Personal Ozone Monitor Manual Rev. E-2
3
Shown on Fig. 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:
www.twobtech.com/dewline.htm
. 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.
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 photometer in the
laboratory over a wide range of ozone mixing ratios. These results are used to
calibrate the POM with respect to an offset and slope (gain or sensitivity). The
corrections for offset and slope are recorded in the instrument Birth Certificate. These
calibration parameters are entered into the microprocessor prior to shipment. The
user may change the calibration parameters from the front panel if desired. It is
recommended that the instrument be recalibrated at least once every year and
preferably once every six months. The offset may drift due to temperature change or
chemical contamination of the absorption cell. As discussed below, an accurate offset
correction can be measured from time to time using the external ozone scrubber
(Zeroing Cartridge) supplied with the instrument.
An animated video explaining how a UV-absorbance ozone monitor works is posted
on the 2B Tech website at:
http://twobtech.com/videos_general.htm
