Environnement Envea MIR 9000, Техническое руководство, Страница: 34 / 212

Environnement
S.A
MIR 9000
Duplication prohibited
2–4
JULY 2009
Some milliseconds later, the cell filled with nitrogen is positioned on the optical path. Absorption of the
infrared energy is due to the gas in the measurement chamber, according to the Beer Lambert law.
Therefore, after absorption by the gas, the ratio between the reference energy and the "I" energy is
known at any moment. Hence, the gas concentration can be deduced, using the following formula:






=
R
I
I
f C
2.1.3 OTHER ADVANTAGES OF CORRELATION
The gas in the reference cell "extinguishes" all lines that are specific to it.
If there are gases on the optical path that are liable to interfere, i.e. that they might absorb in the same
wavelength area, it is not very probable that their absorption lines are exactly identical to the
absorption lines of the gas to be measured. The gases that are liable to interfere will absorb the
reference energy and the measurement energy in the same way. Therefore they will have no effect
whatsoever on the measurement.
To confirm the efficiency of correlation, let us take the example of carbon monoxide CO:
A CO measurement by IR absorption without correlation gives a H
2
O water sensitivity ratio of about
1/350, in spite of an optimum optical interference filter. Because of correlation, this ratio is now only
1/15000, which proves its efficiency.
However, for some gases, only a simple optical reference filter is used for correlation. It is centred on
a wavelength that is not absorbed by the gas to be measured. This is the typical case of the CO
2
when
it is highly concentrated.
In order to further improve metrological characteristics and, more specifically, to correct any deviation,
the monitor readjusts its zero periodically, which means that it defines the ratio between the reference
energy and the measurement energy when there is no gas on the optical path.
2.2 PRINCIPLE APPLIED TO THE MIR 9000
The clean gaseous sample is introduced into the multireflection chamber by the internal pump. The
optical path of the chamber increases sensitivity (path: 12 meters for standard version).
A ray emitted by an infrared source and measured by a detector passes through the optical chamber.
The semiconducting detector receives the optical beam after it has gone through a series of
interference filters and gaseous cells, mounted on a wheel turning at high speed.
The signal triggered by the detector is amplified and electronically processed and the concentration is
displayed in the desired unit.
The concentration value is displayed in real time, according to a response time which has been
programmed by the user beforehand.
Because of correlation by a gaseous filter, interferences that existed in any traditional infrared
measurement, no longer need to be taken into account.