Teledyne API T703/T703U Calibrator Operation Manual Principles of Operation
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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.
Absorption Path Length = 42 cm
O-5 VDC
analog signal
to
Motherboard
Analog current
signal is output by
Detector
Figure 9-22: O
3
Photometer Absorption Path
Interferent Rejection 9.6.1.4.
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.
PHOTOMETER LAYOUT 9.6.2.
The Photometer is where the absorption of UV light by ozone is measured and
converted into a voltage. It consists of several sub-assemblies:
A mercury-vapor UV lamp. This lamp is coated in a material that optically
screens the UV radiation output to remove the O
3
producing 185nm
radiation. Only light at 254nm is emitted.
An AC power supply that supplies the current for starting and maintaining the
plasma arc of the mercury vapor lamp.
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