O
PERATION
M
ANUAL
VKL
10
/100
V
ERSION
V005-06-2020
20
6.4. Impact of Pressure and Temperature
The dilution stages are constructed at the plant in such a way that the dilution factors
indicated in the calibration certificate are only reached, if:
- the pressure inside the mixing chamber is equal to the
pressure at the place of sampling,
- these two pressures are about 1013 hPa,
- the temperature of the dilution air is equal to the temperature of the aerosol to be
diluted,
- these two temperatures are about 20 C.
The previously described operating condition is, according to experience, the most frequent.
6.4.1. Pressure Impact
The impact of the pressure at the sampling point is described in detail in the publication (see
figure 1 with pressure P1, equations 5 and 6, figure 5).
Equation 5 demonstrates that, with increasing underpressure dP0, the impact of the
pressure difference (P1 - P0) decreases. Consequently, higher pre-pressures should possibly
be used, if the pressure conditions at the sampling point are unstable (but: remember the
isokinetic conditions).
6.4.2. Temperature Impact
The temperature of a gas influences its density. It follows that with a stable operating
volume flow rate, the mass flow rate of a gas changes with the temperature. In addition,
according to the Bernoulli equation (dP = 1/2 * rho * v²), the inlet flow rate depends directly
on the gas density. Therefore, the gas temperature influences the dilution factor in two
ways. An estimation of this influence under different operating conditions can be given by
the manufacturer upon demand.