Model NGA2000 PMD

Instruction Manual

760007-A July 2003

SECTION 1

DESCRIPTION AND SPECIFICATIONS

1-1 OVERVIEW

This manual describes the Paramagnetic Detector (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 O2.

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 sample 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., capable 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 displacement forces, resulting in a displacement torque proportional to the magnetic susceptibility 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.

Shaded

Pole

Piece

Sphere

(Magnetic Susceptibility = ko )

Fk

Sample Gas

(Magnetic Susceptibility = k )

Note:

As percentage of oxygen in sample gas increases, displacement force (Fk ) increases.

Figure 1-1. Spherical Body in Non-Uniform

Magnetic Field

Rosemount Analytical Inc. A Division of Emerson Process Management

Description and Specifications

1-1