MBE AG - Messtechnik Engineering

1 Glass dumbbell
2 Pole shoe (N)
3 Pole shoe (S)
4 Measuring cell
5 Light source
6 Mirror
7 Photo diodes
8 Wire loop

Oxygen is one of the few gases with very strong paramagnetic properties. The oxygen molecules are drawn into an inhomogeneous magnetic field inside the measuring cell. Corresponding to the varying strength of the magnetic field, oxygen partial pressures occur which exerts a torque on the displacement body of glass dumb-bell. When oxygen is present in the gas mixture, the position of the glass body shifts slightly. A visible light beam, located at the optical bench, shines on a mirror which is mounted on the dumbbell. The light is reflected uniformly and symmetrically onto two opposing photo diodes which are arranged side by side so that their position produces a voltage difference assumes a value of zero. When the dumbbell even slightly moves, the reflected light beam shines asymmetrically onto the two photo diodes so that it results a voltage difference.

This voltage difference causes a very small current to flow through the wire loop mounted on the outer surface of the glass body. On the other hand this current flow, inside the inhomogeneous magnetic field, causes a torque to the dumbbell compensating the opposing torque caused by the partial pressure of the oxygen, thereby returning the dumb-bell to its original position.

The current flow required to maintain this zero position is directly proportional to the oxygen concentration.