A Practical Guide to Particle Counting for Drinking Water Treatment - Chapter 6 doc

In the case of the particle counter, this refers to the smallest particle size that can be reliably detected.. In the case of the particle counter, noise is produced by the electronic de

PART II Understanding the Technology

In this part of the book we turn our attention to a more in-depth look at particle counters, from the basic construction of the instrument to the features of the complete particle counting system Particle counters are presented generically in this part, while in Part III the actual manufacturer’s product offerings are presented The chapters for Parts II and III are designed to run in parallel, to allow for easy cross-reference

CHAPTER 6 Specifications

All instruments are designed for a specific task Specifications are the measure

of how well the instrument performs the various parts of that task Some specifica-tions are common to all types of instruments (dimensions, weight, power require-ments, etc.) and are easily understood Specialized instruments have specifications that are unique to the task for which they are designed In many cases, common specification terms such as resolution and sensitivity must be understood in relation

to the unique properties of the application

In this section, the primary specifications employed for evaluating particle counter performance are defined and discussed Understanding of these few speci-fication parameters is essential to making informed decisions about particle counters

If these are not clear at first reading, readers should refer back to this section as they continue through the book It will become evident that these specification parameters are interrelated, and will only be understood completely when the whole picture of particle counting is clear

A SENSITIVITY

Sensitivity is the smallest measurable amount that an instrument can detect In the case of the particle counter, this refers to the smallest particle size that can be reliably detected The general rule in the industry is that a particle can be “reliably” detected and measured if it produces a signal with a minimum 2 to 1 signal-to-noise ratio

B SIGNAL-TO-NOISE RATIO

All electronic instruments are subject to various forms of “noise,” or random, meaningless electrical signals The term noise comes from the analogy to sound levels in a given space Noise is meaningless sound, and every space where sound can exist will have some level of noise present Our ability to discern specific sounds L1306/frame/pt02 Page 75 Friday, June 23, 2000 1:54 PM

76 A PRACTICAL GUIDE TO PARTICLE COUNTING

depends upon those sounds being at a higher level than the noise present The amount

by which the specific sound exceeds the noise level is called the signal-to-noise ratio The sound found between stations on a radio is a good example of electronic noise Even the highest-fidelity audio system produces some of this type of noise, which becomes audible as the volume is increased Every type of electronic device

is affected by this phenomenon In the case of the particle counter, noise is produced

by the electronic detector circuit, the laser light source, the power supply, and is induced by other devices The imperfections in the structure of the flow cell windows cause minute scattering and blurring of the laser light source The com-bination of all these factors results in a noise level that can be observed and measured with an oscilloscope (An oscilloscope displays electronic signals in a manner similar to a television.) See Figure 6.1 for a representation of noise as displayed on an oscilloscope

This noise level is measured with no particles in the water flowing through the particle counter sensor (This is not exactly true There are millions of particles well below the detection limit of the particle counter sensor Some of these contribute to the noise level in a small way.) Once the noise level has been determined, particles

of a known size can be sent through the particle counter sensor, and measured The resulting amplitude produced by the particles is compared with the noise amplitude

to determine the signal-to-noise ratio See Figure 6.2

Figure 6.1 Noise on oscilloscope.

SPECIFICATIONS 77

As mentioned is Section A, the smallest particle that can be reliably measured must produce a signal amplitude at least twice that of the noise, or a 2 to 1 signal-to-noise ratio

C RESOLUTION

This term refers to the degree to which an instrument can distinguish between differences in the object of measurement In particle counting, resolution refers to the accuracy with which the sensor can distinguish and measure differences in particle size This is stated in terms of percentage A (+ or –) 10% resolution error would mean that particles ranging in size from 4.5 to 5.5 µm could be measured as

5 µm in size

Resolution measurement is a somewhat sticky subject in the particle counter industry It will be discussed in further detail in relation to calibration and other issues

D COINCIDENCE

Coincidence was discussed in Chapter 5 in relation to sample dilution Basically,

it is the “coincidence” of two or more particles in the sensor view volume at the same time, which is a function of the concentration of particles in the sample There are other factors that affect this, but they are beyond the scope of this book

Coin-Figure 6.2 Pulse signal-to-noise measurement.

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78 A PRACTICAL GUIDE TO PARTICLE COUNTING

cidence is usually presented as the number of particles for which the measured counts fall within 10% of the actual number of particles present This number is the total number of particles per milliliter that are measurable by the sensor

E SIZING RANGE

This measure may appear under various names, but it is basically the range of measurable particles that can be sized and counted by the particle counter It extends from the minimum size particle (usually 2 µm) up to 400 µm or so The upper limit

is generally not too important, as most particles of interest will fall well below 50 µm

F SAMPLE FLOW RANGE

All particle counter data must be collected for a known volume of sample, which usually means the flow rate must remain constant or be measured continuously The specified sample flow range refers to the flow rates at which the particle counter sensor can measure particle size accurately If the particles are flowing through the sensor at too high or too low a velocity, they will not be sized properly by the detector electronics The reasons for this are covered in Chapter 7

Figure 6.3 Resolution error as a function of cell width.

Cell Illumination

SPECIFICATIONS 79

G FLOW CELL DIMENSIONS

The flow cell is the pathway through which the sample must flow A larger flow cell will clog less often and be easier to maintain On the other hand, a smaller flow cell will permit a higher concentration sample to be measured with less coincidence error A trade-off is necessary

Resolution is also a function of flow cell size The laser light source will not maintain an even intensity across the entire flow cell, and this error grows with the width of the flow cell Particles passing through the edges of the beam will not produce the same amplitude pulse as particles of the same size passing through the center See Figure 6.3

H VOLUMETRIC

The term volumetric refers to the coverage area of the laser light source If the beam covers the entire width of the flow cell, the sensor is volumetric If only a fraction of the width is covered, then it is non-volumetric, which is sometimes called

in situ An in situ arrangement allows for a much higher concentration of particles

to be measured because it reduces the effective volume of the flow cell The drawback

is that resolution is diminished because particles may partially pass through the beam and thus be undersized Figure 6.4 provides an illustration of this

Figure 6.4 Volumetric and in situ sensors.

Volumetric

In- Situ

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