Optical particle counters

Efficiency, counting The proportion of particles in a volume or mass flow that are counted as they pass through the sensing element of an optical particle counter. 

Optical particle counter An optical-electronic instrument for measuring the numbei" of airborne particles in different size ranges. 

Zero count rate The number of counts recorded in unit time by an optical particle counter when a particle-free gas is passed through the measuring chamber. 

Optical particle counters provide information on the particles present in different size ranges. A beam of light is collimated and focused onto a measurement cell. Light impinging on a particle is scattered and reaches a photomultiplier tube and converted to an output proportional to particle size. Particle size distributions are computed by appropriate software. 

Optical particle counters, 13 150 Optical properties, See also Light properties... 

The number distribution of particles having diameters from 0.03-10 urn was measured twice daily on most weekdays with a Thermo Systems electrical aerosol analyzer (EAA) ( ) and a Royco Model 202 Optical Particle Counter (OPC). These measurements were performed at a site three miles southeast of the air... 

The size distribution of the particulate matter in the 0.01-5 ym size range is analyzed on line using an electrical mobility analyzer and an optical particle counter. Samples of particles having aerodynamic diameters between 0.05 and 4 ym are classified according to size using the Caltech low pressure cascade impactor. A number of analytical procedures have been used to determine the composition distribution in these particles. A discrete mode of particles is observed between 0.03 and 0.1 ym. The major components of these particles are volatile elements and soot. The composition of the fine particles varies substantially with combustor operating conditions. 

Note DMS differential mobility spectrometer, SMPS scanning mobility particle sizer, CPC condensation particle counter, TDMPS twin differential mobility particle sizer, DMPS differential mobility particle sizer, OPC optical particle counter, APS aerodynamic particle sizer, MAS mass aerosol spectrometer, LAS-X optical laser aerosol spectrometer, ELPI electrical low pressure impactor... 

Fig. 15.1. LCM concentrations during Filmix suspension preparation measured using optical particle counters. (Taken from ref. 717. See text for further discussion.)...

In addition, the effect of Filmix suspension age on the size distribution was determined by measuring the LCM size distribution at different times over 37 days, using the M65 optical particle counter. The results are displayed in Fig. 15.3. No change in the LCM size distribution with time (over at least one month) was evident (ref. 717). 

In summary, the size distribution of LCM in deionized water was measured using several analytical techniques. The Filmix agent was found to contain close to 1010 LCM/ml >0.1 pm, when measured using optical particle counters. A large majority ( 90%) of the LCM (i.e., Filmix coated-microbubbles/particles ) were smaller than 0.2 pm in diameter. 

Example 17.9 An optical particle counter samples an aerosol at a flow rate of 175 cm3/min into a sensitive volume of 0.01 cm. What is the implied maximum particle concentration (in particles per liter) that can be sampled ... 

Rearrangement of Eq. 17.16 shows that a saturation count will be reached by the counter where increasing particle challenge does not increase particle count. Since this saturation level is fairly low for most optical particle counters compared to typical aerosol concentrations, some sort of dilution system is usually required for field use. 

Optical particle counters, in the stream-scanning mode, have been used for many years to determine particulate contamination levels in liquids and in aerosols. 

Knollenberg and Veal [80] discuss operation, design and performance of optical counters in general and a review of extinction optical particle counters has been presented by Sommer [81] cit. 82). A comprehensive review of laser-based techniques for particle size measurement, covering both stream scanning and field scanning methods, contains 167 references [83]. 

Fig. 9.12 Schematic diagram of the Particle Sizing Systems Accusizer autodilution apparatus and optical particle counter.

Optical diameter Optical particle counter (OPC) Invert angular distribution of light scattering requires index of refraction -b... 

The most frequently used method for particle size distribution is based on an optical particle counter. Determination of monosize particles, flakes, and fibers is not accurate. In these cases either electron or optical microscopy are the most suitable techniques. 

ASTM F 661-92. Particle count and size distribution measurement in batch samples of filter evaluation using an optical particle counter. 

In this section, we briefly review three types of instruments, the optical particle counter, electrical aerosol classifier, and diffusion battery. These system.s are based on very dilTerent physical characteristics of the aerosols. The optical counters respond to signals from individual particles. The electrical analyzers depend on the measurement of a current carried by a slreaJTi of cbrnged aerosol particles. The ditfusion battery also depends on the behavior of particle clouds. The system often used to cover the size range from about 10 nm to 10 /jm is a combination of (a) the electrical analyzer up to about 0.2 jum and (b) the optical particle counter over the rest of the range. 

The limit of detection of an optical particle counter depends on instrument noise, Rayleigh scattering by the air molecules, and stray light resulting from imperfect optics. For commercial counters with an incandescent light source, the limit of resolution is about 0.3 /tm. Instruments with laser light sources can go down to about 70 nm. 

As a result of the complex aerodynamics of the particle beam and associated skimmers, the size distribution of the particles that reach the ion source differs significantly from that in the gas samples from outside the system. To relate the measured chemical compositions to the outside aerosol, it is necessary to correct for this effect. This can be accomplished in principle by determining the elficiency of transmission of particles from the exterior into the chamber. It is also possible to use data for particle size distributions measured outside the spectrometer to characterize the external aerosol. Because the particle size distribution measured with an optical particle counter does not correspond to the aerodynamic diameter, there will be some difficulties of interpretation. 

For the optical particle counter of Fig. 6.5, the sensing volume is 1.5 x 1.5 x mm. Determine the total energy scattered by the air molecules in the volume. Compare this with the scattering by single particles with diameters ranging from 0.05 to 5 / m and a refractive index of 1.5. Assume k = 0.5 m and express your answers in terms of the incident intensity. The temperature is 20 0,... 

Figure 10.10 Evolution of the size distribution as the nozzle concentration of DBP.. ro increased. Lines are dts to data measured with an electrostatic classifler and an optical particle counter on the jet centerline at 20 nozzle diameters. The area under each curve is proportional to the total particle volume per volume of gas. At low vapor concentrations (.to 2.5 x lO"" ), most of the aerosol ma.ss is in 1- to 3-/rm particles, which form by nudeation in the shear layer and grown by condensation from the gas. At slightly higher vapor concentrations (jtq = 3.1 x 10 ), a submicron mode breaks out, the sign of the onset of particle formation downstream from the shear layer, At intermediate vapor concentrations (zq = 4,3x 10 ), the smaller mode grows. At the highest vapor concentrations (.ro = 4.9 X 10 ), a third mode of larger particles appears due to coagulation of the other modes.

Quantitative analysis of FTIR spectra series recorded during AIDA expansion cooling experiments allows for deriving time profiles of the number concentration of the ice crystals as well as the ice water content (IWC). This provides a unique possibility to validate the independent measurements of these quantities with the optical particle counter as well as the FISH and TDL instruments, as demonstrated on the right side of Figure 4. The upper panel... 

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