Sensing Zone Techniques

Sensing-zone techniques have been applied to oil-in-water emulsions, although they cannot distinguish between dispersed droplets and solid particles [66], Unless an emulsion sample contains no particles, or is very well characterized, the results... 

Figure 2.9 Illustration of a sensing-zone technique. As the suspended particle or droplet passes through the sensing zone, the capacitance or resistance changes in proportion to the size of the particle. These signals can be sorted and interpreted as a size distribution in terms of equivalent spherical diameters.

A particle or droplet-sizing technique in which the flow of dispersed species in a capillary, between charged electrodes, causes changes in conductivity that are interpreted in terms of the sizes of the species. Coulter is the brand name for the automated counter. See also Sensing Zone Technique. 

An example is the Coulter counter. The term sensing zone technique is also used with reference to similar techniques that use light absorption or scattering instead of electrical properties, see Photozone Counter. 

II ASTM E 1772-95(1995) (1997), Particle size distribution of chromatography media by electrical sensing zone technique, 450... 

Electrical sensing zone technique commonly used to determine equivalent volume diameter, required in Eq. (19), might be problematic. The error associated with this technique is contributed by the breakup of aggregates and inclusion of pores in volume measurement. With this technique, an aggregate will have to be suspended in a liquid. The challenge is to preserve the structure of aggregates. Hence the first method is preferred to obtain the mass fractal dimension of aggregates in situ. 

The sensing-zone techniques are not limited to conductivity but may involve the measurement of capacitance. Figure 2 shows an example of the use of capacitance monitoring in a vertical sedimentation vessel. In this case (14) the effective suspension permittivity was measured and used to estimate the solids concentration (expressed in terms of volume fraction). 

Multiple techniques can be used to measure the particle size distribution, for example electrozone sensing, sedimentation, laser diffraction, and microscopy. With the exception of microscopy, they all require calibration and the results depend on the technique. For example, in a round-robin study reported in Reference 4, the commonly used electrical sensing zone technique (Coulter Counter) was compared to microscopy and sedimentation. The average particle size determined by the electrical sensing zone method was by about 25%... 

Number concentrations can be determined by sensing zone techniques (see Section 2.4.2), which provide an overall average number concentration. A condensation particle counter (CPC) can provide number concentrations for solid aerosols having particle sizes as small as a few nanometres in diameter [129]. In such an instrument, the aerosol is saturated with a vapour (usually water or an alcohol) and then taken to conditions of supersaturation. Vapour condensation on the particles produces droplets having diameters in the range 5-15 pm [124]... 

Electrical Sensing Zone Techniques (the Coulter Counter)... 

Besides for particle sizing, the optical sensing technique can also be employed for measurement of the particle number concentration. Discussions mentioned earlier regarding the measurement errors for the resistivity sensing zone technique apply here as well. When more than one particle is present in the sensing zone, error can occur. 

If the particle size distributions are characterized by the average diameters at the cumulative masses of 10, 50, and 90%, the data scattering measured by different techniques can be represented by the diameter ratios at the cumulative masses of 10, 50, and 90%, or DRio, DR50, and DR90. Since the particle diameter measured by the electrical sensing zone technique is to be the equivalent volume diameter and is independent of the particle shape, its particle diameter is defined to be one. The particle diameters measured by other techniques are then ratioed with this diameter. The experimental results of DRjo, DR50, and DR90 are summarized in Table 5. It can be seen that the results of particle analysis from different techniques can be quite different. 

A distinct advantage of the electrical sensing zone technique is that it allows setting up a mass balance for the measurement, since a volume related signal is measured with high resolution. The mass balance can be used for two purposes. The first and older one uses the mass balance to calculate appropriate calibration constants for those materials where latex calibration leads to systematic errors, such as porous and conducting particulates, the so-called mass calibration procedure. 

The work presented clearly indicates that the electrical sensing zone technique offers a good potential both for certification of reference materials and for analysis of materials such as pigments having a significant amount of sub-sized material, provided that a mass balance can be offered with high precision and accuracy. The target should be no systematic deviations in the mass balance with a precision in mass of 3X. 

One question which then arises is, What do the various particle analyzers measure From the electrical sensing zone technique, we get a number distribution, in spite of the fact that the method measures the volume of the individual particles. Prom the particle volume the instrument computes a volume equivalent particle diameter (see Chap. 2). The instrument then counts and reports the particles within a series of narrow (volume equivalent) diameter ranges. 

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