Toxic gas sensor with monitoring of diffusion barrier

A toxic gas sensor includes a sensor body 1 with a top plate 2 provided with a toxic gas diffusion barrier 3 leading to a chamber 7 having an outlet aperture 5. An oxygen sensor 4 with diffusion barrier 6 communicates with the outlet 5 of the toxic gas sensor. The diffusion barriers may be capillary holes, solid membranes, or Knudsen barriers. If the diffusion barrier 3 of the toxic gas sensor becomes partially or totally blocked, the supply of ambient oxygen to the oxygen sensor is reduced or cut off and indicates a fault in the barrier 3. A further oxygen sensor, external to the toxic gas sensor can also be provided to remove errors caused by changes in ambient oxygen concentration. The first oxygen sensor may be releasably mounted on the toxic gas sensor. Both sensors may be electrochemical and contain a sensing electrode, counter electrode, electrolyte chamber, wicking arrangements to provide contact between electrodes and electrolyte and current collecting means.