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Instruction Manual
760007-A
July 2003
Rosemount Analytical Inc. A Division of Emerson Process Management Description and Specifications 1-1
Model NGA2000 PMD
Shaded
Pole
Piece
Sphere
(Magnetic Susceptibility = k
o
)
F
k
Sample Gas
(Magnetic Susceptibility = k
)
Note:
As percentage of oxygen in sample gas increases,
displacement force (F
k
) increases.
SECTION 1
DESCRIPTION AND SPECIFICATIONS
1-1 OVERVIEW
This manual describes the Paramagnetic De-
tector (PMD) Analyzer Module of Rosemount
Analytical's NGA2000 Series of gas analysis
components.
The PMD Analyzer Module is designed to
continuously determine the concentration of
oxygen in a flowing gaseous mixture. The
concentration is expressed in ppm or percent
volume O
2.
The entire Analyzer Module is designed as a
slide-in module (if configured in stand-alone
instrument fashion), removable from the front
of the Platform, with gas connections made
from the rear. All electronics relative to sam-
ple detection and conditioning are included in
this module.
1-2 TYPICAL APPLICATIONS
PMD Analyzer Module applications include:
• process control
• continuous emissions monitoring systems
(CEMS)
• industrial gas production
• fermentation process monitoring
1-3 THEORY OF TECHNOLOGY
Oxygen is strongly paramagnetic (i.e., capa-
ble of becoming a temporary magnet when
placed in a magnetic field) while most other
common gases are weakly diamagnetic (i.e.,
tend to be non-magnetic). See Figure 1-1
below.
The Magnetic susceptibility of the flowing gas
sample is sensed in the detector/magnet as-
sembly. As shown in Figure 1-2 on page 1-2,
a dumbbell shaped, nitrogen-filled, hollow gas
test body is suspended on a platinum/nickel
alloy ribbon in a non-uniform magnetic field.
Because of a "magnetic buoyancy" effect, the
spheres of the test body are subjected to dis-
placement forces, resulting in a displacement
torque proportional to the magnetic suscepti-
bility of the gas surrounding the test body.
Measurement is accomplished by a null-
balance system, whereby the displacement
torque is opposed by an equal and opposite
restorative torque. The restoring current is
automatically maintained at the correct level
by an electro-optical feedback system. A
beam of light from the source LED is reflected
off the square mirror attached to the test body
onto a bi-cell (dual photodiode).
The current required to keep the test body to
the null position is a linear function of the total
magnetic susceptibility of the sample gas.
See Figure 4-1 on page 4-1, Figure 4-2 on
page 4-2, and Figure 4-3 on page 4-3 for
component configuration.
Figure 1-1. Spherical Body in Non-Uniform
Magnetic Field