Fig. 2-1: Magnetic Field Cell Structure of paramagnetic method dumb-bell type oxygen analyzer (General View)
Figure 2-2: Top view of magnetic field cell in dumb-bell type oxygen analyzer
The paramagnetic method is a measuring principle that utilizes the extremely strong paramagnetism (*1) of oxygen. The dumb-bell type of the oxygen analyzer uses this property to detect the oxygen attracted by the movement of a dumb-bell connected to two nitrogen-filled glass spheres with a glass rod.
The characteristic structure of the analyzer consists of a glass dumb-bell with a attached mirror suspended horizontally by a suspending wire in a small magnetic field cell (about 2 cm3), which magnetizes the oxygen in the sample gas (Figure 2-1). Figure 2-2 shows a top view of the magnetic field cell, omitting the N and S poles, to make it easier to see the explanation of the operation of the dumb-bell sphere and other devices.
Fig. 2-1: Magnetic Field Cell Structure of paramagnetic method dumb-bell type oxygen analyzer (General View)
Figure 2-2: Top view of magnetic field cell in dumb-bell type oxygen analyzer
When sample gas containing oxygen passes through a magnetic field cell, oxygen with strong paramagnetism pushes the dumb-bell sphere and approaches the strongest part of the magnetic field. As a result, the force is generated to push the dumb-bell sphere in response to the oxygen concentration. This generates a force to rotate the suspended mirror (Figure 2-3, Figure 2-4).
F= (X1 - X2)・V・H
F: Force acting on dumb-bell X1: Magnetic susceptibility of sample gas X2: Magnetic susceptibility of dumb-bell V: Volume of dumb-bell H: Strength of magnetic field
The change in the direction of the mirror with the rotation of the dumb-bell changes the reflection angle of the light (incident light) incidented from the light source to the mirror, and the position of the reflected light (received light spot) at the light-receiving unit changes.
The light-receiving unit, which consists of two photovoltaic cells with uniform characteristics, has a difference in the electromotive force of the two photovoltaic cells depending on the position of the receiving light spot. A current corresponding to this difference is applied to the feedback coil around the dumb-bell in the magnetic field to generate a force, and the dumb-bell is returned to the neutral position by feedback control using this force (Figure 2-5).
Since this current is proportional to the oxygen concentration, signal processing is performed to convert the current to the concentration. In the actual analyzer, the force to push the dumb-bell in response to the oxygen concentration is always balanced by the force generated by the controlling the current to keep the dumb-bell in a neutral position at all times. The oxygen concentration is calculated by measuring the current required for this control.
Figures 2-3, 2-4, and 2-5: Operating principle of paramagnetic method dumb-bell type oxygen analyzer
Analyzers using the paramagnetic method dumb-bell type are used to continuously measure an oxygen concentration in exhaust gases and process gases in various fields.
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