A miner's personal gas alarm can be mounted in a helmet powered by a rechargeable battery for a light or be self-contained. A visual indicator will generate an alarm when the concentration of gas detected by the gas sensor triggers an alarm condition. An audio alarm can also be generated by the alarm condition. The gas sensor is a non-dispersive infrared (“NDIR”) gas sensor. When the gas sensor detects methane, the alarm condition is triggered by either an abnormally high rate of increase of methane concentration level or by an elevated concentration of methane that is above approximately 500 ppm and substantially below a lower explosion limit of methane (e.g., approximately 10,000 ppm), and the gas sensor is recalibrated whenever the sample concentration of methane falls below an ambient threshold level of methane.