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Standard Reference Calibration for Explosion Detectors

ASTM F 2069, Standard Practice for Evaluation of Explosives Vapor Detectors, establishes a method for characterizing explosives vapor detectors in the laboratory. It was developed through the efforts of manufacturers, users, and engineers in ASTM Subcommittee F12.60 on Controlled Access Security, Search, and Screening Equipment, part of Committee F12 on Security Systems and Equipment.

“A standard was needed because users and manufacturers were making performance claims based on different methods of calibration and evaluation. Manufacturers were comparing apples and oranges and the ultimate user was confused by the different performance claims,” said John R. Hobbs, Ph.D., physical chemist, Hobbs Consulting, Boxford, Mass.

“This standard practice requires comparisons of performance to an accepted reference calibration and results in performance specifications on a level playing field.”

Hobbs led the working group of manufacturers and users of explosives detection equipment, and engineers from X-ray and metal-detector companies that developed F 2069.

“This standard practice provides manufacturers with the procedures to relate the performance of their detectors to criteria that the ultimate users can use to determine if a particular explosives vapor detector satisfies their requirements,” he explained. “Previously, manufacturers claimed lower limits of detection based on noise, minimum detectable level, or lowest sensitivity. These claims did not tell a prospective user how this machine would perform in his operational system.

“This standard provides a probability of detection at a determined minimum- alarm level and the false-alarm rate related to the performance at that minimum-alarm level. With this information, the prospective user can decide if the performance of a particular detector meets his detection and false-alarm requirements for his particular operational scenario.”

According to Hobbs, F 2069 “requires the comparison of a manufacturer’s secondary calibration unit to one of three acceptable ‘primary standards.’ Only manufacturers and builders of the calibration equipment have the facilities or expertise to carry out this comparison. The users are not equipped to do this.” When manufacturers report performance information in their product literature according to F 2069, the user can make intelligent choices, he said.

“This standard will benefit users because if the standard is followed, the manufacturers report a probability of detection at a minimum alarm level and an attendant false alarm rate,” Hobbs affirmed. “They now can choose a detector with a higher probability of detection, but with a higher false-alarm rate or choose one with a lower probability of detection and a lower false-alarm rate. The user can now trade off performance with detection probability, false-alarm rate, and cost. Previously, manufacturers exposed real high sensitivity without a false-alarm rate, and the user had no way to determine if the detector would meet his requirements.

“For instance, a detector with a real low limit of detection and a false alarm rate of 20 percent may not be as good as a detector with somewhat higher limit of detection, but with a false alarm rate of only two percent,” he concluded. “The user can decide if he needs the lower limit of detection or the lower false alarm rate, since both of these affect the cost of performing security.”

F 2069 provides a method for evaluating 11 parameters, including interference equivalent, temperature and humidity effects, sample and response times, overload level, etc.

For further technical information, contact John R. Hobbs, Ph.D., Hobbs Consulting, Boxford, Mass. (phone: 978/887-6082). Individuals are welcomed to attend the next Committee F12 meeting, Oct. 23-24, in Dallas, Texas. For details, contact staff manager Jim Olshefsky, ASTM (phone: 610/832-9714). //

Copyright 2001, ASTM