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Tuesday, September 27, 2011

#27 - Analysis of Oxygen by Paramagnetic Detection


Nova makes the Model 322BT oxygen analyzer. This instrument is designed for continuous analysis of up to 100% oxygen (O2) in process gas streams that do not contain any condensable water vapor or oil vapors.

The analyzer utilizes a paramagnetic type sensor, which responds quickly to process O2 changes.


The Nova Model 322-BT Oxygen Analyzer utilizes a high stability, magneto dynamic paramagnetic cell for the detection of oxygen in the sample gas.

Oxygen has the unique property of being attracted to a magnetic field, as discovered by Michael Faraday. Most other gases are diamagnetic (repelled by a magnetic field).

A strong magnetic field passing through the magneto-dynamic cell attracts oxygen which enhances the magnetic field. This in turn increases the force on a diamagnetic body suspended in the field.

The diamagnetic body consists of two glass spheres filled with nitrogen and suspended on opposite sides of a taut platinum ribbon between the poles of a strong permanent magnet assembly. A small mirror is also attached to the platinum ribbon which will detect any angular motion.

As the oxygen content in the sample increases, the increased magnetic field increases its repulsion force against the spheres. An infrared light beam focused on the mirror reflects onto a pair of matched infrared detectors mounted side by side. Any angular movement of the platinum ribbon by the repulsion force against the spheres causes the light to reflect more on one IR detector than the other, causing an increased current output from the amplifier. The current is fed to a feedback coil consisting of a single turn of fine platinum wire secured around the spheres. The magnetic field created by this current opposes the force due to the oxygen present in the sample gas and consequently the spheres remain in their original position.

The current required to hold the spheres in the neutral position is directly proportional to the amount of oxygen in the sample gas. By measuring this current, we have a direct measurement of oxygen. An amplifier built right into the cell, converts this current to a 0-1 VDC output representing 0-100% O2. This 0-1V output is then sent to the main amplifier board which then has calibration adjustments, output circuits, and connections to the front panel display meter.

Check out the Model 322 brochure…

While the paramagnetic detector is a quite accurate method of oxygen analysis, we have found that it is somewhat sensitive to external vibration. For example, the constant handling and jostling implied in a portable analyzer can affect the stability of the readings. For most applications, we manufacture this analyzer in a bench top style cabinet. It is basically a sheet metal box with a carry handle on top and a readout on the end facing the user. (See above picture) This cabinet style usually finds it home on a laboratory bench or some other stable surface.

Added March 23, 2013 - We have also noticed that paramagnetic detection is very sensitive to sample pressure and flow. It is very important that we have a clear idea of what the pressure and flow characteristics are of the sample gas. This also applies to the vent flow. It is preferable that this type of detector have an unobstructed vent to the ambient atmosphere.

Alternatively, you might consider an electrochemical O2 sensor. The accuracy and range may not be as high as a paramagnetic detector, and may be a little slower in some cases. However, it is generally quite rugged and well-suited for many applications. Check out the Model 320 brochure…

We make gas analyzers for oxygen, hydrogen, carbon monoxide, carbon dioxide, methane, and many other atmospheric gases. We provide gas analysers for syngas, landfill gas, purity gas, biogas, and others.

Give Mike or Dave at Nova a call, or send us an e-mail.
sales at nova-gas dot com
websales at nova-gas dot com

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