Selecting Air Flow and Pressure Sensors for Medical Applications

Just as there is a real science behind every medical procedure, there is also a science behind determining the airflow and silicon-based pressure sensors used in complex medical devices to aid in the diagnosis and treatment of disease. Three medical applications that use airflow and silicon-based pressure sensors are: anesthesia machines, sleep apnea machines, and hospital diagnostic equipment.

Anesthesia Machines
The agents used in anesthesia machines present unique challenges for the equipment. These chemicals are often viscous and can cause buildup inside the equipment. This factor must be taken into account and resiliency measures implemented when manufacturing such equipment.

Because medical devices are so important to patients, it is important to select a sensor that provides excellent sensitivity and accuracy throughout the life of the device. Selecting a sensor that meets performance requirements and maintains stable performance over time will ensure that the sensor will function properly throughout the life of the medical device. If the right sensor is selected, a 10-year sensor life cycle is achievable.

The engineer should consider the patient's respiratory rate. Patients can range from adults in poor health with slow respiratory rates to healthy adults with fast respiratory rates and large lung capacity. The sensing section needs to be very sensitive to correctly measure the patient's inspiration and exhalation and the anesthetic gas delivered to the airflow. Therefore, 2 (sometimes 3) airflow sensors are used to measure a specific pneumatic subsystem. The designer should probably have a dedicated airflow sensor for all important subsystems in the device.

Because the combination of anesthetic agents and high humidity creates a harsh environment for sensors, a differential pressure sensor can be considered superior to an airflow sensor in the exhalation circuit. Differential pressure sensors are more resilient to contamination of the gas medium caused by moisture, anesthetic agents, and other materials.

In situations where a high pressure delivery source is required, or where the sensor is in direct contact with concentrated oxygen or anesthetic agents, a media-isolated sensor should be used. In this case, a media-isolated stainless steel pressure sensor may be ideal because it is more durable.

If the device needs to be portable, then low-power airflow and pressure sensors should be considered, which can reduce the size of the power supply required and help limit the overall weight of the unit. Mounting the sensor on the manifold of the airflow path can help reduce the design size and weight.

Another consideration is the output. Digital outputs, such as I2C and SPI protocols, can optimize the resolution of the sensor and integration with a microprocessor. However, there is still a need for analog outputs, mainly because the need for rectification in some safety circuits does not allow software involvement. Users may want to use the raw output of the sensor to trigger an alarm or safety condition. The ability to provide both digital and analog options is important.

Finally, the response time of the sensor is critical to effectively delivering the anesthetic agent to the patient. Using current technology, anesthesia machine manufacturers are able to achieve a response time of 1ms.

Sleep apnea machines
For sleep apnea machines, when it comes to total error band, the focus is usually more on patient comfort and convenience, rather than performance. Because they are often used in conjunction with a humidifier, they need to operate at higher humidity conditions and maintain stable performance. They must be durable because they are often operated by a variety of people in a home environment.

User requirements for the device focus on accuracy, stability, portability, and unit size/weight. Noise requirements are also important because the device is used during sleep. An airflow sensor with a low pressure drop is required because if the pressure drop is too high, the motor will work harder (pressure drop equals impedance in the sensor), which increases noise and shortens the life of the motor.

So engineers should choose sensors that can sense differential pressure or airflow at very low rates. The sensor should be able to measure the peak of the patient's breathing, or the turning point between inspiration and exhalation. For more complex sleep apnea machines, airflow sensors are sometimes chosen instead of pressure sensors because the ventilator needs to be more sensitive at low airflow levels.

Once pressure, airflow, and media requirements are determined, it is time to consider accuracy and stability requirements, including total error band. Home CPAP machines must be durable because external factors can affect unit performance.

The enhanced digital product is often chosen for price reasons. It is more expensive to add components later to adjust the signal size than to purchase the sensor already amplified. At the same time, if this is done, the time spent on integration will be shorter, which allows designers to implement the sensor faster and bring the final device to market faster.

Depending on the type of sleep apnea machine, mechanical requirements such as size, mounting, and drilling may also have an impact on the design of the device, as customers want smaller, more aesthetically pleasing, and more portable devices. Customer calibration capabilities may also need to be considered, especially for CPAP (continuous positive airway pressure) applications, to maximize product performance and best match the patient's breathing pattern.

Hospital diagnostic equipment
Hospital diagnostic equipment includes mass spectrometers, chromatographs (e.g., for gas, liquid, and high-performance liquid chromatography), laboratory automation systems, and analyzers, such as those used for blood, hematology, immunoassays, and clinical chemistry.

When selecting sensors for hospital diagnostic equipment, high resolution, high accuracy, and high stability are all key factors to consider. The equipment needs to be able to detect even the smallest amount of material. Therefore, diagnostic equipment generally has the highest resolution requirements, usually 16 bits or higher. The accuracy and stability of the sensor are important for obtaining accurate data, which is crucial for laboratory test results and directly related to the safety of patients.

Determining the full range and increments of pressure and airflow that need to be detected is the first factor to consider. Hospital diagnostic applications may also require compatibility with media other than clean, dry air. In some diagnostic equipment, gases released from plastics or adhesives, even in very small quantities, can contaminate samples and bias test results. Accuracy, especially for linearity and hysteresis errors, is important. Because the sensitivity of the entire system is related to the sensitivity of the sensor used, hysteresis should be minimized. 0.25% accuracy error is optimal (non-linearity and hysteresis errors), and 0.5% is usually the maximum value allowed.

As mentioned above, high resolution is critical, which is why users of diagnostic and analytical equipment may choose to use unamplified sensors to get as much of the core/raw sensor output as possible and create their own compensation and amplification algorithms. Some sensor manufacturers offer products with amplification via high-resolution A/D converters. Note that a high-resolution A/D converter is not the resolution of the sensor - the resolution of the sensor itself needs to be considered. If the sensor resolution is lower, then the extra bits the A/D has will only provide extra useless data.

Stability is very important because drift can unexpectedly affect the sensor readings. If the sensor drifts after the device is manufactured (calibration is done before the device is shipped), the results will be biased. During the manufacturing and installation of the sensor, consideration must be given to how to prevent the effects of thermal and mechanical stress, as this will affect the stability of the device performance. In hospital diagnostic applications, 0.5% or less is the maximum acceptable maximum for errors related to drift and instability.

Unlike other medical applications, size is not a particularly critical factor for diagnostic equipment, as most are large, immobile devices that are stationary in laboratories. Resolution, accuracy, and stability should be considered first, followed by physical factors such as output, size, mounting, aperture, and power requirements.