Teledyne T100, Instruction Manual, Page: 231 / 404

Teledyne ML - T100 UV Fluorescence SO2 Analyzer Theory Of Operation
06807A(DCNxxxx) 231
9.1.9.5. LIGHT POLLUTION
Because T100 measures light as a means of calculating the amount of SO
2
present,
obviously stray light can be a significant interfering factor. The T100 removes this
interference source in several ways.

The sample chamber is designed to be completely light tight to light from sources other than
the excitation UV source lamp.

All pneumatic tubing leading into the sample chamber is completely opaque in order to
prevent light from being piped into the chamber by the tubing walls.

The optical filters discussed in Section 9.1.7; remove UV with wavelengths extraneous to
the excitation and decay of SO
2
/SO
2
*.

Most importantly, during instrument calibration the difference between the value of the most
recently recorded PMT offset (refer to Section 9.1.6) and the PMT output while measuring
zero gas (calibration gas devoid of SO
2
) is recorded as the test function OFFSET . This
OFFSET value is used during the calculation of the SO
2
concentration.
Since this offset is assumed to be due to stray light present in the sample chamber is also
multiplied by the
SLOPE and recorded as the function STR. LGT . Both OFFSET
& STR. LGT are viewable via the front panel (refer to Section 4.2.1).
9.2. OXYGEN (O
2
) SENSOR THEORY OF OPERATION
The O
2
sensor applies paramagnetics to determine the concentration of oxygen in a
sample gas drawn through the instrument.
9.2.1. PARAMAGNETIC MEASUREMENT OF O
2
The oxygen sensor used in the T100 utilizes the fact that oxygen is attracted into strong
magnetic field while most other gases are not, to obtain fast, accurate oxygen
measurements.
The sensor’s core is made up of two nitrogen filled glass spheres, which are mounted on
a rotating suspension within a magnetic field (refer to Figure 9-7). A mirror is mounted
centrally on the suspension and light is shone onto the mirror that reflects the light onto
a pair of photocells. The signal generated by the photocells is passed to a feedback loop,
which outputs a current to a wire winding (in effect, a small DC electric motor) mounted
on the suspended mirror.
Oxygen from the sample stream is attracted into the magnetic field displacing the
nitrogen filled spheres and causing the suspended mirror to rotate. Therefore, the
amount of light reflected onto the photocells and therefore the output levels of the
photocells. The feedback loop increases the amount of current fed into the winding in
order to move the mirror back into its original position. The more O
2
present, the more
the mirror moves and the more current is fed into the winding by the feedback control
loop.
A sensor measures the amount of current generated by the feedback control loop which
is directly proportional to the concentration of oxygen within the sample gas mixture.