3.3 O2 Analyzer

3.3.1 Paramagnetic O2 analyzer

Principle of measurement 

All gases have positive or negative magnetic susceptibilities. Among these gases, O2, NO and NO2 have high positive magnetic susceptibilities and are attracted strongly by magnetic field, whereas the others are diamagnetic substances. NO and NO2 may be contained in combustion gas at negligibly low concentrations. It is therefore possible to measure O2 contents in various types of gases by utilizing its magnetic susceptibility.

There are two types of O2 Analyzer:

1. Dumb-bell type

2. Wind type 

1. Dumb-bell type

In the cell, two glass spheres filled with nitrogen gas are suspended with strong metal wire. At first, the spheres are kept in balance in an in homogeneous magnetic field. When oxygen molecules having a large magnetic susceptibility flow there, the molecules are pulled toward the stronger magnetic field zone and the spheres are moved away from the zone. The resulting deviation of the spheres is detected with the light source, reflecting mirror and light receiving element, and a current is flowed through the feedback loop to control so that the spheres can return to the initial balanced state. The current flowing through the feedback loop is proportional to oxygen concentration. Thus, oxygen concentration is converted into an electric signal. 

2. Wind Type

The sample gas flows into the glass ring tube through the gas inlet, then flows through the tube, and out the gas outlet. If oxygen is present in the sample gas, the gas will be pulled from the left to the right within the tube by the magnetic field of the magnet poles. If this gas is heated by a heating coil, the magnetic characteristic of the oxygen will be weakened, and the gas will be pushed out to the right. In this way, a gas flow (magnetic wind) that corresponds to the concentration of oxygen in the sample will be generated in the middle part of the tube.

Thus the left resistor becomes colder than right one & bridge becomes unbalanced. The bridge unbalanced current will be proportional to the % of oxygen in the sample gas.

Advantages:

Scale is almost linear.

In comparison to paramagnetic method, it is rugged in design.

Dissolved O2 Analyzer

Measured in mg/l.