Para Magnetic Oxygen Analyzer- Examine Oxygen Concentration

                    

The advice of Robert H. Schuller was strongly taken by Instrumentation Engineers and they took all the flaws in Zirconia Oxygen Analyzer (discussed in the previous article) as guidelines and came up with a new instrument to analyze Oxygen- “PARA MAGNETIC OXYGEN ANALYZER”.

The para magnetic oxygen analyzer can be used for measurement of oxygen in the flammable and corrosive gases.

Para magnetic Oxygen Analyzer (Indirect Method)

This method gives the amount of oxygen using the fact that oxygen alone among common gases is para magnetic.  Oxygen exhibits a very strong para magnetic susceptibility with respect to most other gases and thus is attracted to the magnetic field (tends to move from weaker to stronger part of the magnetic field) due to the presence of 2 unpaired electrons in the outer shell.

Let us have more insight on each component of the Para magnetic Oxygen Analyzer:

1.       Dumbbell:

There is a small dumbbell-shaped body made of glass and filled with nitrogen (Diamagnetic with negative magnetic susceptibility). The dumbbell is mounted horizontally on a vertical suspension made up of platinum.

2.       Permanent magnet:

 Specially shaped poles of a permanent magnet produce a non-uniform magnetic field in which the dumbbell body is free to rotate.

3.       Mirror:

A very light flat mirror is attached permanently to the center of the dumbbell which is co planar to the platinum suspension.

4.       Feedback Coil:

Feedback coil is a single turn electrical coil made with the help of a platinum wire which is wound lengthwise on the surface of the dumbbell.

5.       Light source:

A well-defined beam of light is obtained with the help of a light source and optics which is incident on the mirror. The reflected light from the mirror is detected by the pair of photocell detectors connected to the auto balancing amplifier. The auto balancing amplifier takes a counteractive action for the dumbbell movement via the feedback coil.

6.       Measurement Cell:

The measurement cell is completely gas-tight enclosing the dumbbell unit. It allows the suspension and connection wires to pass through without gas leakage. The sample gas comes via an inlet allows the concentration of the oxygen to be measured and exit via the outlet in the measurement cell.

Principle of Measurement

Para magnetic oxygen analyzer works on the “NULL BALANCED SYSTEM” as here the current flow in the dumbbell is maintained such that the “ZERO POSITION” of the dumbbell is sustained. As the dumbbell position changes a feedback action takes place via varying the current in the coil and the amount of this corrective action gives the concentration of oxygen.

Nitrogen is diamagnetic so initially the dumbbell moves in the weaker magnetic field direction, this change is detected and instrument zero is calibrated with respect to nitrogen. Now the sample gas is made to flow through the measurement cell. Oxygen being para magnetic in nature, it tends to move towards the stronger magnetic field, thus upsetting the balance. This displaces the mirror attached to the dumbbell; the strength of the displacement on the suspension is proportional to the oxygen content of the sample gas. Thus the current output of the photocell changes which causes the auto balancing amplifier to take a counteractive action against the position change via the change in the current in the feedback coil and maintain the dumbbell at zero position. This amount of current, which is proportional to the volume magnetic susceptibility of the gas mixture which is calibrated to the partial pressure of oxygen, is measured and gives the accurate measure of the oxygen concentration.

Bit of basics…

Magnetic Susceptibility:

Also called volume susceptibility, is a quantitative indication of the extent to which a material is magnetized when placed in the magnetic field.

                                    Χm= M/H

Where;

M is the magnetization within the material 

H is the applied Magnetic Field strength

Χm is the volume/magnetic susceptibility

 Paramagnetic Substances:

These are the substances that exhibit Para magnetism, which is a form of magnetism in which the materials are attracted by the external applied magnetic field and form internal, induced magnetic fields in the direction of the applied magnetic field due to the unpaired electrons in these materials.

The volume susceptibility for oxygen under a pressure of one atmosphere at the temperature 20°C is +0.1447×10−6. As the volume susceptibility is positive this gives an indication that oxygen will be attracted in the external magnetic field.

Diamagnetic Substances:

These are the substance which create an induced magnetic field in the direction opposite to the external magnetic field and are thus repelled by the applied magnetic field as they don’t have an unpaired electron.

The volume susceptibility for Nitrogen under a pressure of one atmosphere at the temperature 20°C is

-4.91. As the volume susceptibility is negative this gives an indication that nitrogen will be repelled in the external magnetic field.

The paramagnetic oxygen analyzer may be used for the measurement of oxygen at any level between 0-100% in gas mixtures.