Model 755A

Instruction Manual

245364-V May 2002

SECTION 4 THEORY

4-1 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 755A, the volume magnetic susceptibility of the flowing gas sample is sensed in the detector/magnet assembly. As shown in the functional diagram of Figure 4-2, (page 4-3), 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. As soon as the test body begins to rotate, the amounts of light becomes unequal, resulting in application of an error signal to the input of the amplifier circuit. The resultant amplifier output signal is routed through the current loop, thus creating the electromagnetic forces required to restore the test body to null position.

a.Magnetic Displacement Force

Because the magnetic forces on the spherical ends of the test body are the basis of the oxygen measurement, it is worthwhile to consider the force acting on one of these spheres alone and to disregard, for the present, the remainder of the detector. A small sphere suspended in a strong non-uniform magnetic field, Figure 4-1 (page 4-2), is subjected to a force proportional to the difference between the magnetic susceptibility of this sphere and that of the surrounding gas.

Magnitude of the force is expressed by the following (simplified) equation:

Fk = c (k - ko)

Rosemount Analytical Inc. A Division of Emerson Process Management

Theory

4-1