Unless otherwise expressly indicated herein, none of the material presented in this section is prior art to the claims of this application and is not admitted to be prior art by having been included herein. 
Gas sensors may be tested periodically to ensure sensor accuracy and system integrity.  One way to know whether a gas sensor's readings are accurate is to expose the sensor to a known amount of testing gas.  Once exposed to the known amount of testing gas, the sensor output can be evaluated for accuracy of the readings, and can be evaluated for the proper performance of the sensor's alarms.