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<13. MEASUREMENT PRINCIPLE>
13-1
IM 11P03A03-01E 6th Edition : Jan. 26, 2022-00
13. MEASUREMENT PRINCIPLE
13.1 Measurement Principle
A diagram of the principle is shown in figure 13.1.
When the sample gas contains no oxygen molecules, the right and left flow rates of the auxiliary
gas (N
2
), Q
L
and Q
R
are equal, or Q
R
= Q
L
.
If oxygen molecules are included in the gas, flow rate Q
L
on the magnetic field-generating side
is less than Q
R
due to the force with which the magnetic field acts on the oxygen molecules,
establishing Q
R
> Q
L
.
The difference in the flow rate,
D
Q = Q
R
- Q
L
is proportional to the percentage of oxygen
molecules in the measuring gas.
The thermister sensor is operated in the constant-resistance circuit to convert the flow rates
(velocity) into electric signals, the difference of which is obtained by amplifier A as an oxygen
signal.
R.IN
Q
L
V
R
V
L
A
O
2
signal
Magnetic
Field
Thermistor
F
M.IN:Sample gas inlet
R.IN:Auxiliary gas inlet
OUT :
Gas outlet
F:
Magnetic force acting on oxygen molecules
M.IN
Q
R
OUT
V
R
:Q
R
flow signal
V
L
:Q
L
flow signal
F16.1E.ai
Thermistor
Constant-resistance
circuit
Constant-resistance
circuit
Figure 13.1 Measurement Principle
13.2 Principle of Interference-gas Compensation
Although measurements by the paramagnetic oxygen analyzer utilize the intense paramagnetic
property of oxygen, gases other than oxygen also have a slight paramagnetic property. Hence, a
small indication error occurs due to their magnetic susceptibilities for various process gases.
This error may become a problem at low concentration ranges. The model MG8 paramagnetic
oxygen analyzer cancels this error caused by paramagnetism of the process gas by utilizing the
difference in density between the measuring and reference gases.
The error is compensated by changing the cell angle (inclination). The measuring gas stream is
divided into stream A and stream B as shown in Figure 13.2. The auxiliary gas (normally nitrogen
gas) is introduced from the center of the cell and its stream is divided into the right and left. Since
a magnetic field exists at only the one auxiliary gas outlet, if there is a gas other than nitrogen
having a different magnetic susceptibility, then an error occurs because it affects the auxiliary gas
flow ratio of the two streams. The analyzer compensates for this interference by changing the cell
angle.