minimum concentration of detection. Data from field tests, ambient breeze tunnels, and open-air sites are then related back to component tests for overall system evaluation. To expand detector evaluation capabilities, the DOD is also planning a Whole System Live Agent Testing (WSLAT) chamber to conduct live agent tests of entire systems. A brief overview of testing in laboratories, ambient breeze tunnels, and open-air sites is provided in Box 4.1.
Characteristics of Different Testing Facilities
In the laboratory setting, aerosol wind tunnels and test chambers with different biosafety levels (BSL)(Centers for Disease Control and National Institutes of Health 2007) to accommodate simulants and live agents have been used. Laboratory experiments provide a test environment with well-controlled challenge aerosol, wind speed, wind direction, and background interferent aerosol. Depending on the sizes of the detector system and the test chamber, the whole detection system or individual components can be evaluated. A well-controlled test environment is especially useful in the initial stage of development, when laboratory tests can provide information to modify, redesign, and improve the system.
Tests in aerosol wind tunnels provide information about sampling acquisition, including the aspiration efficiency and transmission efficiency as a function of particle size, wind speed, and sampling flow rate.(Cheng et al. 2004; Cheng and Chen 2001) BSL-2 or BSL-3 test chambers enable the use of live biological agents or their simulants and are used to assess the performance of biological agent detection. In current test chambers, biological agents are usually released as a well-mixed aerosol of a constant concentration level or as spikes of agent concentration.(Jensen et al. 1992; Li and Lin 1999; Xu et al. 2003; Semler, Roth, and Semler 2004; Kesavan 2005) By varying the biological agent concentration, one can determine the minimum concentration of detection as well as response time. One can also introduce interferents into the chamber to determine discrimination capability and estimate the probability of false positive detection.
Open Air Testing
Eventually operational tests are conducted on an open air range to evaluate detector performance under real-world conditions. In these tests, simulants and agent-like organisms are released in the open air test. Active agent is never released during these tests due to the health hazard these organisms pose to the population.(National Research Council 2005) Ideally these tests would further characterize the detector’s response time and false-positive detection rate in the presence of natural background. However, field tests can be time consuming and costly because of variable experimental conditions and environmental impact regulations. Highly variable environmental conditions (e.g., wind speed, wind direction, temperature, and relative humidity) affect the aerosol dispersion and introduce variability in the test data, which can require many replicate tests to average out.
Ambient Breeze Tunnels (ABT) provide an environment for controlled dispersion of challenge aerosols, while maintaining certain characteristics of open air conditions, including temperature, relative humidity, turbulent mixing, and even background aerosol. These tunnels are usually longer than 100 ft and have large cross-sectional areas. The wind is unidirectional, with speeds up to 5 mph. The tunnels—like a laboratory aerosol wind tunnel—have been designed to provide uniform aerosol concentration at the test section for the specified wind speed range. Because of the nature of remote detection, standoff detectors, such as LIDAR, can only be tested and evaluated in an ABT or an open air field. Both the open air field and ABT can provide performance test data on operational, full-scale detection systems.