Standard Laboratory Evaluations

Sensors that demonstrated a nominal level of performance in the field are brought back to the AQ-SPEC laboratory and challenged with known concentrations of gases and particles under controlled temperature (T) and relative humidity (RH) levels. These controlled tests are performed in highly specialized chambers known as Sensor Environmental Test Chambers, or SEnTeC chambers. The sensors are evaluated in a group of triplicate to provide better statistical information about the comparability with FRM/FEM instruments or, widely accepted Best Available Technology instruments measuring the same pollutant(s). A Laboratory Testing Protocol has been developed to properly characterize sensor performance and the paper discussing these efforts for SEnTeC-1 (implemented in 2016) has been published in Atmospheric Environment. The SEnTeC-1 chamber is used to evaluate sensors that measure one or more of the National Ambient Air Quality Standards (NAAQS) criteria pollutants and sensors that measure Volatile Organic Compounds (VOC). The SEnTeC-1 chamber was designed and developed in collaboration with American Ecotech (Warren, RI) with software controls developed in collaboration with DR DAS LTD. (Granville, OH). The optimization and implementation of the chamber system was performed by the South Coast AQMD AQ-SPEC staff.
As of July 2021, SEnTeC-1 has been enhanced and dedicated to VOC sensor performance evaluations. Criteria pollutant sensor performance evaluations are now conducted with a new chamber further described below.

Advanced Laboratory Evaluations and Specialty Tests

Recent advances in low-cost sensor applications (i.e., sensor networks and mobile deployment) have necessitated more complex testing capabilities. Therefore, AQ-SPEC has developed a second larger Sensor Environmental Test Chamber (SEnTeC-2), implemented in 2021, capable of performing more comprehensive test procedures. The SEnTeC-2 system was designed using the knowledge and experience gained from SEnTeC-1 and expands the testing capabilities of AQ-SPEC. The development and execution of highly specialized testing scenarios by the SEnTeC-2 system (i.e., vibration, wind speed, and altitude tests) can provided additional insights into the performance of low-cost sensors under extreme conditions. In addition to performance evaluation of criteria pollutant-measuring low-cost sensors, the SEnTeC-2 chamber will assist with sensor networks/sensor loan program calibration, performance target testing, and supplementary testing for low-cost sensors developed on mobile platforms. The SEnTeC-2 chamber was developed in collaboration with RJ Lee Group (Monroeville, PA) with comprehensive pollutants/software controls developed in collaboration with IEStechno (Morgantown, WV). The optimization and implementation of the chamber system was performed by the South Coast AQMD AQ-SPEC staff.

Several parameters are measured for evaluating the performance of a sensor technology under AQ-SPEC including:

• Coefficient of Determination (R²): correlation between co-located sensor and FRM/FEM/BAT concentration measurements
• Intra-model variability: describes how close the measurements from three units of the same sensor type are to each other
• Accuracy: degree of closeness of sensor concentration measurements to the actual (true) concentration value measured using FRM/FEM/BAT instruments
• Precision: variation around the mean of repeated measurements of the same pollutant concentration
• Co-pollutant interference: positive or negative measurement response caused by a pollutant other than the one being measured
• Temperature (T) and Relative Humidity (RH) influences: positive or negative measurement response caused by variations in T and RH

Laboratory Evaluation Methods Paper

Papapostolou V, Zhang H, Feenstra B, and Polidori A. Development of an environmental chamber for evaluating the performance of low-cost air quality sensors under controlled conditions. Atmospheric Environment, 171: 82-90, 2017 (Abstract, DOC. 23 KB)

Full article available on request.

Disclaimer

All documents, reports, data, and other information provided on this website are for informational use only. The majority of sensors evaluated as part of the Air Quality-Sensor Performance Evaluation Center (AQ-SPEC) were field-tested inside a custom-made aluminum enclosure to protect the sensors from windblown rain, harsh sunlight, and animals. The field evaluations reports included on this website contain data collected at our monitoring station during a specific 30- to 60-day period and may not be duplicated at the same or different location, season, time period, or weather conditions (e.g. temperature, relative humidity, pressure, wind speed/direction). As the overall sensor performance may be affected by the specific environmental conditions experienced at our location during the time of testing, replication and/or duplication may not possible to achieve. The laboratory evaluation was conducted in the AQ-SPEC’s environmental chamber with simulated pollutant and interferent concentrations that were generated from nebulizer solutions, dust dispensers, and gas dilution calibrators. Generated environments may not be able to fully replicate the conditions that may be experienced under ambient settings. The sensor assembly, installation, and use can also impact the reliability of the products evaluated by AQ-SPEC. South Coast Air Quality Management District (South Coast AQMD) makes no claim, warranty, or guarantee that these devices will or will not work when operated by other users for their specific applications.

South Coast AQMD's AQ-SPEC aims at providing information to and for the benefit of the public to make informed purchasing decisions on commercially available “low cost” air quality sensor devices. In accordance with this mission, the general policy of the Governing Board of the Agency is to exclude from the AQ-SPEC website all commercial advertising and promotional material, including links which provide exclusive private or financial benefit to commercial, non-public enterprises and which do not promote or enhance a public benefit to the general public. As a Government Agency, the South Coast AQMD neither endorses nor supports individual private commercial enterprises through display of their products on the AQ-SPEC website or through providing links to the sites of such commercial enterprises.