Rosemount 755, Instruction Manual

Page: 53 / 94

5
I
NSTRUMENT
T
HEORY
748183-H
5-1
Rosemount Analytical
December 1997
5.1 PRINCIPLES OF OPERATION
Compared with other gases, oxygen is strongly paramagnetic. Other common gases,
with only a few exceptions, 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, analogous to the magnetization of a piece of
soft iron. Diamagnetic gases are analogous to non-magnetic substances.
With the Model 755, 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 ies 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 this combination 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 output remains constant. As soon as the test
body begins to rotate, however, the amounts of light become 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. Additionally the output from the
amplifier is conditioned as required to drive the meter, and recorder if used. The
electronic circuitry involved is described briefly in Section 5.3 and in greater detail in
Section Six.