ARTICLE FOCUS:
• Pressure sensor stability
• High burst and overpressure
• Sensor packaging

There are many types of pressure sensors available to designers of products used in medical applications. Factor in the vast number of medical applications themselves, and the task of finding the right pressure sensor for the job can indeed be daunting. Understanding how the specific application needs relate to sensor specifications is imperative in the selection process, especially in critical-care applications. When it comes to medical applications, designers may want to consider specifying an ultra-low pressure sensor. This class of sensor offers three key features that are particularly beneficial to critical-care applications.

Stability
The ability for a sensor to retain performance characteristics over time is one of the most important attributes of a board-mount pressure sensor, the variety most commonly found in medical applications. Sensor stability is significant because even a small amount of drift can lead to a large amount of error over time.

It is also important to understand which errors are included in a sensor’s specifications. Some sensor stability specifications only include offset error. Total error band is the deviation from the ideal transfer function over the entire compensated pressure and temperature range (see Figure 1). A stable sensor remains within total error band over time and after exposure to environmental conditions. Using a sensor with tighter error band and stability ensures repeatable and reproducible readings every time. This significantly reduces the need for end-user calibration and saves time in critical-care applications by reducing the need for autozero cycles. In ventilator applications, ultra-low pressure sensors are often used to detect when a patient’s breathing changes from inhalation to exhalation. Measuring this accurately allows precisely controlled operation based on patient feedback.

High burst pressures (over pressure) and high working pressure ranges
High burst pressure above one atmosphere allows the pressure sensor to endure a wide range of conditions while still maintaining a high level of sensitivity. High working pressure ranges also allow ultra-low pressure sensors to be used continuously, well above the calibrated pressure range that they are designed to measure while allowing measurement of very small changes in pressure. Due to the very sensitive elements within an ultra-low pressure sensor, high burst pressures and high working pressure ranges are important. The sensor diaphragm, required to sense small changes in pressure, is vulnerable to overpressure damage. Choosing a pressure sensor that has a high burst and working pressure reduces the threat of failure, allowing continuous use of the medical device over its life.

For medical applications, technicians often need to clean the instrumentation between uses. Frequently this activity results in a higher pressure than the pressure used during the diagnostics itself and can cause damage unless the sensor is specifically designed to resist this type of high pressure.

Flexibility in packaging and output options
Finally, pressure sensors must meet specific system integration requirements. Several components frequently occupy the same space in a medical device, so size and footprint should be considered to maximize space effectiveness. A pressure sensor manufacturer that offers design engineers a modular, flexible design with a variety of packaging styles simplifies integration into the application. Options for electrical connections (surface mount and through-hole), signal type (analog and digital), and port configurations–all having the same industry leading stability–are critical to the designer during the sensor selection and integration process.

See Table I for a complete overview of critical pressure sensor attributes and function in medical applications.

High-accuracy pressure sensors

This company offers pressure sensors that are comprised of piezoresistive silicon and that provide ratiometric output for reading pressure over the specified full-scale pressure span and temperature range. The Honeywell HSC Series TruStability pressure sensors measure absolute, differential, and gauge pressures. They measure pressure ranges from 6mbar to 10bar (1–150psi). The high-accuracy HSC Series features analog/digital output with ±1% total error band, as well as SMT and DIP. The sensors are used for a wide range of medical and industrial applications, and although they were originally intended for use with noncorrosive, nonionic gases such as air and other dry gases (to meet ISO 9001), a liquid media option is now available that allows one port of the sensor to be used with condensing humidity and noncorrosive liquid media such as deionized water. This option eliminates the burden and expense of the end user having to protect the sensor from this type of media. Ultralow pressure sensors offer sensing performance of ±2.5 to ±40 mbar (±1 in H2O to ±30 in H2O). Features include low operating voltage, extremely low power consumption, and ratiometic 12-bit analog output. The RoHA-compliant sensors are virtually insensitive to mounting orientation. Applications include airflow monitors, anesthesia machines, and blood analysis machines among others.
Mouser Electronics, Mansfield, TX, (800) 346-6873

Tactical-grade DoF sensor

A tactical grade 10-degree-of-freedom(DoF) sensor integrates a tri-axis gyroscope, tri-axis accelerometer, tri-axis magnetometer, and a pressure sensor into a single package. The ADIS16488 is a third-generation iSensor MEMS IMU (inertial measurement unit). The MEMS IMU provides a stable and complete integrated sensor suite, supporting mission-critical requirements in high-performance stabilization applications. According to the company, the ADIS16488 outperforms other gyro/IMU offerings on the critical specifications of g-effect, temperature coefficient, and bandwidth by up to 100X. The IMU offers performance equal or better than legacy high-end IMUs on key measures such as bias stability and angular random walk, and better performance on the often more-critical parameters of vibration-rectification, linearity, and bandwidth. Operation is fully autonomous, including all embedded compensations, with valid data available from the SPI interface 500 milliseconds after applying power. With all calibration done at the factory, and with an ADI Blackfin processor-powered configurable interface for tuning embedded filtering and other diagnostics, a major source of design integration time and risk is virtually eliminated. Key specifications include gyro bias in-run stability of 6o/hr, gyro dynamic range of 450°/s, and gyro angular random walk of 0.3 deg/rt-hr.
Analog Devices Inc, Norwood, MA, (781) 329-4700

Starter Kit

The FlexiForce Starter Kit for identifying the appropriate tactile force sensor search is designed for engineers who prefer the convenience of one-stop shopping. The kit contains all of the necessary components to build any of the company’s three recommended circuits. It also includes a guide with detailed, step-by-step instructions on how to build the circuits. The FlexiForce sensors are paper-thin, flexible, and accurate piezoresistive force sensors used to detect and measure a relative change in force or applied load, detect and measure the rate of change in force, identify force thresholds and trigger appropriate action, and detect contact and/or touch. For a limited time, the kit includes two sample FlexiForce tactile force sensors. A free eBook on tactile force sensors is available for download at www.tekscan.com/flexiforce/force-sensors-for-design.
Tekscan Inc, Boston, MA, (617) 464-4500

Embeddable core sensors

An augmented line of embeddable C-Series sensors now includes synchronous serial interface (SSI) and longer lengths. Medical applications such as contrast injection machines, infant training tools, and surgical theater systems have used the company’s Temposonics magnetostrictive linear measurement technology. The C-Series sensors and now Model CL embeddable sensors offer a method of using magnetostrictive technology while retaining all the no-wear advantages of its noncontact nature. SSI is a digital interface method whereby each clock pulse from a controller shifts out one bit of a 24-bit data word from the sensor. The SSI interface has a parity bit option and a choice of binary or Grey-coded output. This new interface augments the standard 5V analog output. The new Model CL sensors offer additional lengths up to 1500mm in 25mm increments. Model CL sensors are also available in 5V analog output and SSI. Both run off of 12V. According to the company, the new products offer medical equipment and device designers more options for elegant, ergonomic designs without the heavy industrial packaging and bolt-on look characteristic of heavy industrial-intended magnetostrictive sensors. The sensors are suitable for devices that are high volume and at the lower cost range of the medical device spectrum, such as hospital beds, dental chairs, and desktop medical analysis devices. Sensor preproduction prototypes are available currently, and production is scheduled to begin in the first quarter of 2012.
MTS Sensors Division, Cary, NC, (919) 677-2314

Detectors

This company offers noninvasive, ultrasonic air bubble detectors, continuous-liquid and point-level sensors, optical blood component detectors, and pressure/occlusion detectors for device manufacturing.
Introtek, Edgewood, NY, (631) 242-5425