Where
l
is the path length (6.2 cm) and
σ
is the absorption cross section for ozone at
254 nm (1.15 x 10-17 cm2 molecule-1 or 308 atm-1 cm-1), which is known with an
accuracy of approximately 1%. The Eco Sensors instrument uses the same absorption
cross section (extinction coefficient) as used in other commercial instruments.
The logarithm of equation 1 is approximated in the microprocessor of the instrument with
sufficient accuracy to provide five orders of dynamic range; ozone mixing ratios are
measured up to 1,000 ppmv, as compared to 1 ppmv for most commercial ozone
instruments.
The pressure and temperature within the absorption cell are measured so that the ozone
concentration can be expressed as a mixing ratio in parts-per-million by volume (ppmv).
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). Calibration data is
supplied with the instrument. It is recommended that the instrument be recalibrated at
least once every year and preferably more frequently. The offset may drift due to
temperature change or chemical contamination of the absorption cell. An accurate offset
correction can be measured from time to time using the ozone scrubber ("zero ozone
filter") available for use with the instrument.
16
Hg Lamp
Temperature Sensor
Air Pump
Photodiode
Absorption Cell
Pressure Sensor
Solenoid Valve
Ozone
Scrubber
Air Inlet
Basic UV Instrument System
