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Oxygen is unique among common gases. It is strongly paramagnetic by virtue of its magnetic susceptibility, and this property has been utilized in gaseous oxygen monitoring.
A lightweight, dumbbell-shaped, glass rotor is suspended by a torsion spring in a non-uniform magnetic field.
Oxygen tends to accumulate in this field to displace the nonmagnetic rotor.
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This rotation is opposed by the torsion spring and occurs to a degree proportional to the oxygen tension. Readout depends on the position on a translucent scale of a slit of light reflected from a mirror that turns with the rotor. This meter is accurate, requires no external power, and will function indefinitely if not abused. However, the gas to be analyzed must be pumped through the chamber, and the response to changes in oxygen concentration is slow.
An analog signal could be obtained from this type of analyzer by arranging for the light reflected from the motor mirror to strike a photocell. A mask with a triangular aperture overlies the light-sensitive area of the cell. As the mirror rotates, an increasing amount of the reflected slit of light passes through the widening aperture, and the electrical output of the photocell rises.
Accuracy can be increased by the use of a null balance method. Wound on the rotor is a coil so oriented that current flows through it, producing a magnetic torque opposed to torque resulting from the presence of oxygen. A photocell detects the rotor movement. The amplified output of this cell adjusts coil current to a level just sufficient to balance the initial torque and prevent rotation. The magnitude of balancing current required is proportional to the oxygen tension.
Oxygen Analyzer:
One of the paramagnetic oxygen analyzers that have been used in investigations for a long time is DCL Servomex Oxygen Analyzer, type 83, from the United Kingdom. This is strictly a linear measuring device and is a dumbbell-type paramagnetic oxygen analyzer.
Standardization of Oxygen Analyzer:
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Since it is a strictly linear measuring device, it can therefore be standardized for all scales by checking at two points only. For convenience, 0% oxygen (pure nitrogen) and 21% oxygen (air) are used. After switch-on, the instrument is allowed to warm up at a temperature of 35°C.
It is held at this temperature for 2 hr. The following steps are observed. Pure nitrogen (oxygen free) is passed through the instrument at a set flow rate. With the help of control switches provided on the instrument, the meter indicator is adjusted accurately at zero following the standard set procedure.
Next, drier air at 29°C (room temperature) is passed through the instrument at the selected flow rate. When the indicator needle shows a steady reading, the standardized control switch on the instrument is adjusted until the meter reads exactly 21 %. The instrument is now standardized. It is connected through a sterile rotameter and CaCl2 absorber to the bioreactor exit gas line. Percentage oxygen in the gas stream is measured periodically during bio-reaction.
