Sharp JX-9400 Technical Information PDF herunterladen (Seite 168)

By making the prescr ibed number of strokes of the hand-held bellows, the

correct amount of air is drawn through the tube. This enables the tracer gas

evaluation to be made. The glass tube has graduation marks on it, and the

length of the discolouration caused by the reaction indicates the concentration

of tracer in the room air. Detector tubes can only be used once and must be

discarded after each sample taken.

This single shot method is not very accurate but it is cheap and easy to

operate. Therefore, it is suitable for a rough ﬁrst estimate of the air change

rate. The interpretation of the result is performed using the integral decay

method (see Chapter 1, ‘Pulse injection’).

Characteristics of chemical indicator tubes:

Analysable tracers – CO

O and many toxic gases that are not useable as

tracers.

Sensitivity – 0.01–0.3 per cent range for CO

Analysis time – one minute.

Accuracy –  5 per cent or 10 per cent of full scale.

Calibration of the analysers

Any analyser should be periodically calibrated by analysing standard samples,

which are mixtures of the tracers in air or other inert gas. The calibration

mixtures containing N

O and CO

must be moistened when a photo -acoustic

detector is used. To transfer the calibration mixture from the containers to

the analyser, never use valves or tubes that were previously used with pure

or high concentration tracers.

During the measurements, it is recommended to periodically sample and

analyse the outside air as a convenient zero refe rence, even if no tracer is

expected in the outdoor air.

Identiﬁcation methods

Identiﬁcation is to assess the values of some parameters in formulae from values of

variables involved in these formulae. There are several identiﬁcation techniques.

Only some of them are presented below. The rationale of the presented methods

can be found in the literature, so only the ﬁnal formulae are presented here.

Linear least square ﬁt

The problem is the following: given N pairs of data (x; y), ﬁnd the straight

line:

y ¼ a þ nx ð7:12Þ

ﬁtting these points at the best. That means that the coeﬃcients a and n should

be such that the sum of the ‘distances’ of the measured points to the line is a

minimum.

Common Methods and Techniques 147

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Sharp JX-9400 Technical Information Seite 168