where l is the path length (0.1 cm) and . 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 2B Technologies instrument 

uses the same absorption cross section (extinction coefficient) as used in other 

commercial instruments. 

When there is no ozone in the cell, a difference in light intensity at the reference 

and sample photodiodes can be expected, and it will cause a zero offset in the 

readings. This light intensity difference is inherent in the instrument due to the 

different properties of the two photodiodes, drift in the angle of the beamsplitter, 

contamination in the cell and thermal drift in any of the optical components. 

The zero drift has been determined to be less than or equal to 2% of full scale 

per month, non-cumulative and can easily be corrected for by using the zero 

function described below. The zero drift does not affect the slope calibration 

factor in any way, and the instrument does not need to be recalibrated after 

performing the zeroing function. 

The pressure and temperature within the absorption cell are measured so that 

the ozone concentration can be expressed as a mixing ratio in percent by 

volume (vol%). The fraction of the sample that is ozone can then be calculated 

and used to determine the ozone concentration in percent by weight in either 

air (Wt.% air) or oxygen (Wt.% O2). The instrument displays and records the 

cell temperature and pressure in addition to the ozone concentration. The cell 

pressure is displayed and logged in units of mbar, torr or psia and the cell 

temperature in units of either oC or K. 

In principle, the measurement of ozone by UV absorption requires only a zero 

calibration to account for the different light intensities at the reference and 

sample photodiodes. However, non-linearity of the photodiode response and 

electronics can result in a small measurement error. Therefore, each 

instrument is compared with a reference ozone monitor calibrated to the 

International Ozone Association (IOA) buffered KI method. These results are 

used to calibrate the Ozone Monitor with respect to a slope (gain). The 

correction for slope is recorded in the instrument Birth Certificate. The slope 

parameter is entered into the microprocessor prior to shipment. The user may 

change the slope parameter from the front panel if desired. It is recommended 

that the instrument be recalibrated at least once every year and preferably 

more frequently. The zero may drift due to temperature change or chemical 

contamination of the absorption cell. As discussed below, an accurate zero 

correction can be measured from time to time using zero gas.