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M400E Ozone Analyzer Operator’s Manual Theory of Operation
11.1.2. THE PHOTOMETER UV ABSORPTION PATH
In the most basic terms, the photometer of the Model 400E 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 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 then passes through 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 assembly reacts to the UV light and outputs a voltage that varies in direct relationship with the
light’s intensity. This voltage is digitized and sent to the instrument’s CPU to be used 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
Figure 11-1: O
3
Absorption Path
04315 Rev. C1 191