Teledyne 6200E - Sulfides Analyzer - Manual (Page 210)

Theory Of Operation Model 6200E Instruction Manual
210 M6200E Rev: A1
10.2.5. Optical Lenses
Two optical lenses are used to focus and optimize the path of light through the sample chamber.
214 nm
Filter
330 nm
Filter
UV Source
Lens
PMT Lens
Reference
Detector
If source UV is unfocused, PMT
receives fluorescence from area
outside Reference Detector’s view
PMT
When source UV is focused, PMT
and Reference Detector view
similar volume of SO
2
*
When source UV is focused,
Reference Detector sees
most of the emitted light
If source UV is unfocused,
Reference Detector only sees a
small portion of emitted light
Figure 10-6: Effects of Focusing Source UV in Sample Chamber
A lens located between PMT and the sample chamber collects as much of the fluoresced UV
created there as possible and focuses it on the most sensitive part of the PMT’s photo cathode.
Another lens located between the excitation UV source lamp and the sample chamber collimates
the light emitted by the lamp into a steady, circular beam and focuses that beam directly onto the
reference detector. This allows the reference detector to accurately measure the effective intensity
of the excitation UV by:
Eliminating the effect of reflected light from the UV lamp reaching the PMT.
Making sure that all of the light emitted by the source lamp, passed though the 214 nm filter and
not absorbed by the SO
2
reaches the reference detector. Conversely, this also makes sure that the
volume of sample gas affected by the excitation beam is similar to the volume of fluorescing SO
2
*
being measured by the PMT, eliminating a possible source of measurement offset.
10.2.6. Measurement Interferences
It should be noted that the fluorescence method for detecting H
2
S is subject to interference from a
number of sources. The M6200E has been successfully tested for its ability to reject interference
from most of these sources.