5
T
HEORY
748213-P Rosemount Analytical June 1997
5-1
Model 755R Oxygen Analyzer
5.1 PRINCIPLES OF OPERATION
Oxygen is strongly paramagnetic while most other common gases are weakly
diamagnetic. The paramagnetism of oxygen may be regarded as the capability of an
oxygen molecule to become a temporary magnet when placed in a magnetic field.
This is analogous to the magnetization of a piece of soft iron. Diamagnetic gases are
analogous to non-magnetic substances.
With the Model 755R, the volume magnetic susceptibility of the flowing gas sample is
sensed in the detector/magnet assembly. As shown in the functional diagram of Figure
5-1, a dumbbell-shaped, nitrogen-filled, hollow glass test body is suspended on a
platinum/nickel alloy ribbon in a non-uniform magnetic field.
Because of the “magnetic buoyancy” effect, the spheres of the test body are subjected
to displacement forces, resulting in a displacement torque that is proportional to the
volume magnetic susceptibility of the gas surrounding the test body.
Measurement is accomplished by a null-balance system, where the displacement
torque is opposed by an equal, but opposite, restorative torque. The restorative torque
is due to electromagnetic forces on the spheres, resulting from a feedback current
routed through a titanium wire conductor wound lengthwise around the dumbbell.
In effect, each sphere is wound with a one-turn circular loop. The current required to
restore the test body to null position is directly proportional to the original displacement
torque, and is a linear function of the volume magnetic susceptibility of the sample
gas.
The restoring current is automatically maintained at the correct level by an
electro-optical feedback system. A beam of light from the source lamp is reflected off
the square mirror attached to the test body, and onto the dual photocell.
The output current from the dual photocell is equal to the difference between the
signals developed by the two halves of the photocell. This difference, which
constitutes the error signal, is applied to the input of an amplifier circuit that provides
the restoring current.
When the test body is in null position, both halves of the photocell are equally
illuminated, the error signal is zero, and the amplifier is unequal. This condition results
in application of an error signal to the input of the amplifier circuit. The resultant
