MONITOR FOR DETECTING HYDROCARBONS AND OTHER GASES IN AN OPEN AREA

摘要

An improved area monitor for detecting gas concentration is provided that requires a very infrequent calibration, and works reliably in detecting a particular target gas using only two semiconductor sensors. One of these sensors will exhibit a drift that is constantly monitored and automatically compensated for, while the other sensor exhibits a stable, repeatable response over very long periods of time. Once the 'stable' has been initially calibrated, the concentration of the target gas can be discerned directly from the resistance of this sensor, although more than one particular chemical gas may cause the stable sensor to change resistance. The second sensor is then used to discern whether or not the change in resistance of the stable sensor was due to the actual target chemical, or due to some other similar chemical. The second sensor is selected to be of a type that responds in one direction (i.e., its change in resistance either increases or decreases) when emersed in the particular target chemical of interest. For all other similar gases, the resistance of the second sensor will change in the opposite direction. In the illustrated embodiment, the second sensor is not a highly stable sensor, since it exhibits a relatively large drift over short time intervals. However, this drift can be compensated for once an initial value of the resistance for the second sensor is known under the condition where the second sensor is emersed in an open area that does not contain the target gas of interest. Once this initial value of the second sensor's resistance is known, its resistance can be periodically measured at relatively short time intervals while the first sensor also periodically samples the same area. If the resistance of the second sensor begins to drift, and if the first sensor is not detecting a concentration of the target gas above a certain threshold (e.g., 'a minimum' threshold below which the area monitor does not concern itself with concentration of the target gas), then the 'new' resistance value of the second sensor (caused by its drift) can be stored in the microprocessor's memory system. This latest value of the second sensor's resistance can be used, for all practical purposes, as the latest calibration value of the resistance of the second sensor.