The Photometer Uv Absorption Path; Figure 6-1. O 3 Absorption Path - Teledyne T400 User Manual

Finally, to convert the result into parts per billion (PPB), the following change is made:
C =
Briefly, the Model T400 Ozone Analyzer:
• Measures each of the above variables: sample temperature; sample pressure; the
intensity of the UV light beam with and without O
• Inserts known values for the length of the absorption path and the absorption coefficient,
and
• Calculates the concentration of O
6.1.2 T P
HE HOTOMETER
IIn 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
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 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.
instrument's CPU to be used in computing the concentration of O
Window
Sample Gas OUT
UV
Source
083730400A DCN8130
( ) ( ) (
I -9
10
0
X X
ln
I
αL
present in the sample gas.
3
UV A
BSORPTION
ABSORPTION TUBE
Absorption Path Length = 42 cm
Figure 6-1. O Absorption Path
3
Teledyne API – Model T400 O
)
T
29.92 inHg
X
P
273K
present,
3
P
ATH
and transparent to UV radiation
3
This voltage is digitized and sent to the
in the absorption tube.
3
Window
Sample Gas IN
Analyzer
3
Equation 6-4
UV Detector
141