Emerson Process Management GmbH & Co. OHG 3-5
X-STREAM XE
Instruction Manual
HASXEE-IM-HS
04/2010
3Principles
3.2 Oxygen Measurement
3.2 Oxygen Measurement
Two different principles are used for mea-
suring oxygen concentrations. The principle
used in your specic 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 (N2 is
not paramagnetic) are arranged in a dumb-
bell conguration and, hinged to a platinum
wire, placed inside a cell. Fixed to the wire a
small mirror reects 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 mag net of specic
design.
Oxygen molecules within the sample gas now
due to their paramagnetical characteristics
are deected 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 deected, 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 Preamplier
9 Display
10 Gas inlet
11 Gas outlet
4 1 5 3 6 8
11 7 9
2
10