Table of Contents
Advertisement
where k is the spatial decay constant and ω is the frequency of the light source. This equation applies for
large particles in the geometric scattering region.
(Note: Because of the dependencies on both angle and wavelength, varied incident angles and
wavelengths can provide even more of a complete picture of the distribution.)
To complete the picture for the particle size distribution, parSYNC® also incorporates an ionization sensor.
7.4.3
Ionization
Ionization sensors are far more sensitive to ultra-fine particulates than opacity and scattering sensors.
Ionization sensors have been an important part of emission analyses (see diffusion charging sensors).
If we assume that the particles in the air are of uniform size, then the diffusion motion of ions are shown
to have a linear response to the particle surface area concentration detected in the sample. If not, then
an approximation typically is be made over the range of particle sizes; typically, the models and
experimental data show that particles' sizes would be in a log-normal distribution. For ease of use,
virtually all ionization sensors have compensators to make their response linearly proportional to the
particle size.
Specifically, the following system of equations govern the detector response:
Define
Where
And
I
= measured current in the detector
1
Define
Where
Define
Where
And
Then
y = x * (2 – x) / (1 – x)
And
Where
And
When we plot the theoretical/mathematical responses of the three sensor types against particle diameter
in one graph (re-scaled each response to illustrate them all on the same graph, we get relative sensitivities
as shown in the next figure.
Note: Wavelengths (and angles) of incident light used in the illustrated calculations match the 3DATX
current sensors.
ΔI = I
– I
1
0
I
= original current in the detector
0
x = ΔI / I
0
ΔI and I
are defined above
0
N
= f
/ (18.62 * r
2
)
t
v
m
N
= total number of particles passing through the chamber
t
rm = mean particle radius
* rm
y = k * N
t
y = detector response
k = constant (determined empirically)
36
Advertisement
Table of Contents
