Air sampling system for smoke detection

Claim 1

An air sampling system for diposition in a cabinet having a source of cooling air, for detecting the presence of smoke in cooling air directed over the planar surfaces of a plurality of printed circuit cards interconnected in a backplane, comprising: at least a first air sampling tube oriented normal to the direction of flow of said cooling air, said first air sampling tube being positioned in proximal, transverse relation to the edges of said cards, said last mentioned edges being those farthest removed from said source of cooling air, said first air sampling tube being closed at one extremity thereof and open at its opposite extremity, said first air sampling tube having a plurality of spaced-apart holes of predetermined orientation formed therein along its length, at least a second air sampling tube arranged in coplanar, spaced-apart relationship with said first air sampling tube and being positioned outside the flow of said cooling air and in a proximal, parallel relationship to said backplane, said second air sampling tube being closed at one extremity thereof and open at its opposite extremity, said second air sampling tube having a plurality of spaced-apart holes formed therein along its length, a detector box comprising first and second chambers, a smoke detection unit interposed between said chambers, a manifold having a plurality of input ports and an output port, the open extremities of at least said first and second air sampling tubes being connected respectively to a pair of said input ports, said output port being connected to said first chamber of said detector box, said predetermined orientation of said holes formed in said first sampling tube effecting a pressure on the surface of said last mentioned tube which is substantially the same as that on the surface of said second air sampling tube, whereby the respective pressures on said first and second tubes are balanced, a draft tube located in a low pressure area of said cabinet as effected by said source of cooling air, means coupling said draft tube to the second chamber of said detector box, whereby the pressure differential present between said first and second air sampling tubes and said second chamber of said detector box causes a Plurality of air samples to be drawn into said last mentioned air sampling tubes and through said smoke detection unit.