresistive sensing technology instead, for miniature package design and flexible pressure ranges. The P6050 surface-mount pressure sensor is compatible with standard PCB assembly methods and is suitable for pressures ranging from 0–13 in. H20 to 0–75 psi. It is designed to measure air or dry gases in such medical applications as oxygen concentrators, ventilators, and respirators. Accuracy specification is ±0.5% for linearity, repeatability, and hysteresis at 25°C.

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