ASTM D 991 TEST FIXTURE
Model 831
Operating Manual
7/08
1.0 GENERAL DESCRIPTION
The Model 831 D 991 Test Fixture, shown in Figure 1.0-1 is designed specifically to
test material in accordance with ASTM D 991 – RUBBER PROPERTY - VOLUME RESISTIVELY OF ELECTRICALLY CONDUCTIVE AND ANTISTATIC PRODUCTS.
Figure 1.0-1: Model 831 D 991 Test Fixture
This method is ud to evaluate the electrical behavior of rubber products (also applicable to other types of rigid and sheet material) that are ud in applications such as safety, static charge accumulation and dissipation, current transmission, etc. This test method is uful in predicting the behavior of such products having resistance up to approximately 100 megohms.
D 991 utilizes the measurement of current (i) through a material and the voltage
drop (V) across a ction of the material to calculate the volume resistivity in Ohms-
cm. It is designed for a standard 3”x5” (76x127mm) specimen, but can measure specimens from 0.4 - 4” (10 - 102mm) wide to 5-6” (127 -152mm) long.
The Fixture requires either a 4-pole resistance meter where the voltage source and Sen functions are contained within the same instrument such as the ETS Model 863-6487 Wide Range Resistance Meter, or parate adjustable voltage source and digital voltmeter. The voltage source is ud to apply a potential across both sides of the test specimen (A-A’ shown in Figure 1-2) causing current to flow through the specimen. The DPM is ud to measure the voltage drop across a ction of the
specimen (B-B’ shown in Figure 1-2). The Milliammeter is ud to measure the current from the voltage source.
Figure 1.0-2: Model 831 meter connections
Using the following calculation from the D 991 test method, the volume resistivity of the material can be determined:
ρv = Vwd
iL
where ρv = Volume resistively in Ohm-cm
V = Potential difference across potential electrodes (B-B’) I = Current through specimen (A-A’) w = width of specimen (7.62cm) d = Thickness of specimen (cm)
L = distance between potential electrodes (6.35cm)
When using a standard size sample the volume resistivity then becomes
ρv = 7.62Vd Ω-cm
6.35i
Voltage Source + -
DVM + - (Sen)
A
A’
B B’
The Fixture has a fixed mass bad on a 4.0” (7.62cm) specimen width.
Mass between current electrodes and specimen = 6.67 lb (3kg).
Mass between potential electrodes and specimen = 1.34 lb (0.6kg)
2.0 SET-UP
The Model 831 Test Fixture requires either a 4-pole resistance meter or individual instruments as shown in Figure 1.0-2. The red and black jumper cables connect the upper and lower electrodes together. All connections u standard 0.161 (4mm) banana plugs.
When possible sample size should be 3x5” (7.62x12.7cm)
3.0 TEST PROCEDURE
3.1 Characteristics of Static Dissipative and Conductive Material
Thermoformed plastics that are rendered static dissipative or conductive
consist of a plastic resin filler with very high resistance properties loaded
with a small percentage of a conductive material such as stainless steel
fibers, or carbon powder or fibers. The materials have bulk resistance
properties vers the surface only resistance properties found in other ESD
materials. When a voltage is applied either across or through the material
the dielectric of the filler breaks down and current flows from particle to
particle. As the loading of the conductive medium decreas there is greater
distance between particles that requires a higher voltage to break down the
incread dielectric. At some point, once a higher voltage is applied to
establish continuity the resistance of the path created may become altered
permanently. Loaded thermoplastic materials are effective in reducing the
upper resistance limit to approximately 108 Ohms.
Another characteristic associated with loaded thermoplastic materials that
affects measuring resistance is the microscopic insulative layer that
develops on the surface of the molded part. The dielectric of this layer must
be broken down before a resistance measurement can be made. Once this
occurs the actual resistance of the part may be lower than the measuring
range of the instrumentation ud.
In esnce, the materials are non-linear and voltage dependent. Hence,
different test voltages may give different results.
Loaded thermoelectric material is generally not adverly affected by
humidity, as long as it is reasonable such as less than 75%.
At prent, ESD materials are classified as follows:
Insulative
Conductive
Dissipative
Surface <104104 to <1011≥1011 Ohms
Volume same
Materials with bulk resistance characteristics can also be classified by
specifying its volume resistivity. Increasing or decreasing the thickness of
the material will change the measured resistance of the part with a specified
volume resistivity. This is a common technique ud in ESD products to
achieve a particular resistance. It is the actual resistance of the part, not its
resistivity that determines how a part dissipates a static charge.
3.2 Test Procedure
Measure the thickness and width of the test specimen in cm.
Place the test specimen in the Test Fixture. Verify that the electrodes are
making good contact with the specimen surfaces.
NOTE:
Molded plaques may not be sufficiently flat to ensure good electrode
contact. Application of additional pressure may alleviate this problem.
The less electrode contact the higher the measured resistance.
If the samples have identification marks the sheets shall be normal to the
calendar grain and shall not be in contact with, nor lie between the current
electrodes.
Adjust the current through the specimen after connection of the voltage
source so that the power dissipation in the specimen between the potential
electrodes is approximately 0.1 Watt. The following voltages should not be
exceeded for the maximum current specified.
Current-ma
Pontential-Volts
3 50
6 25
10 15
30 5
75 2
150 1
300 0.5
When the current has stabilized or after 5 conds, measure the potential
difference across the current electrodes to the nearest 1% of the respective
values.
Calculate the volume resistivity using the formula listed in Section 1.0.
