Aircraft Cabin Environmental Quality Sensors

Identification of aircraft cabin environmental quality concerns for which sensors may be useful

• The highest priority environmental indicators identified are ozone and cabin air pressure, followed by carbon monoxide and carbon dioxide with moderate priority, and then relative humidity, airborne particles, and organic contaminants, including engine oil byproducts and pesticides. This list is based on the Congressional requirements and recent scientific literature, starting with information from recent studies (NAS/NRC, ASHRAE/Battelle), and continuing by seeking input from a variety of stakeholders.
• The parameters that can be monitored routinely with off-the-shelf sensor technology are ozone, cabin pressure, CO, CO₂ and relative humidity. These formed the prioritized list of environmental parameters for in-flight sensing. Definition of requirements for sensor and sensor systems LBNL investigators deduced that sensors intended to provide data for routine use by stakeholders must emphasize simplicity, ruggedness and satisfactory performance with limited attention by the crew and maintenance staff. In order to guide maintenance of environmental control systems and document exposure to contaminants, sensors should be installed at multiple locations in the bleed air and cabin air supply/recirculation system, including the return duct. Packaging requirements for installation and operation on aircraft emphasize simplicity, ruggedness and satisfactory performance with limited attention by the crew and maintenance staff. Within these limits:
• Specific requirements or benchmarks for performance emphasize accuracy (±15%), sensitivity (low ambient levels), and sampling interval (≤60 s).
• Suggested requirements include limitations on the size of sensor elements (≤ 3/8 in diameter), weight of sensor systems (≤1 kg), power (28 V), frequency of maintenance (coincident with service schedules), required operator skill (minimal) and target cost for replaceable sensor elements (≤ $100).

Sensor systems most capable of meeting current requirements

• A survey of sensor systems, parameter by parameter, from highest priority to lowest, is included as Appendix A. Systems chosen for testing were based on principles that are representative of the main approaches that are utilized currently for real-time monitoring of the prioritized parameters. The use of COTS systems simplified the experimental approach because data from their sensors could be acquired without project staff designing and building prototype units. However, no COTS system met all the specifications or benchmarks. To proceed under this limitation, sensors judged to be most capable of meeting requirements were tested in COTS systems based on IR or UV spectroscopy, electrochemical cells, and metal oxide semiconductors. The selected technologies were based on light absorption (UV for ozone and non-dispersive IR for carbon dioxide) and electrochemistry (electrochemical cells and metal oxide semiconductors for ozone and carbon monoxide). Pressure sensors are already standard in aircraft, although the output needs to be logged.

Performance of sensors

• Representative sensor technologies were tested in the laboratory under conditions that occur in-flight (cabin air pressure 0.7 to 1 atm; temperature from 65 to 85 °F) and at ground level (relative humidity from 20 to 80%). The results show that neither the EC nor MOS-based sensors responded with at least ±15% accuracy, primarily due to poor reproducibility and hysteresis. The sensors based on light absorption (UV for ozone and IR for CO₂) performed better under the influence of changes in pressure, temperature and humidity than the sensors that depend on electrochemistry (for ozone and CO, analyte-induced redox reactions at sensing and counting electrodes (EC) or at the gas sensing surface of the (HMOS sensor)). The UV-based sensor gave unacceptable performance (> 15% change) only when the relative humidity exceeded 65%, but this condition does not occur in flight because RH rarely exceeds 30%. The optical sensors, both UV and NDIR, need further miniaturization before they can be installed routinely in aircraft.

Recommendations for sensor development

• Circulate the main findings of this study among sensor manufacturers to stimulate development of improved technologies.
• Implement ASHRAE's recommendations for routine monitoring to encourage aircraft-specific sensor designs. When large markets exist for monitoring aircraft cabin environmental quality (ACEQ), developers and manufacturers will have more incentive to miniaturize optical sensors and tailor materials for EC, MOS and other to meet the performance specifications. Costs could approach the benchmark of ≤ $100 per sensor element. (Current EC and MOS sensor elements cost at least twice the target amount.)

Recommendations for future sensor testing

• Broaden the usefulness of the results by evaluating the performance of a larger selection of existing sensors, including GOTS and sensors in the research phase. This would provide stronger guidance for both sensor development and industry-wide monitoring of ACEQ.
• Evaluate the performance of improved sensor materials and assess the performance of systems that become candidates for widespread use in aircraft. A body such as an ASHRAE committee of stakeholders and sensor developers could use LBNL's protocol as a starting point for evaluating improved sensors.
• Use LBNL's protocols to screen sensor systems before more rigorous testing intended to overcome the limitations of this study.
• Test sensor systems for cross-sensitivity to ozone, CO, then add VOC (toluene, terpenes) ethanol and halogen-containing species (pesticides and flame retardants)
• Evaluate using CO/CO₂ ratios to signal incidents caused by pyrolysis products.
• Collaborate with aircraft engineers for in-flight testing of the best available sensor technologies.