Method and apparatus surface ionization particulate detectors

Abstract

Surface ionization technique for detection of airborne particles whereby each particle is pyrolyzed on a hot surface, releasing its chemical constituents, some of which are ionized at the surface, creating a burst of ions that denote the particle's presence. The hot surface is a catalytic material deposited on an inert substrate heated by an internal heating element. Inert substrates are selected to provide mechanical strength, reduce microphonic noise and make a large catalytic surface area achievable, and hence permit high sensitivity while employing reduced quantities of catalytic materials. By locating the heater within the substrate, its electrical parameters are such that the heater power supply can be simplified. The pulses during 'on' parts of the 'on-off' cycles are filtered out and not counted. In one embodiment the hot sensor surface is biased to a high voltage by a high bias resistor and is coupled to a pulse-counting preamplifier through a capacitor. When there is a burst of positive ions from a heated sensor surface, it causes an immediate drop in the bias voltage which cannot be immediately replaced through the biasing resistor. The result is a negative pulse at a preamp proportional to the number of ions in the pulse which is not affected by what ultimately happens to the ions in a turbulent airstream. Heating is accomplished by a current which is alternating 'on' 'off'.