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Emerson Process Management GmbH & Co. OHG 3-5
X-STREAM XE
Instruction Manual
HASXEE-IM-HS
04/2010
3
Principles
3.2 Oxygen Measurement
3.2 Oxygen Measurement
Two different principles are used for mea-
suring oxygen concentrations. The principle
used in your specic instrument is given by
the channel code (sample gas designator)
on the nameplate label (
gure on page
2-22):
pO2 = paramagnetical sensor
eO2 = electrochemical sensor
Oxygen measurement is based on the para-
magnetical characteristics of oxygen mole-
cules:
Two quartz spheres lled with nitrogen (N
2
is
not paramagnetic) are arranged in a dumb-
bell conguration and, hinged to a platinum
wire, placed inside a cell. Fixed to the wire a
small mirror reects a light beam to a photo
detector (Fig. 3-4).
The measuring cell is placed inside an in-
homogeneous magnetical eld, generated
by a strong permanent magnet of specic
design.
Oxygen molecules within the sample gas now
due to their paramagnetical characteristics
are deected into the area of highest eld
strength. This generates different forces on
both spheres and the resulting torque turns
dumbbell and mirror out of the rest position.
This generates a photodetector signal becau-
se the beam is deected, too.
Initiated by the photodetector signal a pre-
amplifier drives a compensation current
through a loop surrounding the dumbbell to
turn back the dumbbell into the rest position
by effect of a magnetic eld
So the current compensating the torque affec-
ting the dumbbell is a direct measure for the
oxygen concentration within the sample gas.
3.2.1 Paramagnetic Measurement
The paramagnetic oxygen detector also con-
tains a temperature sensor and a heating ele-
ment to hold the detector at approx. 55 °C.
Several variations are available including
corrosion resistant, solvent resistant and/or
intrinsically safe (for measuring ammable
gases) versions.
Fig. 3-4: Paramagnetic Oxygen Sensor -
Assembly Principle
1 Permanent magnet
2 Platinum wire
3 Mirror
4 Glass ball
5 Loop
6 Photodetector
7 Light source
8 Preamplier
9 Display
10 Gas inlet
11 Gas outlet
4 1 5 3 6 8
11 7 9
2
10