Measurement Method; Illustration1 Diagram Of The Measurement Cell, And Optical Signal Processing - M&C PMA 05 Manual

7

MEASUREMENT METHOD

Oxygen is a gas with strong paramagnetic properties. The molecules of the oxygen will be
influenced more than that of most other gases by a magnetic field.
The measuring method presented below takes advantage of these properties of oxygen. The great
advantage of the paramagnetic measuring principle is the greatly reduced cross-sensitivity of
measurement to the other components in the sample gas.
Figure 2 shows the diagram of the measuring cell, as well as the optical system, for detecting
motion of the dumbbell.
Illustration 1 Diagram of the measurement cell, and optical signal processing
The measuring cell consists of two nitrogen-filled hollow microspheres, which are formed by a
web on the dumbbell. In the centre of rotation of the dumbbell there is a small mirror. The
dumbbell is surrounded by a wire loop which is required for the compensation method. The above
system is fixed by a platinum straprotationally symmetrical in a glass tube and screwed with two
polepieces.
Two permanent magnets generate an inhomogeneous magnetic field. When oxygen flows in, the
oxygen molecules are drawn into the magnetic field. There is compression of the field lines of the
wedge-shaped pole pieces. The nitrogen-filled diamagnetic hollow microspheres are pushed out
of the magnetic field. This produces a rotational movement of the dumbbell. The rotary movement
is detected by means of an optical system consisting of mirrors, projection LEDand
photocell.
If the dumbbell is forced out of the magnetic field, the voltage of the photocell changes
immediately. The amplifiers (7) and (8) generate a corresponding current, which generates via
the wire loop on the dumbbell an electromagnetic countermoment. The countermoment returns the
dumbbell back to its zero position.
8 Gas sampling and gas conditioning technology 9-3.20-MD