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Emerson Process Management GmbH & Co. OHG 3-5
X-STREAM X2
Instruction Manual
HASX2E-IM-HS
02/2012
3
Measuring Principles
3.2 Oxygen Measurement
3.2 Oxygen Measurement
Two different principles are used for mea-
suring oxygen concentrations. The currently
used principle is given by the channel code
(sample gas designator) on the nameplate
label (
see gure on page 2-21):
pO2 = paramagnetical sensor
eO2 = electrochemical sensor
Oxygen measurement is based on the para-
magnetical characteristics of oxygen mole-
cules:
Two nitrogen lled quartz spheres (N
2
is not
paramagnetic) are arranged in a dumbbell
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
(g. 3-7).
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
In addition to measuring cell, permanent
magnet, electronics and enclosure the para-
magnetic oxygen detector contains a tempe-
rature sensor and a heating element 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