The Brechbuehler H Sensor series 9000 is a semiconductor based flow-through safety monitor for hydrogen concentration in any GC-oven. With a lower limit of detection of 0.5% H2 by volume, the alarm threshold is freely adjustable between 0.5 and 4% H2 (the lower explosive limit of hydrogen). H2 concentration is continuously reported by a bar graph LED display. In the event that the alarm threshold is exceeded, the user will be alerted by both flashing LED and audible alarm signals. At instrument level, the GC oven is shut off and cooling vents are opened. If equipped with a gas switching valve, the carrier gas will be switched to nitrogen. Once triggered, the hydrogen sensor will not reset until the H2 concentration has fallen to an acceptable <0.8% by volume level. Interruption of power results in the same shut down procedure.