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Appendix C ELECTRICAL RESISTIVITY BACKGROUND The committee's Panel on Dust Test Equipment reviewed two reports describing equipment used to determine the electrical resistiv~ty of combustible dusts: (1) a paper on the measurement of electrical conductivity and resistivity of dust samples prepared by S. H. Chiang in 1976 and submitted to the committee in 1977 and (2) a Recommended Practice of the Instrument Society of America (ISA)~1973~. The method proposed is essentially the same as the ISA method except that a direct-current rather than an alternating-current power supply is used to reduce the likelihood of instrument error and reactance effects. RECOMMENDED METHOD OF ME: ASUREMENT OF DUST RE SI STIVITY Scope The objective of this test method is to provide a convenient, albeit not precise, determination of the resistivity of dust layers for the purpose of ~ ~ The results are used to determine whether dusts nave res~st~v't~es similar to metallic dusts or to agricultural dusts; therefore, minor differences in measurement technique and experimental conditions are not likely to lead to differences in classification. Classitying dusts with regard to Dire and explosion nazaras. Sample Chamber The sample chamber should be of open construction with rectangular electrodes similar to the chamber described in the Instrument Society of America report (19731. This chamber uses stainless steel bars approximately 14 by 14 mm and 100 mm long, placed approximately 12.S mm apart (Figure C-1~. The exact dimensions of the cell are not crucial. -~ ~~ ~~ Nat r - ~~mm~n~l - ~1 h - a: - Theo are difficult to fill and clean. Smaller cells are A rectangular_ cell is recommended because it is simpler to construct than a cell made from concentric tubes and because calculations of resistivity can be made more easily. The open, rectangular cell construction also may be filled and cleaned with less difficulty. 43
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44 / ~ --L ~ ~ FIGURE C-1 Sample chamber. The electrodes should be mounted on a material so that the empty cell resistance will be higher, by at least a factor of 10, than that of the cell filled with sample. Any metallic mounting screws should be recessed or countersunk to avoid shunting effects of table tops, etc., during measure- ment. The dimensions of the cell are not standardized (i.e., they are left to the discretion of the laboratory making the resistivity determination). Direct-current measurement is recommended to avoid possible complications of capacitance effects. The measurement of resistance can be carried out with any convenient instrumentation suitable for resistance measurement in the range of 104 to 1011 ohms, depending upon specific cell dimensions. The method of resistance determination may be based either on voltage and current measurement or use of a do ohmmeter. If voltage and current are measured, the internal resistance of the voltmeter or ohmmeter must be considered in calculating the cell resistance. (Note the following exception. If metallic dusts with oxidized surfaces are being measured the voltage gradient applied to the cell should be higher than the highest voltage gradient to which the dusts may be exposed in use (see page 71~. Preparation of Dust Sample The sample should be dry sieved through a 200-mesh sieve to exclude large particles and should be representative of dust that may deposit on the surface of electrical apparatus. No drying or other pretreatment of the sample is required, however, if drying is necessary to facilitate sieving, the sample should be allowed to equilibrate at ambient humidity for at least 24 h. For thus purpose, the sample should be exposed in a thin (less than 6 mm) layer.
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45 Procedure The procedure involves the following steps: measured. measured. The resistance, Ro, of the empty sample chamber should be 2. The dust should be poured into the sample chamber and the sample chamber tapped several times to settle the dust. Excess dust should be removed by running a straightedge along the top of the sample chamber. If the dust does not remain in place at the ends of the sample cell, tape may be used to keep the dust in place. If tape is used, the measurement of Ro should be made with tape in place. 3. The resistance, Rs, of the filled sample chamber should be 4. If Ro is greater than 10 Rs, dust resistivity should be computed from the equation: ~ = R HW where H. W. and L are cell dimensions in cm (see Figure C-1), Rs is measured resistance of the sample-filled cell in ohms, and is resistivity of the dust sample in ohm-cm. 5. If Ro is less than 10 Rs, dust resistivity should be computed from the equation: p R Ro HW R -R L 0 s VALIDATION OF TEST METHOD Since there is no data base for the recommended test method for determining electrical resistivity of dust, two panel members constructed three different test cells and tested four dusts (cornstarch, powdered sugar, activated charcoal, and powdered graphite) under various conditions to validate the method. Except for some differences in treatment before the test, the dust samples were the same. The results obtained are given in Table C-1. The results of the recommended method might be considered subject to excessive error when compared to the well-controlled laboratory experiments. However, the intent of the testing is to be able to group dusts of similar resistivity. Data in ISA-S12.10 of the Instrument Society of America (1973) and the panel's tests do not indicate that the effect of moisture, aging, testing, test voltage, etc., are likely to be sufficiently great to cause misclassification of dusts.
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46 TABLE C-1 Resistivity of Test Dusts Under Various Conditions Dust Resistivity, ohm-cm Test Cell 1 Test Cell 2 Test Cell 3 As Received Cornstarcha 2 x 106 2 x 1011 1011 _ 1ol2 Powdered sugarb 6 x 1014 lol4 1012 _ loll Activated charcoals 1.2 x 104 ln3 g Graphited 10~ g Dry Cornstarcha 4 x 1012 5 x 1012 g Powdered sugarb 8 x 1013 ol5 g Activated charcoals 8 x 103 105 g Graphited 2 x 103 4 x 103 g 2 x 103 104 Moisturef Cornstarcha 6 x 108 7 x 101° g Powdered sugarb 7 x 107 6 x 106 g Activated charcoals 6 x 103 7 x 103 g Graphited 6 x lO1 8 x 102 g NOTE: On Test Cell 3 only, voltage varied 10 to 1000 volts dc for cornstarch and powdered sugar. Test Cells 1 and 2, testing cornstarch and powdered sugar, used General Radio No. 1644A Megohm Bridge at 500 volts dc; Test Cells 1 and 2, testing activated charcoal and graphite, used Fluke digital ohmmeter. Test Cell 3, testing charcoal and graphite, used Triplett multimeter. a Argo pure cornstarch. b Domino confectioners 10-X powdered sugar." c Apache Chemicals No. 1599 carbon powder, activated decolonizing, Lot No. 10077. d Wickes Engineered Materials No. 205 lubricating graphite. e Dried in dessicator at least 24 h. f Stored over water in closed container at room temperature at least 24 h. g ~^ t t PCtP~ .
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47 REFERENCES Instrument Society of America, Area Classification in Hazardous Dust Locations, Report ISA-S12. 10, Pittsburgh, Pennsylvania, 1973. !
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Representative terms from entire chapter: