Must be recovered or disconnected. Paramagnetic sensor Oxygen is paramagnetic, and measuring the force of attraction of a gas filled vessel by a magnetic field can be used to measure oxygen content. Generally large and complex mechanics. The resonant frequency of a crystal is affected by a mass attached to it. Coat a crystal with a layer that absorbs volatile compounds in proportion to their concentration.
Frequency shift is proportional to added mass.
Need to know which gas is present, and calibrate for its partition coefficient and mass. Based on the motion of charged particles in electric or magnetic fields. Mass affects the motion. Requires an evacuated chamber to prevent collisions. Use a reciprocating pump to get to about 0.1 torr, then oil diffusion pump or turbomolecular pump to get low pressures.