This fixture is intended to be used with an ordinary hand-held multimeter to quickly and conveniently perform simple volume resistivity and "surface-" or "sheet"-resistance measurements on specimens of tapes, films, composites, fabrics, and other thin, flat materials ranging up to about 5mm (0.2in) thick and 3.5cm (1.5in) wide. The unguarded electrode arrangement is consistent with the specifications of ASTM D257 FIG 3, ASTM D4496, DIN-IEC 60167 and 60093, and IEC 62631-3-3.
The extent to which measurements made with the fixture can be certified to be compliant with these standards depends upon the resistance range of interest. Materials sold or used by coaters, laminators, convertors, and other suppliers of tapes and membranous materials for electrical and electronic applications are typically categorized as conductive, 10-10^5 ohm·cm, static dissipative, 10^5-10^12 ohm·cm, and insulative, >10^12 ohm·cm. Standard multimeters, such as the Fluke Model 114 we offer, have a usable range of 10 to about 5x10^7 ohms, with an accuracy of ±2%, and so may be used with our fixture for certifying products in the first two categories but not the third, insulative materials. An exception may have to be taken regarding the voltage applied to make the measurement as well. ASTM D 247, for example, calls for the applied voltage to be 500V. The report must indicate the measurement was made instead using an applied voltage of 9V, typical for most battery-powered multimeters.
The electrodes are stacked pairs of brass bars one-half-inch thick and one-inch wide spaced one inch apart. The specimen is clamped between them by means of plastic knobs with thru brass inserts rotating on 1/4-20 stainless-steel studs passing through holes in the upper electrodes. Four 7/16" bronze hex nuts are also supplied should the user wish to employ a wrench or socket to accomplish the clamping. A 20 in-lb torque-limiting wrench, the use of which ensures the clamping force is consistent test-to-test and operator-to-operator, is available at small additional cost. When the fixture is open the upper electrodes are supported by springs that maintain clearance between the electrodes ample for easy insertion of specimens. The insulating polycarbonate base of the assembly is mounted on a hardwood sub-base to facilitate holding the device firmly when clamping the specimens. Two standard banana-jack leads to connect the binding posts on the device to the terminals of the multimeter are furnished.
The fundamental geometry of the fixture, clearly exhibiting the principle of the measurements and the rationale for the calculations, renders it particularly useful as a teaching or training tool.
The volume resistivity measurement is performed by clamping a specimen of uniform width and thickness between the electrodes and observing the resistance indicated by the multimeter after one minute. The volume resistivity, ρ, in units of ohm·cm (Ω·cm), is calculated,using the formula ρ = R·t·w/2.54, where R is the measured resistance in ohms, t is the average thickness of the specimen in centimeters (cm), and w is the width of the specimen in centimeters. The 2.54 in the denominator is the distance between the electrodes in this fixture in centimeters, symbolized by l in the general formula. Although the terminology and symbols used may differ, this calculation, taken from ASTM D257 Standard Test Method for D-C Resistance or Conductance of Insulating Materials (http://www-eng.lbl.gov/~shuman/NEXT/CURRENT_DESIGN/TP/MATERIALS/ASTM-D257_resistance_meas.pdf) is essentially that shown in all the standards listed above.
The surface resistance or sheet resistance measurement is performed similarly except that after the specimen, which for ease of calculation is most conveniently 2.54cm (1.00in) wide, is placed in the fixture, a 250µm (10mil)-thick sheet of PTFE-coated glass fabric (furnished) is placed upon it, as shown in image 4 above. When the resulting PTFE-specimen sandwich is clamped in the fixture the uppermost face of the specimen is thereby insulated from the upper electrodes, and the resistance indicated by the multimeter is that of the face in contact with the lower electrodes only. The resistance indicated after one minute is reported as the surface resistance or sheet resistance of that face. No calculation is required when the specimen is 2.54cm (1.00in) wide. The value may be expressed in units of ohms (Ω) or "ohms per square" (Ω/sq), depending on individual preference or accepted practice within the user's organization. For homogeneous materials the surface resistances of both faces should be measured and the average reported. If the specimen is known or appears to be non-homogeneous face-to-face, the surface resistances of both faces should be measured and reported separately.
