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MICRONANOSYSTEMS
particles per millilitre at 50 nanometers starting in 2007 and
0.2 particles per millilitre at 50 nanometers by 2012.
Particle distributions in high purity water systems often follow
an inverse third power law relationship. Fig 1 shows a typical
particle distribution on a high purity water system. Because of
the power law relationship between particle counts and size it is
often best to plot it on a graph with a log-log scale.
Liquid optical particle counter technology
Optical particle counters measure particles individually one at a
time. The method of single particle detection used in monitoring
high purity water is referred to as light scattering. Here the
particle redirects light from a light source when the particle
intersects the light. This redirected or scattered light then causes
an increase in the light that hits a photodetector thereby creating
a signal for particle detection.
Light scattering is typically broken into two regimes referred
to as Mie scattering and Rayleigh scattering. Rayleigh
scattering describes scattering of small particles present in high
purity water. More specifically Rayleigh scattering describes light
scattered from small particles as compared to the wavelength of
light. Here the signal is proportional to the sixth power of the
particle diameter. Therefore the signal decreases dramatically
for particles of smaller sizes.
In light scattering liquid particle counters a laser light source The counter circuitry then counts and bins the voltage pulses
is focused on an optical flow cell that the fluid travels through. into discrete channels associated with the particle’s size through
As a particle in the fluid travels through the laser beam, it a calibration process. At the end of the sample time each bin
scatters light. The scattered light is then gathered by collection contains the number of particles for that particle size.
optics and relayed onto a photodetector. The photodetector in It should be noted that the light scatter signal is a function of
turn generates a current that is converted to a voltage and the shape of the particle. The signal is also proportional to the
output to the counter circuitry. The voltage output is in the form optical index of refraction of the particle divided by the optical
of a pulse, the amplitude of which is proportional to the size of index of refraction of the liquid. Therefore the signal changes
the particle. depending on the exact composition of the particle and the fluid
type being monitored. Liquid particle counters report an
equivalent size to a calibrated spherical particle (typically
polystyrene latex) in water and produce very reproducible
measurements in liquid applications, including high purity water
monitoring.
There are other specifications that define a liquid particle
counter’s performance. One parameter is referred to as the view
volume. It describes how much of the flow path is illuminated by
the laser and therefore what fraction of the flow is monitored by
the particle counter. Today there are two types of particle
counters for liquids. The first type of particle counter is a full
stream particle counter that has a view volume close to 100%
and measures particles throughout nearly the entire cross section
of the flow path. The second type of particle counter is a partial
stream particle counter which has a very low view volume and
counts particles confined to a small fraction of the cross section
of the flow path. Partial steam particle counters have a small
view volume to maximise sensitivity or the smallest size particle
they can detect. Today’s particle counters for monitoring high
purity water are partial stream sensors.
Another parameter to consider when selecting a liquid
particle counter is the flow rate which defines the rate of fluid
Fig 1. Typical distribution of particles in high purity water flow through the sensor. Higher flow rate particle counters can
www.micronanosystems.info July 2008
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