Sharp JX-9400 Informations techniques télécharger pdf (Page 167)

Gas chromatography

A puﬀ of the sampled air is injected into a separating (chromatographic)

column, a tube in which adsorbent material is packed. This column is heated

and the pulse of sample is pushed with a ﬂow of inert carrier gas. The various

components of the sample pass through the column at various speeds according

to their aﬃnity for the adsorbent material. At the end of the column, the

components emerge in sequence and can be quantitatively detected with a suit-

able detector. Both ﬂame ionization detectors (FIDs) and electron capture

detectors (ECDs) have been used for tracer gas analysis.

In the FID, a pair of polarized electrodes collects the ions produced

when organic compounds are burned into an hydrogen ﬂame, and the

current produced is ampliﬁed before measurement. This detector is rugged,

reliable, easy to maintain and operate, and is by far the most used in gas

chromatography, since it combines a good sensitivity to organic compounds

(limit of detection of about 10

9

g) with a good linearity within a ra nge of up

to 10

However, organic compounds are not very good tracer gases and the ECD,

which is much more sensitive to halogenated

compounds, is the most common

in tracer gas analysis. In this detector, a radioactive nickel cathode emits

electrons, which are received on an anode. Halogens capture these electrons,

lowering the received current and thereby indicating the tracer concentrations.

ECDs are popular since they can measure halogenated tracers to exceptionally

low concentrations.

Characteristics of gas chromatographs:

Analysable tracers (with ECD) – any halogenated compound like SF

freons, perﬂuorocarbons or perﬂuorocycloalkanes.

Sensitivity – from ppb (10

9

) range for SF

down to 10

14

for the PFTs.

Interfering gases – H

O, O

(oxygen traps and desiccators are used to

suppress these eﬀects).

Analysis time – a few minutes but can be lowered down to 20 seconds by

shortening and back ﬂushing the column, if high selectivity is not needed.

Accuracy – depends on the quality of the calibration, but can be 2 per cent of

reading.

Chemical indicator tubes

This is a single shot method to estimate the air change in a single zone by the

decay or constant injection technique with some tracers.

Detector tubes ar e glass tubes packed with a selective solid absorbent, which

gives a colour reaction to some gases. The tubes used are sensitive to CO

in the

0.01–0.30 per cent range. Tubes as supplied by the manufacturer are sealed at

both ends. To make a measurement the seals are broken, one end of the tube

(the correct end is indicated on the tube) is inserted into a pair of specially

designed hand bellows, the other end being left open to sample the air tracer

mixture.

146 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Informations techniques Page 167

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