OZONE MONITOR BMT 932 Manual, Rev. 02/2021
8
The law of BOUGUER-LAMBERT says that the product of the concentration, e.g. of ozone in
air, and the cuvette length, is proportional to the logarithm of the quotient of both light intensi-
ties, at the input to the cuvette and the output. This logarithm is also referred to as the extinction.
The coefficient of proportionality for ozone at 254 nm wavelength has been investigated thor-
oughly by many scientists. The different results agree to about plus/minus one percent. The actu-
al number of the extinction coefficient depends upon the definition of "concentration", and upon
the dimensions chosen. The number recommended by the IOA (International Ozone Associa-
tion) is 3000 ltr/cm mol with the concentration in mol/ltr at 1 atm and 0°C.
To measure ozone content in a sample gas, an ozone monitor has to measure the extinction of
UV radiation of high spectral purity at 254 nm wavelength during its passage through the cuvette
of well known length. At the same time the monitor has to measure temperature and pressure of
the sample gas in the cuvette, because the ozone "concentration" to be evaluated would be mean-
ingless without the knowledge of the thermodynamic condition of the sample. Actually the tem-
perature during measurement usually is around 293 K (20°C), but the pressure may vary dramati-
cally with the altitude and with the inevitable pressure drop in the sampling tube.
6
Units of Measure: ppm
v
, µg/m
3
(1 atm/different temperatures)
The internationally agreed upon definition of ozone content in the ambient air is
ppm
v
(parts per
million, volume of ozone per volume of sample gas). The International Standard
ISO/FDIS 13964 of 1998 defines ambient ozone content as
µg/m
3
at 1 atm/25°C
, or at 20°C,
or at 0°C. Actually the definition of 1mg/m
3
(1000 µg/m
3
) at 1 atm/20°C (usually used in the
European Community) is practically identical with 0.5 ppm
v
.
7
The OZONE MONITOR BMT 932
The OZONE MONITOR BMT 932 has a cuvette 285 mm in length. The cuvette is an
6.3 mm ID PTFE tube. The tube is mounted inside a thick-walled aluminium tube as the rigid
optical bench. Sample gas temperature is measured in the highly thermally conductive aluminum
tube. Sample gas pressure is measured directly inside the cuvette.
The low pressure mercury lamp is mounted directly to the aluminium optical bench. The
UV radiation passes a semi-reflective mirror, through which the radiation intensity is measured by
the reference detector just before entering the cuvette through a quartz window. At the other end
of the cuvette the measuring detector is mounted behind another quartz window. The reference
intensity signal is used to auto control the power to the lamp to precisely stabilize the
UV radiation. A LAMP LOW WARNING is activated when the UV lamp is near the end of its
service life.
Sample gas is sucked through the cuvette by a silent membrane pump. The flow rate is measured
by an electric flow meter. A control loop holds the mass flow rate constant at 1 l/min by control-
ling the power to the pump, automatically stabilizing the flow.
