CLIMET INSTRUMENTS CI-154, User Manual, Page: 77 / 98

CI-154 User’s Manual
Page 67
4. Theory of operation
4.1 How particles are detected
As you enter an old barn, you see a shaft of bright light shining down from the
space between two of the boards that are part of the barn’s wall. A myriad of specks
of light dance about within that shaft of light. Some of the specks are very dim,
while others shine brightly. These points of light are specks of dust floating in the
air, and the largest specks of dust are the brightest points of light. In fact, the shaft
of light is only visible because millions of specs of dust collectively reflect the
sunlight, like drops of water in a fog.
What we can remember seeing in an old barn is, in essence, how a particle counter
works. A beam of laser light replaces the shaft of light from the sun. The darkness of
the barn is replaced by the darkness of a sensor cavity. But the
dust that the
particle counter sensor detects is much smaller, too small for the human eye to see.
To provide a little perspective, human hair has a diameter of about 80
 m, and the
human eye can only resolve objects down to 50  m. The range of sizes that a
particle counter detects is as small as 0.1  m, and usually less than 25  m.
Lenses or mirrors collect the light scattered by the microscopic particles, and the
light is reflected onto a semiconductor photodetector. The photodetector turns the
light energy into an electrical current that is proportional to the brightness of the
light. Since the light was proportional to the size of the particle that scattered the
light, the amplitude of this electrical pulse is also proportional to the size of the
particle. We now have a device that not only detects the presence of something
invisible to our eyes, we also have information about the relative size of those
invisible particles.
To complete our particle counter, we must add specialized electronic circuits.
Amplifier circuits and filtering circuits amplify the signal and pull the smallest
particles out of the noise that would otherwise obscure them. A patented digital
processor compares the pulse amplitude to a voltage
threshold . If the amplitude
exceeds the voltage threshold for a known reference particle, the detected particle is
converted as greater than that size. Additional digital circuits provide us with the
means of displaying and printing these counts, and manipulating the data. We now
have a particle counter.
4.2 The systems of a particle counter
There are certain basic building blocks required to make a particle counter:
 An airflow system to bring the air sample into the particle counter at
a controlled rate,
 A light source to illuminate the particles in the air stream,
 An optical system to gather light scattered by the particles in the air
stream,
 Electronic systems to detect the light scatter, and to amplify,
measure, and count the resulting signal pulses.