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Mixing tracer gases

Perfect mixing of tracer gas in the air of the measur ed zone or in the measured

duct is essential when determi ning the airﬂow rates, but not for experiments to

determine the age of the air. We found that in a closed, quiet, isothermal room,

it may take severa l hours to mix a tracer gas into the air. In a 60 m

room, this

time is shortened to less than half an hour if a 100 W heat source (such as a quiet

person or a light bulb) is present. A small 20 W fan like those used to ventilate

the power supplies of computers reduces the mixing time down to ﬁve minutes.

Several methods can be used to improve or accelerate the mixing of tracer

gases. The most widely used method is to inject the tracer upwind of a

mixing fan, which can be a small 20 W cooling fan used in the electronic

industry. Alternatives include portable oscillating fans. This method works

perfectly but changes the thermal gradients in the measured zone, and may

aﬀect the air exchange rates. It should not be used during the measurement

of the age of the air.

Mixing fans are not necessary if the injection nozzles are located at the

locations where natural convection or mechanical ventilation provides signiﬁ-

cant air currents. Moreover, a continuous injection ﬂow rate greatly assists

the attainment of a uniform tracer gas concentration and is therefore preferred

to pulse injection.

Quick mixing with the air around the injection port is obtained if the

velocity of the tracer gas at the injection nozzle is large enough to create a

turbulent jet (Silva and Afonso, 2004). For this purpose, the ﬂow controlling

valve and nozzle should be at the end of the injection tube, with the tube

Table 7.4 Qualities of some tracer gases

Name Compliance with the quality

No ﬁre

hazard

Low

toxicity

Density

close to

air

reactivity

Ease

use

Back-

ground

conc.

local

sources

Low

cost

Helium þþ þþ — þþ þ – þþ

Neon þþ þþ þþ þþ þþ þ þþ —

Carbon dioxide þþ – þ – þþ ——þþ

Nitrous oxide



– þ – þþ þþ

SF6 yþ– þþþþþþþ–

Freons R11, R12, R13 yþ– þþþ þþ

Freons R111 to R115 yþ— þþþ þþ

Halon BCF yþ— þþ þ þþ þ –

Halon R13B1 yþ— þþ þ þþ þ þ

Perfreons (PFT) þþ þþ ——þþþ þþþ

Note: þþVery good for that property; þGood; – Not so good; — Poor;



Is not combustible but

a strong oxidant at high concentration and temperature; yIs not combustible but decomposes in a

ﬂame, producing toxic chemicals.

Common Methods and Techniques 137

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