Standard Laboratory Evaluations

Sensors measuring volatile organic compounds (VOC), which includes air toxics, are evaluated in the AQ-SPEC environmental chamber and challenged with known concentrations of target VOC and gasous interferent pollutants under controlled temperature (T) and relative humidity (RH) levels. These controlled tests are performed in a modified and enhanced environmental chamber (SEnTeC-1) that was previously used for routine evaluations of sensors measuring criteria pollutants. The sensors are evaluated in triplicate to provide better statistical information about the comparability with Best Available Technology (BAT) instrument(s) measuring the same pollutant(s). A VOC Laboratory Testing Protocol has been developed to properly characterize sensor performance and has been published. 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. The SEnTeC-1 chamber was further optimized for VOC sensor testing to ensure air-tightness of the chamber and safe operation.

Several parameters are measured for evaluating VOC sensor performance under AQ-SPEC including:

• Coefficient of Determination (R²): correlation between sensor and 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 BAT instrument(s)
• 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
• Mean Absolute Error (MAE): the absolute difference between the sensors and the reference instruments. The larger MAE values, the higher measurement errors as compared to the reference instruments.
• Mean Bias Error (MBE): the difference between the sensors and the reference instruments. MBE indicates the tendency of the sensors to underestimate (negative MBE values) or overestimate (positive MBE values).
• Root Mean Square Error (RMSE): another metric to calculate measurement errors.
• Sensor Detection Limit (SDL): the lowest VOC concentration reported by the sensors with confidence
• Analysis of Variance (ANOVA): to determine the importance of explanatory variables in explaining sensor responses to VOC concentrations
• Short Term Sensor Response Change: to investigate the change in sensor response after subjecting the sensors to various climate conditions and interferents.  

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.