Model 202 Ozone Monitor Manual Rev. J-4
3
1.2. Calibration Overview
In principle, the measurement of ozone by UV absorption requires no external
calibration; it is an absolute method. However, non-linearity of the photodiode
response and electronics can result in a small measurement error. Therefore, each
instrument is compared with a NIST-traceable standard ozone spectrophotometer in
the laboratory over a wide range of ozone mixing ratios. These results are used to
calibrate the Ozone Monitor with respect to an offset and slope (gain or sensitivity).
The corrections for offset and slope are recorded in the instrument Birth Certificate
and on a calibration sticker that can be viewed by removing the top cover of the
instrument. These calibration parameters are entered into the microprocessor prior to
shipment. The user may change the calibration parameters from the front panel or the
serial menu if desired (see Sections 3.14 and 4). The offset may drift due to
temperature change or chemical contamination of the absorption cell. As discussed
below (Section 3.3 and Section 7), an accurate offset correction can be measured
from time to time using the ozone scrubber supplied with the instrument. It is
recommended that the instrument be recalibrated at least once annually and
preferably more frequently. The calibration procedure is described in detail in Section 6.
1.3. Use of DewLine to Remove Humidity Interference
Shown on Figure 1-1 is the DewLine, which serves to make the humidity entering
the detection cell identical during I and I
o
measurements. Please see our website for
a technical discussion of the DewLine™ and its importance to ozone measurements:
https://www.twobtech.com/dewline.html. Briefly, water vapor adsorbed to the inner
wall of the detection cell changes the reflectivity of the cell. If humidity is not the same
during I and I
o
measurements, an offset in the ozone measurement will occur and can
be up to several tens of ppb for sudden changes in ambient humidity. The offset will
change with time as the internal ozone scrubber equilibrates with water vapor. Even
for fixed-site ozone monitors, an offset measurement error will occur if the instrument
is zeroed with dry tank air and then used to measure ozone in humid air. The
DewLine solution to this often-ignored problem is unique to 2B Tech instruments.
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