O
PERATION
M
ANUAL
VKL
10
/100
V
ERSION
V005-06-2020
10
4.3. The Dilution Factor
The dilution factor W describes the ratio between the total volume flow rate V
ges
and the
inlet volume flow rate V
an:
W = V
ges
/ V
an
with:
V
ges
= V
r
+ V
an
(V
r
: clean air flow rate)
The enclosed publication describes this equation in more detail. It demonstrates in theory
and practice that the dilution ratio remains constant, independently of the pre-pressure,
within a large range. The equations 1 - 4 and diagram 4 describe the correlations.
4.4. The Outlet Nozzle and Isokinetic Conditions
Since most particle measuring instruments have a sampling flow rate that is lower than the
flow rate of the diluted aerosol, a portion V
ab
of this flow has to be taken out of the mixing
chamber for the measurement. This is done via the outlet nozzle. When taking a portion of
the flow you must see to it that isokinetic conditions prevail. Isokinetic conditions are given,
if, at the suction location, the flow velocity inside the nozzle corresponds to the velocity
inside the flow channel. Different nozzle diameters permit the adaptation of different outlet
flow rates to different total flow rates. Following the isokinetic sampling of a part of the
flow, the remaining air flows to the outside via the excess air outlet. For aerosols, for
example, which constitute a danger to health, the excess air has to be led away via a tube. It
is absolutely necessary not to produce any counter-pressure during this process, because an
increase of the pressure P
O
inside the mixing chamber would influence the inlet flow rate
and thus the dilution ratio. See also equations 5 - 6 and diagram 5 of the publication.