Abstract: A sealed sensor device (104) comprising: an internal atmosphere comprising a gas pressurised to a predetermined pressure, the predetermined pressure being below atmospheric pressure when the internal atmosphere is hermetically sealed from ambient. A sensor cavity (214) is also provided having a periphery and is in fluid communication with the internal atmosphere, thereby comprising the gas and the gas having a mean free path at the predetermined pressure associated therewith. A thermopile (256) is disposed in the sensor cavity (214) for detecting a change in pressure of the internal atmosphere and detecting failure of the hermetic seal. A membrane structure (234) disposed within the cavity comprises the thermopile (256). The membrane structure (234) also comprises a heating element, and a shortest distance from substantially any point on the membrane structure (234) to the periphery of the sensor cavity (214) is less than the mean free path of the gas at the predetermined pressure.