Malvern Instruments Spraytec, User Manual

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Multiple scattering
In cases where the particle concentration is high, the measurement process is
complicated by scattered light being re-scattered by other particles before it
reaches the detector. The Spraytec software can apply a patented ‘multiple
scattering’ algorithm to correct for this.
Background factors
The measured scattering signal is made up of these components:
.
An electronic background component.
.
An optical background component.
.
A scattering component.
The electronic and optical backgrounds are measured and automatically
subtracted from the total signal to leave the scattering pattern that is caused by the
presence of spray in the beam.
The two background components are measured as follows:
.
Electronic background (noise) – the laser beam is automatically switched
off by the software to make this measurement, termed the
Dark back-
ground measurement. At this point there is no transmission reading and
there should also be minimal light diffraction as no light should be able to
enter the sealed unit.
.
Optical background – this measures the laser power, the scattering caused
by contamination on the optics, the scattering caused by daylight or artifi-
cial illumination, and flare caused by the optical surfaces. This measure-
ment, termed the
Light background , is made when the laser beam is
switched on but no spray is present.
'
N o t e
. Excessive dirt on the optics degrades the instrument’s sensitivity.
Chapter 8 gives instructions on cleaning the windows and advice on
how to keep them clean for an acceptable amount of time.
Avoid excessive background light, for example strong sunlight.
The light scattering components are:
.
Raw light scattering – this is the scattering signal from the detector with
the dark background subtracted from it. This pattern can be viewed at any
time, regardless of the instrument status.
C H A P T E R 2
S p r a y t e c
P a g e 2 . 5