Sharp JX-9400 Informations techniques télécharger pdf (Page 164)

up to several minutes for gas chromatographs or multi-tracer infrared

analysers. Faster analysis will enable more frequent sampling of each zone

and hence provide more detailed data. Frequent sampling (for example,

every ﬁve to ten minutes) is essential for the constant concentration technique

to maintain accurate control of concentration.

Accuracy – last but not least, the accuracy of the concentration measurement

directly inﬂuences the accuracy of the results.

There are several principles employed for analysis for tracer gas concentrations

that diﬀer in the gases analysed, the ra nge of con centration detecte d, accuracy,

speed, ease of use and cost. These principles are discussed and speciﬁc

examples are given below.

Infrared absorption spectrometry

Any polyatomic gas molecule exhibits vibration modes, which are excited by

infrared radiation. The wavelengths of the infrared radiation correspondi ng

to molecular vibration frequencies

are absorbed in proportion to the number

of tracer gas molecules present in the infrared beam. This absorption is used

as a measure of the concentration of tracer gas molecules in the path between

an infrared source and detector. This technique is referred to as absorption

spectroscopy.

Infrared absorption spectrometers may be either dispersive or non-dispersive

types, and both are in common use. Dispersive spectrometers use a diﬀraction

grating that reﬂects the electromagnetic waves of a light beam into diﬀerent

directions, each direction corresponding to a given wavelength. The instrument

is tuned by the operator to a narrow band of wavelengths speciﬁc to the gas of

interest and any convenient infrared detector measures the absorption. Some

modern instruments use tun eable laser diodes that emit the appropriate

wavelength.

In non-dispersive devices, all the infrared radiation present in the absorption

bands of the tracer is used. The infrared light beam is sent through both a refer-

ence channel containing nitrogen or pure air and an analysis channel that

contains a sample of room air. A chopper is used to alternately pass the radiation

from each channel to an analysis chamber containing a pure sample of the tracer

gas. This gas heats and cools in response to the modulated beam. The heated gas

expands through a measuring channel into an expansion chamber. The resulting

alternating ﬂow through the measuring channel is measured by a highly sensitive

gas ﬂow detector, which transmits an electric signal.

Characteristics of infrared absorption spectrometers:

Analysable tracers – N

O, SF

, halon 1301, CO

. Other detectable gases,

such as H

O, benzene, alkenes and so on, are not suitable as tracers.

The sensitivity depends on the analysed gas. For example, full-scale devia-

tion is 200 ppm for N

O, or 20 ppm for SF

and CF

Br.

Interfering gases – care must be taken to eliminate the eﬀects of other gases

absorbing at similar frequencies (cross-sensitivity), particularly water

Common Methods and Techniques 143

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