User's Manual Part 3

10-17
measurement accuracy.
In addition to the measurement filters, two optical filters appropriately located within the 4
to 10 µm range are used as references.
E.2.2. Oxygen measurement, Paramagnetic
Oxygen does not absorb infrared light to the same extent as other breathing gases and is
therefore measured using alternative methods. The ISA OR+ analyzer is fitted with a
paramagnetic oxygen sensor.
Paramagnetic oxygen analysis
Paramagnetic oxygen analysis is based on measurements of the attractive force exerted
by a strong magnetic field applied to the oxygen molecules in a gas mixture. The
paramagnetic analyzer distinguishes oxygen from other gases as a function of their
magnetic susceptibility. Due to its paramagnetic nature, oxygen is attracted into the
magnetic field, while most other gases are not. On a scale, where oxygen is assigned the
value 100, most other gases have a magnetic susceptibility of close to zero.
The Servomex sensors
The oxygen sensor included in the 3886 Multi-Gas system as part of the ISA OR+ gas
analyzer is the Pm1116 paramagnetic oxygen sensor from Servomex. In these sensors, a
symmetrical non- uniform magnetic field is created. If oxygen is present, it will be attracted
into the strongest part of this field. Two nitrogen-filled glass spheres are mounted on a
rotating suspension within the magnetic field. Centrally on this suspension, a mirror is
mounted. A light beam projected on the_mirror is reflected onto a pair of photocells.
Oxygen attracted into the magnetic field will push the glass spheres from the strongest
part of the magnetic field, causing the suspension to rotate.
When this rotation is detected by the photocells, a signal is generated and passed to a
feedback system. The feedback system will pass a current around a wire mounted on the
suspension, causing a restoring torque that keeps the suspension in its original position.
The current flowing around the wire is measured. This current is directly proportional to
the oxygen concentration.
Figure 4-2. Oxygen measurement with Servomex paramagnetic oxygen sensors.
The most important benefits of the paramagnetic oxygen sensor are:
• Fast rise time
• High stability and accuracy
• No chemicals to replace or renew
• Low maintenance requirement