Optical Particle Counter

:: Section 3

Components and Operational Principles of the OPC

The OPC contains a laser illuminated optical system which allows single particle sampling by collecting the scattered light from each particle with a solid state detector. Electronics provides amplification of the low level signals received from the photodetector and converts each scattered light pulse to a corresponding size category (depending on the pulse height), which is then accumulated in a data logger. Each scattered pulse corresponds to a particle count, and this is incremented in the appropriate size category to obtain particle concentration in a given size interval. There are three major sections in an OPC (i) the airflow system; (ii) the optical system, and (iii) the electronics system. The following animated schematic describes the operation of the instrument.

Figure 1:

i) Air Flow System

The airflow system in an OPC is used to isolate the particles from the optical components, and more importantly confine the particle stream to the boundaries of the laser beam. The aerosol stream is aerodynamically focused and this is done by means of a sheath airflow.

The inlets of the instrument are designed to minimize aerosol losses. Typically bends are avoided, especially as the OPC is used to sample relatively large size particles also. The aerosol flow is merged with the filtered sheath air flow, prior to its entering the optical sampling chamber. Clearly the aerosol sample flow must isokinetically match the sheath air flow while merging to avoid turbulence and avoid losses. Clearly, this is achieved by carefully control of the flow rates of these two streams, and is an important parameter. The flow rates also are important in determining the rate at which particles enter the optical view volume, and thus converting the particle counts to particle concentrations (# / cc)

(ii) Optical System

Typically a laser is used as the light source, and in many commercial instruments it is a He-Ne laser (wavelength of 632.8 nm). Optical components such as mirrors and lenses are the highest quality available to ensure minimal loss in signal. Particles that pass through the laser beam scatter light that is collected by the optics and is then directed to the photodiode. The collecting optics is typically a parabolic mirror with a very high reflectivity (> 95%) at 632.8 nm wavelength.

(iii) Electronics System

The photodetector electric signal is preamplified and pulse height measurement electronics classifies the signal into a size bin. Each pulse recorded increments the particle counter by one in the appropriate size bin. The display system increments the display in each channel on a real time basis as the sample aerosol flow continues to enter the measuring chamber. At the end of the run (set by the User as a sampling time), the full size distribution is displayed and the results can also be printed out. Each register can record a maximum number of particle counts, and this is also established by the maximum range selected by the User.

An important aspect of the OPC is that it is a single particle counting instrument, ie, a single particle is counted at a time. This is ensured by adjusting the flow rates (aerosol sample and sheath flow). However, if the concentration at the inlet is higher than a prescribed value (typically 10⁷ particles /cc), no selection of flow rates will ensure that only a single particle enters the optical viewing volume at a given instant. If multiple particles are present in the optical viewing volume (or scattering volume) at the same instant, the signal will be biased (both in terms of the size and the count). This is indicated by a overloaded signal in the display. The way to avoid this problem is then to dilute the aerosol prior to its being inlet into the instrument. Dilution sampling systems are typically used for measurements in combustion systems where concentrations of aerosols are high.