3.1 Visual study with visible smoke
In order to assess air change
rate the movement of air and the existence of poorly ventilated
areas, visible smoke can be released into the space. With the
ventilation system operating, the movement of air and the
dissipation of smoke can be studied and the air change rate
with tracer gas
By use of tracer gas it is
possible to estimate air change rate and air distribution in chosen
points in the ro-ro cargo space.
Measurement with tracer gas involves mixing a gaseous component
with the air. The atmosphere in the space is examined to determine
how dilution of the tracer gas is tracked at chosen points in the
ro-ro cargo space whilst the ventilation system is operational.
This method should be carried
out with and without vehicles.
3.2.1 Test procedures
The placing of the measurement
probes shall be chosen with regard to the purpose of the
measurement. The probes are not to be placed near to the supply air
terminal devices or at places where a so-called ventilation shadow
can be expected, such as behind pillars, webs etc. As a rule the
probes are placed at the head height and in the vicinity of persons
working on the deck.
gas should be spread and mixed in the air as completely as possible.
The mixing may be done by the ordinary ventilation plant or with
help of external fans. In order to reach an adequate accuracy, the
concentration of the tracer gas ought to reach at least 50 times the
detection limit of the analytical instrumentation.
When the tracer gas concentration is
adequate the ventilation plant as well as the measurement equipment
should be started. Tracer gas concentration should be recorded until
the detection level is reached.
With a dilution ventilation
system the logarithm of the concentration of tracer gas will be
linear with regard to time (see figure below).
The relation between the
concentration of tracer gas and time (the inclination of the graph)
is a straight measure of the effect to the ventilation expressed in
number of air changes according to the following formula:
N = (ln (Co/C1))/(t1 -
N = number of changes
Co = the
concentration at the beginning of the effective dilution
C1 = the concentration
at the end of the effective dilution
to = the point of time at the beginning of
the effective dilution
t1 = the point of time at the end of the effective
As an alternative to the
tests in paragraph 3.1 and 3.2, air flow distribution in the
ro-ro cargo space may be evaluated by use of an anemometer.