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Part 2 Analysis Unit
6.2 Paramagnetic Sensor
The paramagnetic sensor is sensitive to pressure and temperature
variations. The sensor used in the model 3010MA includes a preamp and
an internal temperature controller set to 55 C. The entire sensor assembly
is housed in an oven to further insulate the sensor from thermal
fluctuations.
The sensor produces a linear output from 0-1.2 V dc, and operates
over the range of 95-100% oxygen. The sensor has zero and span trimpots
for calibration.
6.2.1 Principles of Operation
The heart of the 3010MA is a paramagnetic type oxygen sensor that
is maintenance free and has a long lifetime. Oxygen has a very high
magnetic susceptibility compared to other gases and thus takes on a
distinct magnetic behavior when influenced by another magnet or
positioned in a magnetic field. This is referred to as paramagnetic
behavior.
A small glass dumbbell filled with nitrogen and rotating on a taut
platinum wire is suspended in an inhomogneous magnetic field
surrounded by sample gas. This glass dumbbell is diamagnetic (does not
respond or only weakly responds to a magnetic influence) and tends to
rotate out of the magnetic field depending on the magnetic susceptibility
of the sample gas. The rotation of the dumbbell produces a torque that is
proportional to the magnetic susceptibility of the sample gas. This torque
is compensated for by a counter torque induced by an electrically charged
platinum coil on the dumbbell.
The zero position of the dumbbell is controlled by means of an
optical system consisting of a light source, a mirror at the dumbbell axis
and a pair of detectors. The difference between the compensating currents
required to bring the dumbbell to the zero position in the presence of zero
gas (i.e. no O
present) or of sample gas is proportional to the partial
2
pressure of oxygen in the sample gas.
The sensor enclosure is temperature controlled to 55 degrees
centigrade to insure that the magnetic susceptibility of oxygen in the
sample is not affected by the ambient temperature. The measuring cell has
a measuring volume of approximately 3 cm
Teledyne Analytical Instruments
Model 3010MA
3
.
68
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