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3-9
Instruction Manual
HASAxE-IM-HS
05/2006
Emerson Process Management GmbH & Co. OHG
3 Principles
3-2 Oxygen Measurement
3-2 Oxygen Measurement
Two different principles are used for measuring
oxygen concentrations. The currently used
principle is given by the channel code (sample
gas designator) on the nameplate label (
fig 2-7, page 2-13):
pO2 = paramagnetical sensor
eO2 = electrochemical sensor
Oxygen measurement is based on the para-
magnetical characteristics of oxygen mole-
cules:
Two nitrogen filled quartz spheres (N
2
is not
paramagnetic) are arranged in a dumbbell
configuration and, hinged to a platinum wire,
placed inside a cell. Fixed to the wire a small
mirror reflects a light beam to a photo detector
(fig. 3-6).
The measuring cell is placed inside an
inhomogeneous magnetical field generated by
a strong permanent magnet of specific design.
Oxygen molecules within the sample gas now
due to their paramagnetical characteristics are
deflected into the area of highest field 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 because the beam is
deflected, 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 field
3-2-1 Paramagnetic Measurement
So the current compensating the torque affec-
ting the dumbbell is a direct measure for the
oxygen concentration within the sample gas.
In addition to measuring cell, permanent
magnet, electronics and enclosure the para-
magnetic oxygen detector contains a tempera-
ture 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 flammable
gases) versions.
Table 3-1: Solvent Resistant Sensor:
Approved Solvents