Sharp JX-9400 Informações Técnicas descarregar pdf (Página 165)

vapour and CO

present at high concentrations in the air. Filters are used to

minimize the eﬀect but humidity should be measured simultaneously to

some tracers, suc h as N

O, to allow for corrections.

Analysis time – 10–50 s.

Accuracy – 1 per cent of full scale if the zero drift is controlled.

Photo-acoustic detector

This analyser is also an infrared absorption spectrometer, but uses a diﬀerent

detector. An infrared radiation beam is ﬁrst choppe d then optically ﬁltered to

leave only frequencies that are absorbed by the tracer of interest. This beam

then enters a gas-tight chamber containing the air sample. As above, the

sample is heated and cooled in phase with the chopping frequency, creating

sound waves in the chamber. This is the photo-acoustic eﬀect. Microphones

detect these sound waves.

Characteristics of analysers with photo-acoustic detector:

Analysable tracers – N

O, SF6, CO

, freons F11, F112, 113 and 114, halons

(one ﬁlter for each tracer). Other detectable gases are not suitable as tr acers,

such as H

O, benzene, alkenes and so on.

Sensitivity – the detection limit depends on the tracer but is typically

0.05 ppm, and the dynamic range is 10

. The lowest full-scale range may

then be 2 ppm but 10 ppm is recommended with usual tracers. The sensi-

tivity for N

O, CO and CO

drops strongly when these gases are di luted

in dry nitrogen, as is often the case for calibration gases. Adding a special

‘Naﬁon’ tube in the sampling circuit allows for the moistening of the mixture

in or der to recover the normal high sensitivity.

Interfering gases – several gases (which are not necessarily present in the air)

may interfere with each tracer. Therefore, ﬁlters and tracers should be

chosen in accordance with manufacturer’s speciﬁcations.

Analysis time – 30 s for one gas, 105 s for ﬁve gases and H

Accuracy – 1 per cent of full scale.

Mass spectrometry

The pressure of the air sample is ﬁrst lowered to about 10

5

Pa by pumping it

through a capillary tube. The molecules of the sample are then ionized, accel-

erated to a given velocity and passed into a mass spectrometer.

The classical mass spectrometer curves the trajectory of the ions with a

strong magnetic ﬁeld. The radi us of curvature depends on the velocity and

charge-to-mass ratio of the ion; only those having the appro priate combination

will pass a slit placed in front of the detector.

The most suitable spectrometer is, however, the quadrupole mass spectr o-

meter, which is currently used in vacuum processes to analyse the residual

gases. The gases entering into the analyser are ionized and the positive ions

are separated by directing them axially between two pairs of rods creating an

electric ﬁeld at variable radi o frequency. The ions follow a helicoidal path in

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