Page:NIOSH Manual of Analytical Methods - Chapter G.pdf/2

 formed from condensation or combustion processes are generally called fumes or smokes. Some of these aerosols have a significant vapor pressure and will evaporate when aged. The direct-reading photometer may detect these high vapor pressure aerosols while the reference method for respirable dust (Method 0600) will not.

2. PRINCIPLES OF OPERATION

Light-scattering aerosol monitors (also called nephelometers or aerosol photometers) operate by illuminating aerosol passing through a defined volume and detecting the total light scattered by all the particles in that volume (Figure 2). This discussion will not include single-particle counting photometers that are used to measure lower concentrations such as in clean rooms and give information about individual particles. However, there is an instrument listed below (Portable Dust Monitor from A. P. Buck, Inc.) that uses single-particle counting to estimate mass concentration.

The light source of a photometer can be monochromatic such as a light-emitting diode or laser or a broad-wavelength light source such as a tungsten filament lamp. The choice of light source in different instruments has more to do with the ability to control the light output level than with the wavelength of the output. The detector is generally a solid state photodiode but can be a photomultiplier tube. The detection geometry varies from one instrument to another. These instruments generally use a forward-scattering geometry (i.e., less than 90 ). The angle of scattering (theta) is defined with respect to the beam of light passing through the aerosol in the detection volume. The smaller the value of theta the more the detection is weighted toward larger particles.

The amount of light scattered by a particle into the detector is a complex function of the particle size, shape and refractive index. For spherical particles of known refractive index the instrument response can be calculated. However, in general, calibration must be carried out experimentally. An example of instrument response as a function of particle size for spherical particles of two different refractive indices is shown in Figure 1. It can be seen that there is a peak at approximately 0.6 mm and that there is a drop in instrument response to larger particles. For comparison, the ACGIH definition of respirable dust is superimposed on Figure 1 [6]. It can be seen that the size dependent response of the photometer is somewhat similar to the desired response for a respirable sampler.

For quantitative measurements, it is necessary to calibrate with an aerosol similar in refractive index and particle size to the one being measured. This is because aerosols with different refractive indices can produce photometer responses differing by more than a factor of ten. Since these instruments have specific size-dependent response to particles, the size distribution of dust particles is also important in evaluating the mass response of the instrument for a specific dust.

3. SAMPLING CONSIDERATIONS

a.

Some portable photometers have been designed for intrinsic safety, i.e., for use in potentially explosive atmospheres. This must be checked with the manufacturer to ensure that a specific instrument meets the appropriate intrinsic safety requirements (e.g., Underwriter's Laboratory or Mine Safety and Health Administration).

b.

Photometers generally cannot be used to discriminate between different types of aerosol. The instrument will respond to all types of aerosol present simultaneously in the detection volume. Therefore, measurement of a small amount of a specific aerosol in the presence of a large amount of interfering dust is not feasible with a photometer. For example, if lead fume must be measured in the presence of a large percentage of road dust, the photometer would not be the instrument of choice. However if lead fume were the major 1/15/98