The resistivity of materials is the resistance to the flow of an electric current in a material, with some materials resisting the current flow more so than others. Static charge from any material depends on the resistivity of that material. The electrical resistivity of conducting the material is a measure of how strongly the material opposes the flow of the electric current passing through it.
This resistivity factor is sometimes called its “specific electrical resistance,” and it enables the resistance of different types of conductors to be compared to one another at a specific temperature according to their physical properties without regard to their lengths or cross-sectional areas. The resistivity of an insulator is determined by using a recognized voltage. To do so, you measure the resulting current and calculate its resistance using Ohm’s Law.
From the resistance measurement, the resistivity is calculated based on the physical dimensions of the test sample.
The resistivity is dependent on several factors:
- Applied voltage
- Electrification time
- Environmental conditions, such as humidity and temperature
To start with, resistivity is a function of the applied voltage and occasionally the voltage may be varied deliberately to know an insulator’s voltage dependence.
The resistivity sometimes also shows inconsistencies due to the length of the electrification period. The longer the time voltage is applied, the higher the resistivity, due to the fact that the material continues to charge exponentially. Therefore, choosing the right test parameters may need some pre-testing.
Environmental factors also have an effect on an insulator’s resistivity. Generally, the higher the humidity, the lesser the resistivity of a conductor. In making precise and correct comparisons to a specific test, the applied voltage, electrification time, and environmental conditions should be kept constant from one test to the next.
Conductivity, which is also called specific conductance, refers to the ease of which an electric current can flow through a material.
Conductance (G) is the reciprocal of resistance, i.e., (1/R). The unit of conductance is siemens (S) and is symbolized by the upside-down of the ohms symbol, i.e., mho ℧. Thus when a conductor has a conductance of 1 siemens (1S), it has resistance is 1 ohm (1Ω). So if its resistance is doubled, the conductance halves, and vice-versa, as siemens = 1/ohms, or ohms = 1/siemens.
While a conductor’s resistance gives the amount of opposition it offers to the flow of electric current, the conductance of a conductor indicates the ease by which it allows electric currents to flow. So metals such as copper, aluminum, or silver have an enormous value of conductance, meaning that they are good conductors.
Surface resistivity is defined to be the electrical resistance on the surface of the insulating material. It is measured with the use of an electrode to an electrode along the surface of the insulator sample.
This surface resistance meter 9v is most times a lightweight and moveable device, which is simple to make use of and gives precise and accurate results every time it is made use of, given that all things are equal.
- Measuring surface material is simply done by placing the meter on the area to be tested and pressing the test button
- For measuring resistivity to the ground, place the ground lead into the earth leakage socket, connect the crocodile clip to the area to be tested and press the test button
The surface resistivity meter will test conductive, static dissipative, and insulative surfaces to give ohms per square readings as follows:
- Conductive is indicated from 103 -105
- Static dissipative from 106 – 1011
- Insulative from 1012 +
The Digital Surface Resistance Meter Kit is an instrument intended to measure the resistance point-to-point (Rtt) or surface to ground (Rtg).
Its test functions comprise of:
• Resistance measurement accuracy of ±10% (±20% accuracy for 1 x 1012 ohms and greater)
• Resistance range of 1 x 1012 ohms
• Under load voltages of 10 and 100 volts ±5%
• An electrification time of about 15 seconds. The Digital Surface Resistance Meter also measures ambient temperature and relative humidity. Up to 100 measurements may be saved and recollected from the resistance meter’s internal memory. This includes the resistance value, relative humidity, temperature, and test voltage at the time of the measurement.
In Measuring Resistance Point-to-Point (Rtt) on the surface of any material, it is important to take note of these important procedures:
• Do not clean the surface
• Remove from only items that can cause interference with the test from the surface
• ESD sensitive devices should also be removed
• Use two five-pound electrodes, placed in the most commonly used portion of the surface; about 10″ apart (2″ from any edge, 3″ from any groundable point)
• Press the button and hold test button until the measurement is displayed
• If the most used portion is not obvious, use two points near the center of the surface. If the measurement is above acceptable limits, clean surface and re-test to determine if the cause of failure is the insulative dirt layer or the ESD protective product. Note: Use an ESD cleaner containing no insulative silicone (i.e., Reztore™ anti-static Surface and Mat Cleaner or it kind)
Most preferable point electrodes that can be placed include:
• Most commonly used area of a surface
• Most worn area
• Center of surface
• Furthest area from a grounded point
If the surface has to be measured has sections, for instance (floor tiles, garment panels), put the electrodes on different sections for Rtt measurements. Clean the material’s face for test lab measurements, but do not clean the surface for materials that are already in use. Just clean and re-test the installed material if a failure occurs.
Alternatively, Keithley offers software to complete resistivity measurements on insulators using the alternating polarity technique. The kick triggers high resistivity measurement application and allows reliable measurement of volume and surface resistivity of insulators in accordance with relevant set standards. This application is intended for use with Keithley’s 6517B Electrometer/High Resistance Meter and the 8009 Resistivity test fixture.
Use this application to:
• Perform a step response to determine an electrification time appropriate to the materials’ time constant
• Observe resistivity dependency on temperature and relative humidity using optional 6517-TP and 6517-RH probes.
• Use the alternating polarity technique to eliminate inherent background currents for the most accurate resistivity measurements.
Below is the picture of a resistance meter used to measure the surface conductivity of an ESD floor.