A Practical Guide to Particle Counting for Drinking Water Treatment - Chapter 16 potx

147

CHAPTER

16
Particle Sensor Construction

All the manufacturers of drinking water particle counters provide particle sensors
quite similar in operation and construction. The main variation to be found is in the
size of the flow cell cross-section, and in the mounting of the sensor with regard to
sample taps and access for cleaning. The former will mainly determine the sensitivity
resolution and coincidence characteristics, while the latter impacts maintenance and
potential for damage due to leaks.

A. FLOW CELL

Most of the manufacturers except for Met One provide a standard-type flow cell
with synthetic sapphire windows and a stainless steel or aluminum flow cell. Met
One has a sensor of this construction as well, but it is no longer supplied for drinking
water applications unless specifically requested.

1. Met One

The Met One drinking water particle sensors employ a replaceable quartz cell
mounted in a plastic mechanical assembly. The quartz flow cell must be replaceable
because the quartz is more susceptible to scratching than synthetic sapphire. There
is no advantage to this type of design outside of lower cost of manufacture, which
is important in that it helps to lower the cost of particle counting for the end user.
This quartz/plastic flow cell does not have the same structural integrity of the
metal/sapphire units, and is restricted to lower amounts of sample pressure. In this

case, no more than 100 PSI is to be applied, and for less than 1 min. In most cases,
this is many times in excess of what is required. The standard constant-head overflow
weir provides an open system that limits the pressure to a few PSI. Closed systems
should not be used because of the difficulty in maintaining constant pressure, and
could potentially exceed the pressure limits, especially if severe water hammering

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148 A PRACTICAL GUIDE TO PARTICLE COUNTING

were to occur. Pressurized washing to clear clogs could exceed 100 PSI, so care
should be exercised. It is probably better to remove the flow cell in these cases, to
avoid ruining the whole assembly.
Replacing the flow cell does leave the door open to potential problems. Care
must be taken to follow the instructions carefully. If fingerprints are left on the flow
cell, the optical integrity will be altered, affecting the sizing accuracy of the sensor.
If the mechanical alignment is altered or loosened, the calibration will be affected,
and leaks could occur. The directions for replacing the flow cell are not complicated,
but if the person performing the task is not aware of the sensitivity of the instrument,
carelessness could lead to problems. The sensor should be recalibrated when the
flow cell has been replaced. See Figure 16.1.
The flow path for the Met One sensors is 750 µm

×

750 µm. This is the same
size as the stainless steel/sapphire sensors provided with earlier Met One particle
counters and still available as an option. All Met One sensors are volumetric.

2. Chemtrac

The Chemtrac particle counters employ a nituff-coated aluminum flow cell. The
nituff coating provides a tougher, wear-proof finish than standard black anodizing.
The flow path dimensions are 1 mm

×

1 mm (1 mm = 1000 µm) and the sensors
are volumetric.

Figure 16.1

Met One removable flow cell. (Courtesy of Pacific Scientific Instruments, Grants
Pass, OR.)

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PARTICLE SENSOR CONSTRUCTION 149

3. ARTI

ARTI offers two sensors for drinking water application. Each employs a stan-
dard aluminum flow cell housing with a monolithic fused silica flow cell with an
antireflective coating. The WPC 2000 is designed for 2 µm sensitivity, and has an
800 µm

×

800 µm flow path. The WPC 1000, a 1 µm sensor, has a 600 µm

×

600
µm flow path.

4. IBR

The IBR sensor is available in two concentrations. Higher concentration limits
are obtained by shrinking the flow path dimensions from 400 µm

×

400 µm to 150
µm

×

1000 µm.

B. CELL WINDOWS

The Chemtrac and IBR sensors come with synthetic sapphire windows, as well
as the older Met One sensors. The Met One drinking water sensor employs a quartz
flow cell as described above. This flow cell is a single bored piece of quartz, which
is field replaceable. ARTI uses fused silica as opposed to synthetic sapphire.

C. SAMPLE FITTINGS

Sample fittings vary among the different manufacturers, with the simplest being
the nylon barb fittings found on the Chemtrac particle counters. Since the Chemtrac
flow connections are located outside the NEMA enclosure, the type of fitting
employed is less critical, because leaks will not cause any damage or electrical
hazards. The Chemtrac sensor uses standard NPT threads for mounting these fittings,
so they can be easily replaced or changed to a different type if desired.
Met One also uses barb fittings on the sensor, which is located inside the NEMA
enclosure. Short lengths of tubing are then run from the sensor inlet and outlet to
nylon quick-disconnect fittings mounted on the walls of the NEMA box. The external
sample tubing is then connected to these fittings.
IBR uses threaded stainless steel fittings designed for higher pressures. This
sensor was designed for other applications requiring pressures higher than those
found in drinking water plants. The same fittings are used on the older Met One
sensors designed for the same purpose.
ARTI employs nylon twist-lock fittings for smooth internal flow passages
designed to prevent particle buildup.
All of the sample fittings are designed for 1/4 inch outside diameter (or 6 mm
metric) tubing.

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150 A PRACTICAL GUIDE TO PARTICLE COUNTING

D. LASER/OPTICAL ASSEMBLY

All of the manufacturers employ a similar method of mounting the laser/optical
assembly to the flow cell. Mechanical configurations vary, as well as the method of
alignment during calibration. This assembly is not user serviceable, and the design
features are not made public.

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