Electrical conductor free smoke detector pipe networks - includes air flow sensor to indicate blockage of any air sampling holes in network using wheatstone bridge and heating resistors

摘要

The system includes a network of pipes (10) comprising a series of parallel tubes (18-21) with holes of specified size at intervals. A fan operating with a saturated electric motor is arranged to provide a reduced pressure in the network - 10 to 30 millibars below atmospheric pressure. The various holes in the network serve to sample the air at various places and draw the samples towards a single common detector, e.g. of the ionisation type. A sensor is also included to indicate if any of the sampling holes is blocked. A printed circuit carrying a wheatstone bridge circuit has one of its components situated in the air flow. While the flow is correct, the bridge is balanced, but any reduction is flow caused by blockage changes the resistance and unbalances the bridge, initiating a warning.