Instruction manual

NGA 2000 MLT Hardware
MEASURING PRINCIPLE
2 - 7
Emerson Process Management GmbH & Co. OHG
Instruction Manual
90002929
07/2006
2.3 Oxygen Measurement
Depending on analyzer model different two measuring methods will be used.
2.3.1 Paramagnetic Measurement (PO
2
)
Oxygen measurement is based on the paramagnetical characteristics of Oxygen molecules:
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. 2-4).
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 paramagnetic 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 preamplifier 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
So the current compensating the torque affecting 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 paramagnetic
oxygen detector contains a temperature 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.
OXYGEN MEASUREMENT