A Practical Guide to Particle Counting for Drinking Water Treatment - Chapter 7 potx

81

CHAPTER

7
Particle Sensor Construction

A. FLOW CELL

The flow cell is a foundation piece of the particle counter sensor. It is generally
made out of aluminum or stainless steel or even plastic. Stainless steel is more
expensive primarily because it is much harder than aluminum and thus more difficult
to machine. Aluminum must be coated or specially anodized to prevent pitting when
exposed to water. The sample flows up through a channel formed by the flow cell
windows and the slit through the flow cell. The sample tube fittings are mounted
into the flow cell on both ends of the slit. In most cases, the laser/optics assembly
and the detector circuit are bolted to either side of the flow cell. See Figure 7.1.
The flow cell must be rigid enough to hold all these pieces together without
allowing any alteration in alignment, which would result in the particle counter
sensor losing calibration. The cell windows must fit tightly and be sealed to prevent
sample leakage.
The materials of construction are not important, as long as the functional integrity
is maintained. The ability to withstand repeated cleanings and clog removal is a
necessity.

B. CELL WINDOWS

The cell windows employed are usually round disks of synthetic sapphire with
flat, polished surfaces. These windows are seated on either side of flow cell slit and
sealed with O-rings to create the flow cell path. Quartz can be used for this type of

window as well, or can be drilled and polished to produce a single four-sided piece
that contains the flow cell path as well. The laser light beam is sent through the
center of the window through the flow cell path and then out the other side, where
it strikes the detector.

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82 A PRACTICAL GUIDE TO PARTICLE COUNTING

The cell window materials vary in optical quality. Small imperfections in the
material diffuse the light and are a source of resolution and sizing error. Quartz may
have fewer imperfections as far as optical quality is concerned. It is a softer material
than sapphire, and thus can be bored and polished to create the single window piece.
This softness accounts for the fact that it can be scratched from cleaning, or from
contact with particles over a long time period. Different grades of sapphire are
available. The entrance window (the side where the laser enters before passing through
the flow cell) must be of a higher grade than the exit window. The beam must be
tightly focused where it contacts the particles. Since the detector is measuring total
light energy, it is not greatly affected by some diffusion in the exit window.
The entrance window is covered with an antireflective coating to keep the light
energy from reflecting back into the laser. Lasers produce light by bouncing light
energy back and forth at a high frequency between mirrored surfaces, which amplifies
the light intensity to produce the desired output beam. If light is reflected back into
the laser, it can disrupt this process, and cause variations in the beam intensity. This
is another reason that aluminum flow cells are anodized a dark color, as the shiny
metallic surface can increase the problems with reflections. Stainless steel flow cells
cannot be coated in this way, and thus require finer alignment to minimize reflections.

C. SAMPLE FITTINGS

A variety of sample fittings are available, and are usually of concern only as a
matter of convenience or preference. Ease of accessibility is important. Plastic or
nylon fittings should be easily replaceable, as they will break periodically. Some
older sensors still in use have pressure fittings that can alter the optical alignment
if tightened too snugly.

Figure 7.1

Particle sensor. (Courtesy of L & H Environmental, Inc., Roseburg, OR.)
Power Regulator
Sensor Cover
Flow Cell Channel
Flow Cell Housing
Laser Diode Holder
Pin Photodiode
Collimation and Focus Optics
Sapphire Windows
780 nm Laser Diode
Fitting
Optic Holder
X,Y,Z Adjust
Spacer
Laser
Driver
Signal Amplification Board

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© 2001 by CRC Press LLC

PARTICLE SENSOR CONSTRUCTION 83

D. LASER/OPTICAL ASSEMBLY

The laser light source is passed through a series of lenses to produce a thin, even
beam across the flow cell. These lenses must be rigidly mounted to maintain the
proper alignment. This is usually accomplished by mounting the laser and other
optical lenses on a single block or in a tube. This “subassembly” is then bolted to
the flow cell.
The detector electronics assembly is mounted to the opposite side of the flow
cell. Once the entire assembly is in place, final adjustments are made to produce the
desired alignment.

L1306/frame/pt02 Page 83 Friday, June 23, 2000 1:54 PM
© 2001 by CRC Press LLC