How to quantify the resistivity of the material when the iron chain at the rear of the tanker vanishes?

If you have carefully observed the fuel tanker, you will find that a chain is usually installed at the rear of the fuel tanker, but have you ever thought about the meaning of its existence? Could this chain be the same as the music that plays when the water truck passes by, just to remind us that the tanker is coming?

In fact, it is not. As an exclusive vehicle for transporting petroleum derivatives, many types of fuel such as gasoline and kerosene are loaded in its metal body. Although the staff has reduced the possibility of fuel combustion from various aspects such as controlling the oxygen content in the car, but in the entire transportation process, once encountering uneven terrain or congested traffic, it is bound to cause changes in speed, which in turn causes The sloshing of fuel leads to constant friction and collision between the fuel and the wall of the car, which will fill the fuel tank with positive or negative charges. However, the tire material of the car is made of rubber. The rubber itself is a bad conductor and has an excessive resistivity. When a voltage is applied to the rubber, the electrons inside it cannot move smoothly, so the charge cannot be exported in time, and the charge continues to flow. Aggregation will generate static electricity, and when a certain electrostatic potential is reached, it will cause the discharge of electric charges, which is what we often call electric sparks, which is very dangerous for flammable things like fuel, even sporadic sparks , it can also ignite the tanker and cause an explosion, resulting in unpredictable losses and casualties.

　　Why is the importance of iron chains in safety gone?

So it is precisely to prevent such risks, people tie an iron chain behind the car, just like a “ground wire”. The purpose of this chain is to:

Conduct the static electricity induced by the oil tank away to avoid flashover with external substances;

Keep the tank truck and the entire oil loading and unloading equipment at the same potential to prevent potential difference;

·It has the effect of accelerating the leakage of the electric charge in the oil.

This shows the importance of dragging a chain behind the tanker! So why are there no chains behind the fuel tankers we see on the road now?

In fact, it is because the current tire rubber is doped with a certain concentration of carbon black. An important application of carbon black in the plastics industry is electrical conductivity. By controlling the amount of carbon black added, polymers with different electrical conductivity can be obtained. After adding a certain amount of carbon black, the resistivity of these materials can be reduced to below 100Ω*cm, so that the high-resistance polymer material becomes a material with semi-conductive properties or anti-static shielding properties, which can be used to produce high-voltage cables. conductive shielding materials, tire rubber for oil tankers and anti-static packaging materials for making high-precision Electronic products.

　　How to quantify the resistivity of a material?

So how should we quantify the resistivity of a material? There are two basic methods of resistivity determination: volume and surface resistivity.

　　Volume resistivity measurement

The volume resistivity measurement is generally completed using the test fixture shown in Figure 1 below. The voltage source V-Source is applied to the upper electrode to test the small current I flowing through the test sample, and then the volume resistivity is calculated by the formula. where, = volume resistivity, = effective area of ​​electrode contact, = sample thickness, R = V/I (resistance ratio of applied V-Source and tested current value).

Figure 1 Test Fixture

　　Surface Resistivity Test

The surface resistivity measurement generally adopts the electrode configuration shown in Figure 2 below. The voltage source V-Source is applied to the ring electrode, and then the current on the surface of the sample flowing from the guard electrode to the main electrode is tested, that is, the surface current I. The surface resistivity was then calculated using the formula. Among them, = surface resistivity, R = V/I (resistance ratio of applied V-Source and tested current value), P = effective circumference of guard electrode (mm), = distance between guard electrode and ring electrode (mm) .

Figure 2 Electrode configuration

　　Complex polymer materials and ultra-high resistance testing

What test measurement problems arise when the material we are testing is not a homogeneous material, but a rubber-doped carbon black polymer as described above?

We will find that as the +V applied to the sample continues for a period of time, the carbon black particles in the polymer will gradually form a conductive path that is more conducive to the flow of current, resulting in a gradual increase in the test current, which is obviously It is not the resistivity characteristics of the material itself that we want to truly characterize. At this time, we only need to reverse the +V at both ends of the sample to -V to destroy the established guide point path, and then perform the average test and measurement. (Take the volume resistivity test as an example, see the diagram below)

When testing some complex polymer materials or ultra-high resistance and resistivity, in order to improve the repeatability of the test results, we will use a method called alternating polarity measurement method (alternating polarity measurement method). The actual test process is as follows, apply a positive voltage +V, wait for the specified time (wait for 15 seconds in the figure below), and then measure the current; then, apply a negative voltage of the same magnitude -V, wait for the same time (15 seconds) , and perform another current measurement.

　Keithley’s Complete High Resistivity Measurement Solution

High resistivity measurements can be performed using the method specified in standard ASTM D-257 “DC Resistance or Conductivity of Insulating Materials” using the following kit: 6517B Electrometer/High Resistance Meter + 8009 Fixture Box + KickStart HRMA Test Software.

The Keithley 6517B Electrometer/High Resistance Meter is the standard for sensitive measurements in research laboratories around the world. With over 60 years of expertise in weak current measurement, Keithley electrometers can reliably measure currents down to 10 aA (10×10-18A), charges down to 1 fC, and support resistance measurements up to 1018Ω. The 6517B is also capable of measuring a maximum voltage range up to 200 V with input impedance exceeding 200 TΩ. All of these performances are combined in one instrument that is as easy to operate as a digital multimeter.

The 8009 Resistivity Test Fixture complies with the American Society for Testing and Materials (ASTM) D257 Standard Test Methods for DC Resistance or Conductivity of Insulating Materials. Combined with the 6517B, the 8009 provides a complete system for making high quality and safe resistivity measurements. The 8009 comes with 6517B-ILC-3 Safety Interlock Cable, 7078-TRX-3 Triax-to-Triax Cable, and 8607 1 kV Supply Voltage Banana Jack Bundle.

The KickStart High Resistivity Application controls the electrometer and test fixture to perform the required measurements, making ASTM-D-257 standard resistivity measurements. It can test materials at voltages up to 1000V, determine resistivity up to 1018 Ω-cm, analyze step response plots of current versus time, determine how long it takes for the measurement to stabilize, analyze multiple reading plots, Ensure stable and consistent measurements. The KickStart high resistance application uses swapped electrode technology to eliminate the inherent background current for the most accurate resistivity measurements. You can also use this application to observe the correlation of resistivity to temperature and relative humidity using an optional thermocouple and relative humidity probe.