The Absorption Path; Interferent Rejection; Figure 9-17. O 3 Photometer Absorption Path - Teledyne T750 User Manual

The Absorption Path

In the most basic terms, the 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
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 uses this digital data in computing the
concentration of O
Window
Sample Gas OUT
UV
Source

Interferent Rejection

It should be noted that the UV absorption method for detecting ozone is subject to
interference from a number of sources. The photometer has 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 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.
083730750B DCN8606
and transparent to UV radiation at 254nm and into an absorption
3
in the absorption tube.
3
ABSORPTION TUBE
Sample Gas IN
Absorption Path Length = 42 cm
Figure 9-17. O Photometer Absorption Path
3
Window
UV Detector
Analog current
signal is output by
Detector
O-5 VDC
analog signal
Photometer
Pre amp
PCA Motherboard
to
177