Specifications

DETECTOR METHODOLOGIES
2.2 Paramagnetic Oxygen Method
The paramagnetic principle refers to the induction of a weak magnetic field, parallel and
proportional to the intensity of a stronger magnetizing field.
The paramagnetic method of determination of oxygen concentration utilizes nitrogen
filled quartz spheres arranged at opposite ends of a bar, the center of which is
suspended by and free to rotate on a thin platinum wire ribbon in a cell. Nitrogen (N2) is
used because it is diamagnetic or repelled by a magnet.
A small mirror that reflects a light beam coming from a light source to a photodetector,
is mounted on the platinum ribbon. A strong permanent magnet specifically shaped to
produce a strong, highly inhomogeneous magnetic field inside the analysis cell, is
mounted outside the wall of the cell.
When oxygen molecules enter the cell, their paramagnetism will cause them to be
drawn towards the region of greatest magnetic field strength. The oxygen molecules
thus exert different forces on the two suspended nitrogen filled quartz spheres,
producing a torque which causes the mirror to rotate away from its equilibrium position.
The rotated mirror deflects the incident light onto the photodetector creating an
electrical signal which is amplified and fed back to a coil attached to the bar holding the
quartz spheres, forcing the suspended spheres back to the equilibrium position.
The current required to generate the restoring torque to return the quartz bar to its
equilibrium position is a direct measure of the O
2
concentration in the sample gas.
The complete paramagnetic analysis cell consists of an analysis chamber, permanent
magnet, processing electronics, and a temperature sensor. The temperature sensor is
used to control a heat exchanger to warm the measuring gas to about 55 °C.
Rosemount Analytical µCEM Continuous Analyzer Transmitter 2–5