T400 Ozone Analyzer Operator’s Manual Theory of Operation
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13.1.2. THE PHOTOMETER UV ABSORPTION PATH
In the most basic terms, the photometer of the Model T400 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 OUT Sample Gas IN
Window
Window
Absorption Path Length = 42 cm
Figure 13-1: O
3
Absorption Path
