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.

Analyze The Gas That Gives Us Life! - OXYGEN

       All of us know how essential is oxygen for our living…

As this is what happens without oxygen

                

Instrumentation engineers too realized the importance of oxygen thus they made an instrument “OXYGEN ANALYZER” for measuring the proportion of oxygen and displaying the oxygen content.

One among the primarily used oxygen analyzer is discussed

Zirconia Oxygen Analyzer (Direct Method):

This method gives the amount of oxygen using the concepts of electrochemistry. Here there is an electrolyte; 2 electrodes and a generated EMF (tells the oxygen concentration).

Let us have more insight of each component:

   1.     Zirconia sensor:

                                                  

The sensor probe uses a high-reliability zirconia sensor made up of Zirconium Dioxide, which is a special material; its resistance decreases drastically at higher temperatures (500-1000 deg C) and thus allows the conduction of ions as temperature increases. Using the property of zirconia that it conducts only oxygen ions at temperature of 600 deg C; zirconia acts as an electrolyte in the analyzer.

    2.       Reference and Measured Gas:

      The reference gas is usually taken as air which has a fixed amount of oxygen in it (20.9%) and measured gas is the one whose oxygen concentration has to be measured.

                                      

      

      3 .  Platinum electrodes:

It comprises of two electronically conducting, chemically inert electrodes attached to either side of a solid electrolyte tube. The plates provide a catalytic surface for the change in oxygen molecules.

       4.    Electric Oven, Thermocouple and Temperature Controller:

      In Zirconia sensor the temperature has to be above 450⁰C as only after that the zirconia become active as an electrolyte. Thus a thermocouple measures the temperature and controller gives the desired o/p to the electric heater and the desired temperature is maintained.

       

            Principle of Measurement

      There is a difference in concentration of oxygen (consequently a difference in the partial pressure of oxygen) on either side of the measured gas and reference gas which drives the oxygen to move from higher concentration to lower concentration.

                                           

Depending on the concentration on either side following reactions occur on each electrode:

The oxygen ions formed flow through the electrolyte and the electrons through the outer wire as happens in the galvanic cell producing an EMF.

                            

Just a small review of class 8

A galvanic cell is an electrochemical cell which converts chemical energy to electrical energy through a spontaneous chemical reaction. In a galvanic cell there are two half-cells. Each half-cell contains an electrode in an electrolyte. The separation is necessary to prevent direct chemical contact of the oxidation and reduction reactions, creating a potential difference. The electrons released in the oxidation reaction travel through an external circuit before being used by the reduction reaction.

In oxygen analyzer we don’t need to have separate half cells as here zirconia is a solid acting as an electrolyte so no contact of the reaction takes place.

Let us move to high school and review some further concepts

The EMF generated by the galvanic cell in non-standard condition is given by the Nernst Equation:

                                

Where, E _{cell 0} is the cell potential under standard conditions (25^oC, 1.0 M solution concentrations, 1.0 atm gas pressures), E _cell  is the cell potential under nonstandard conditions, R is the gas constant 8.314 J/mol-K, T is the temperature in Kelvin, F is Faraday's constant (96,485 coulombs/mol), n is the number of electrons transferred in the balanced oxidation/reduction reaction, and Q is the reaction quotient ([products]/[reactants]).

Applying the Nernst Equation in Zirconia Oxygen Analyzer

The potential difference across the cell is given by Nernst equation:

                           

Here, E= E _{cell 0} -E _cell all the terms represent the same terms as illustrated above and P1& P2 are the partial pressures of the oxygen on either side of the zirconia tube.

The Nernst equation can therefore be reduced to:

                          

Thus, if the oxygen partial pressure at one of the electrodes is known (reference gas-Air) and the temperature of the sensor is controlled (constant temperature for zirconia to act as a conducting electrolyte) and then measurement of the potential difference between the two electrodes enables the unknown partial pressure to be calculated using above equation. This partial pressure refers to the unknown oxygen concentration.

The following 3 cases can be there:

If P1>P2; ions flow from P1 to P2 i.e. a positive EMF

If P1<P2; ions flow from P2 to P1 i.e. a negative EMF

If P1=P2; no net ion flow i.e. a zero EMF

The analyzer electronically calculates the oxygen partial pressure, and therefore oxygen concentration, of a sample gas with unknown oxygen concentration. This is accomplished by measuring the potential, E, produced across the zirconium cell electrodes, substituting for E in the Nernst equation and anti-logging to obtain P1 (sample).

The zirconia oxygen analyzer may be used for the measurement of oxygen at any level between 

0-100% in gas mixtures.

But the Zirconia Oxygen Analyzer has a few drawbacks

1.       Incapable of measuring flammable gas mixtures due to the high temperature that needs to be maintained for the conductivity of zirconia.

2.       As there is a sensor that comes in direct contact with the reference gas, so can’t be used for corrosive 

      gases.

For overcoming these drawbacks another oxygen analyzer was introduced for knowing more about it stay tuned for the next article…