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15.2 Source Sampling 453

Aerosol particles Gas Stream
U
0
Sampling probe

U
s

Misalignment

Fig. 15.6 Misalignment in air sampling

alignment between the sampling probe and the bulk air flow direction. As seen in
Fig. 15.6, misalignment between the probe and the bulk air velocity may also lead
to sampling error.
Empirical equations were given by Durham and Lundgren [5] to quantify the
misalignment effect for anisokinetic sampling as follows:

0
0
g ¼ 1 þ cosh 1ð Þb Stk ; hÞ for R ¼ 1 ð15:10Þ
ð
s
0 0
where b Stk ; hð Þ is a function of Stokes number and the angle of misalignment,
1
0 0
b Stk ; hð Þ ¼ 1 ð15:11Þ
0
1 þ 0:55 Stk exp 0:25 Stkð 0 Þ
and
0
Stk ¼ Stk exp 0:022hð Þ for 0 h\p=2 ð15:12Þ
Since cosh\1, this equation indicates that misalignment of the probe always
results in the underestimation of the particle concentration.
Figure 15.7 is produced using Eq. (15.10) above for R ¼ 1. It shows that the
sampling efficiency is always less than unity when h 6¼ 0; again, the most dramatic
change in sampling efficiency corresponds to the range of 0:1\Stk\10.
When there are both misalignment and mismatch in air velocities, Eq. (15.13)
can be used for estimating the sampling efficiency.