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expected tracer gas concentration of tracer k:

¼ C

ð2:5Þ

Sampling points for concentration measurements

Tracer gas concentrations are measured at several carefully chosen locations in

order to obtain enough information to determine all the w anted airﬂow rates. It

is important that there is a good mixing of tracer gas in the measured airﬂow.

For this, several criteria should be fulﬁlled. Practice has shown that suﬃcient

mixing is reached when the distance between injection ports and air sampling

location is at least:

10 diameters (or duct widths) in straight ducts;

5 diameters if there is a mixing eleme nt such as bends, droplet catcher or a

fan between injection ports and the air sampling location.

Proper mixing can be checked by looking at the variations of measured concen-

tration with time, and when displacing the sampling location within the duct

(see Figure 2.6). If variations are large and random, change the sampling

and/or injection points, or use multiple in jection ports until variations are

within the usual measurement noise.

If the minimum distances mentioned above cannot be achieved, use

multiple injection (see Figure 2.7) or install obstacles in the airﬂow to increase

the turbulence.

Turbulent ﬂow may transport some tracer gas a little upwind of the

injection point. Therefore, the distance between sampling location upwind of

injection points and the injection nozzle should be at least one duct diameter

when there is no possibility of backward ﬂow, and larger (3–5 duct diameters)

when backward airﬂow is suspected (for example close to T junctions).

When sampling, never use the tubes that were once used for injecting a pure

tracer gas, since some gas absorbed in the plastic of the tubes may be desorbed,

10.7

10.8

10.9

11.0

Concentration [ppm]

11.1

11.2

Time

08:00 08:10 08:20 08:30 08:40 08:50 09:00

Figure 2.6 Evolution of tracer gas concentration versus time

Note: The solid line indicates good mixing of the tracer gas; the broken line indicates

poor mixing.

22 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Technical Information Seite 43