Teledyne API Model 200AH NOX Analyzer Instruction Manual, 06492, Rev. G2
4-3
Figure 4-1: Paramagnetic Sensor
Refer to Figure 4-1. The measuring system is “null-balanced”. First, the ‘zero’ position of the
suspension assembly, as measured in nitrogen. is sensed by a split photo-sensor that receives
light reflected from a mirror attached to the suspension assembly. The output from the photo-
sensor is fed back to a coil wound around the suspension assembly. This feedback achieves two
objectives:
a) First, when oxygen is introduced to the cell, the torque acting upon the suspension
assembly is balanced by a restoring torque due to this feedback current in the coil. The
feedback current is directly proportional to the volume magnetic susceptibility of the
sample gas and hence, after calibration, to the partial pressure of oxygen in the sample.
A voltage output is derived which is proportional to the current.
b) Second. the electromagnetic feedback “stiffens” the suspension, damping it heavily and
increasing its natural frequency, making the suspension resilient to shock.