A copy of the procedure for measuring and calculating the volume-resistivity and surface resistance using the Keeselab® Volume Resistivity/Surface Resistance test fixture is available at the link below.
The extent to which measurements made with the fixture can be certified to be compliant with these standards depends upon the resistance range of interest. Materials sold or used by coaters, laminators, convertors, and other suppliers of tapes and membranous materials for electrical and electronic applications are typically categorized as conductive, 10-10^5 ohm·cm, static dissipative, 10^5-10^12 ohm·cm, and insulative, >10^12 ohm·cm. Standard multimeters, such as the Fluke Model 114 we offer, have a usable range of 10 to about 5x10^7 ohms, with an accuracy of ±2%, and so may be used with our fixture for certifying products in the first two categories but not the third, insulative materials. An exception may have to be taken regarding the voltage applied to make the measurement as well. ASTM D 247, for example, calls for the applied voltage to be 500V. The report must indicate the measurement was made instead using an applied voltage of 9V, typical for most battery-powered multimeters.
The electrodes are stacked pairs of brass bars one-half-inch thick and one-inch wide spaced one inch apart. The specimen is clamped between them by means of plastic knobs with thru brass inserts rotating on 1/4-20 stainless-steel studs passing through holes in the upper electrodes. Four 7/16" bronze hex nuts are also supplied should the user wish to employ a wrench or socket to accomplish the clamping. A 20 in-lb torque-limiting wrench, the use of which ensures the clamping force is consistent test-to-test and operator-to-operator, is available at small additional cost. When the fixture is open the upper electrodes are supported by springs that maintain clearance between the electrodes ample for easy insertion of specimens. The insulating polycarbonate base of the assembly is mounted on a hardwood sub-base to facilitate holding the device firmly when clamping the specimens. Two standard banana-jack leads to connect the binding posts on the device to the terminals of the multimeter are furnished.
The fundamental geometry of the fixture, clearly exhibiting the principle of the measurements and the rationale for the calculations, renders it particularly useful as a teaching or training tool.
The volume resistivity measurement is performed by clamping a specimen of uniform width and thickness between the electrodes and observing the resistance indicated by the multimeter after one minute. The volume resistivity, ρ, in units of ohm·cm (Ω·cm), is calculated,using the formula ρ = R·t·w/2.54, where R is the measured resistance in ohms, t is the average thickness of the specimen in centimeters (cm), and w is the width of the specimen in centimeters. The 2.54 in the denominator is the distance between the electrodes in this fixture in centimeters, symbolized by l in the general formula. Although the terminology and symbols used may differ, this calculation, taken from ASTM D257 Standard Test Method for D-C Resistance or Conductance of Insulating Materials (http://www-eng.lbl.gov/~shuman/NEXT/CURRENT_DESIGN/TP/MATERIALS/ASTM-D257_resistance_meas.pdf) is essentially that shown in all the standards listed above.
The surface resistance or sheet resistance measurement is performed similarly except that after the specimen, which for ease of calculation is most conveniently 2.54cm (1.00in) wide, is placed in the fixture, a 250µm (10mil)-thick sheet of PTFE-coated glass fabric (furnished) is placed upon it, as shown in image 4 above. When the resulting PTFE-specimen sandwich is clamped in the fixture the uppermost face of the specimen is thereby insulated from the upper electrodes, and the resistance indicated by the multimeter is that of the face in contact with the lower electrodes only. The resistance indicated after one minute is reported as the surface resistance or sheet resistance of that face. No calculation is required when the specimen is 2.54cm (1.00in) wide. The value may be expressed in units of ohms (Ω) or "ohms per square" (Ω/sq), depending on individual preference or accepted practice within the user's organization. For homogeneous materials the surface resistances of both faces should be measured and the average reported. If the specimen is known or appears to be non-homogeneous face-to-face, the surface resistances of both faces should be measured and reported separately.
A copy of the procedure for measuring and calculating the volume-resistivity and surface resistance using the Keeselab® Volume Resistivity/Surface Resistance test fixture is available at the link below.
| measuring_volume_resistivity_or_surface-sheet_resistance-10-03-2021.pdf |