Teledyne API Model 401 O
3
Photometric Calibrator Instruction Manual, 01124, Rev. J2
The intensity of light is converted into a voltage by the detector/preamp module. The voltage is
converted into a number by a voltage-to-frequency (V/F) converter capable of 80,000 count
resolution. The digitized intensities, along with the other variables, are used by the CPU to
compute the concentration using the above formula.
Every 8 seconds the M401 Photometer completes a measurement cycle consisting of a 2 second
wait period for the sample tube to flush, followed by 2 seconds of measuring the average light
intensity to determine I. The sample valve is switched to admit scrubbed sample gas for 2
seconds followed by 2 seconds measuring the average light intensity to obtain I
o
. Measurement
of the I
o
every 8 seconds minimizes instrument drift due to changing intensity of the lamp due to
aging and dirt. The 120 sec filter provides 32 averages.
1.3.1 Interferent Rejection
It should be noted that the UV absorption method for detecting ozone is subject to interference
from a number of sources. The Model 401 has been successfully tested for its ability to reject
interference from sulfur dioxide, nitrogen dioxide, nitric oxide, water, and meta-xylene.
While the instrument rejected interference from the aromatic hydrocarbon meta-xylene, it should
be noted that there are a very large number of volatile aromatic hydrocarbons that could
potentially interfere with ozone detection. If the Model 401 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.
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