Vapor Generator for the Calibration and Test of Explosive Detectors

Thousands of explosive vapor trace detectors deployed in the field require frequent verification and calibration to guarantee their accurate operation. In this paper we describe the use of precision micro-dispensing technology in the development of vapor generation instruments for the calibration and testing of explosive detectors. Several digitally controlled systems ranging from bench-top/research instrument to a hand held unit using ink- jet technology have been prototyped and tested. The use of digitally controlled ink-jet dispensing to precisely eject minute amounts of dilute explosive solutions and convert them into vapor has been demonstrated. An electrical pulse applied to a piezoelectric micro-dispenser causes a drop of fluid to be ejected through a precise orifice. These droplets land on a heater that converts them to vapor. The amount of explosive delivered to the detectors can be controlled by the number of drops (dose mode - specified number of drops is generated) and the frequency of the droplet generation (continuous mode - droplets are generated continuously at fixed frequency). An additional control of the amount of explosive is provided by the concentration of the explosive solution that is dispensed facilitating the generation of infinitely small amounts of explosive vapors. The prototypes built have been evaluated with commercially available detectors in both dose and continuous modes. Ultimately, the ability to further miniaturize the vapor generators will lead to units that are embedded into next generation detectors for real-time verification and calibration.