Accurate measurement of ultra-high resistance, resistivity and insulating materials

[Guide]Rubber, plastic, bakelite, etc., as insulating materials, are indispensable in the design of our Electronic and power products. But I wonder if you have thought about the resistance performance of the material you choose, how big is its resistance or resistivity under various working scenarios or temperature conditions, and whether it can meet the design requirements of the product?

As shown in the table below, the electrical resistivity of metals and alloys are very small; while the electrical resistivity of bakelite and rubber are both large. In the power supply and transmission lines, in order to reduce losses, low-resistivity materials such as copper and aluminum should be used to make wires; the outer insulating part should also be made of high-resistivity materials such as rubber.

Resistivity of several commonly used materials at 20℃

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Many engineers, when verifying the resistance characteristics of materials, may first think of using a digital multimeter. But even our highest-performance six-and-a-half-digit digital multimeter has the highest measuring range of resistance, only 1GΩ. But the insulating material sampled in our design is super high 1GΩ at will! Why is it so difficult to measure large resistances?

How to measure the resistance of TΩ or even PΩ?

Must be using Ohm’s law

Accurate measurement of ultra-high resistance, resistivity and insulating materials

When measuring resistance, it is usually excited by applying current to measure the voltage at the resistance end, and the resistance value can be obtained according to Ohm’s law, such as measuring a resistance of 100KΩ, and measuring 1V voltage under 10uA current excitation.

However, if the measured resistance of 0.1TΩ is still applied with 10uA current, how many volts is the voltage?

10 uA x 0.1TΩ = 1MV !!!

Does this voltage value make engineers feel terrified!

Therefore, when measuring high resistance, voltage excitation is used to measure the current value. Traditional multimeters are no longer competent and need to use picoammeters and high resistance meters, such as Keysight’s B2985B. When measuring the resistance of 0.1TΩ, it can apply a voltage of up to 1000V, and the current measurement resolution is 0.01fA! Its resistance measurement capability can reach 10PΩ level, which is 1016 level!

Accurate measurement of ultra-high resistance, resistivity and insulating materials

2985B Picoammeter and High Resistance Meter

Accurate measurement of ultra-high resistance, resistivity and insulating materials

B2985B measured 100GΩ resistance

For insulators or high resistance, the resistivity of the material is often more concerned than the resistance value itself. ASTM D-257 is an industry standard for testing the electrical resistivity of insulating materials. The standard has detailed regulations on testing methods, electrodes, and measurement procedures for electrical resistivity of insulating materials.

Accurate measurement of ultra-high resistance, resistivity and insulating materials

In order to achieve accurate measurement of the volume resistivity and surface resistivity of insulating materials, test fixtures are essential, such as Keysight’s N1424 insulation resistivity test fixture, which can be used with N2985B or N2987B. The test fixture consists of a shielding box, upper and lower electrodes, and a pressure control device.

Accurate measurement of ultra-high resistance, resistivity and insulating materials

N1424 Insulation resistivity test fixture

1. Volume resistivity and electrode equivalent method:

The measured material is sandwiched between the upper and lower electrodes, and voltage is applied to the upper and lower electrodes, and the volume resistivity is obtained according to the measured current, the thickness of the material, the cross-sectional area and other parameters.

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

2. The surface resistivity and electrode equivalent method are:

The measured material is sandwiched between the upper and lower electrodes, and a voltage is applied between the outer ring and the inner ring of the circular electrode underneath. According to the measured current between the inner and outer rings, the thickness of the material, the cross-sectional area and other parameters, Surface resistivity.

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Therefore, the measurement of the resistivity of insulating materials is still the measurement of large resistance in essence, and the resistivity value per unit volume or area is calculated according to the parameters of the electrode through the measured resistance.

During the measurement, the measured current may be extremely small, due to material polarization or interference, which will make the reading a little unstable. This requires the cooperation of related software to obtain the desired result in the end.

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Through the Excel software, input the corresponding parameters, you can automatically obtain the resistivity test curve, and the operation is simpler.

We have specially recorded a video of the test process, so that you can see it more clearly:

Accurate measurement of ultra-high resistance, resistivity and insulating materials

Source: Keysight

The Links:   6MBP100RS120 CM200TU-12F

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