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Theory of Operation M703E Calibrator Operator’s Manual
9.6.1.3. The Absorption Path
In the most basic terms, the M703E photometer uses a high energy, mercury vapor lamp to generate a beam of
UV light. This beam passes through a window of material specifically chosen to be both non-reactive to O
3
and
transparent to UV radiation at 254nm and into an absorption tube filled with sample gas.
Because ozone is a very efficient absorber of UV radiation the absorption path length required to create a
measurable decrease in UV intensity is short enough (approximately 42 cm) that the light beam is only required
to make one pass through the Absorption Tube. Therefore, no complex mirror system is needed to lengthen the
effective path by bouncing the beam back and forth.
Finally, the UV passes through a similar window at the other end of the absorption tube and is detected by a
specially designed vacuum diode that only detects radiation at or very near a wavelength of 254nm. The
specificity of the detector is high enough that no extra optical filtering of the UV light is needed.
The detector reacts to the UV light and outputs a current signal that varies in direct relationship with the intensity
of the light shining on it. This current signal is amplified and converted to a 0 to 5 VDC voltage analog signal
voltage sent to the instrument’s motherboard where it is digitized. The CPU to be uses this digital data in
computing the concentration of O
3
in the absorption tube.
UV
Source
ABSORPTION TUBE
UV Detector
Sample Gas IN Sample Gas OUT
Window
Window
Absorption Path Length = 42 cm
Photometer
Pre amp
PCA
O-5 VDC
analog signal
to
Motherboard
Analog current
signal is output by
Detector
Figure 9-20: O
3
Photometer Absorption Path
9.6.1.4. Interferent Rejection
It should be noted that the UV absorption method for detecting ozone is subject to interference from a number of
sources. The has M703E’s photometer been successfully tested for its ability to reject interference from sulfur
dioxide, nitrogen dioxide, nitric oxide, water, and meta-xylene.
While the photometer rejects interference from the aromatic hydrocarbon meta-xylene, it should be noted that
there are a very large number of other volatile aromatic hydrocarbons that could potentially interfere with ozone
detection. If the M703E calibrator is installed in an environment where high aromatic hydrocarbon
concentrations are suspected, specific tests should be conducted to reveal the amount of interference these
compounds may be causing.
158 05744 Rev B