The effect of resistance or resistivity testing

The main factors that affect the resistance or resistivity test are:

a. Ambient temperature and humidity

The resistance of general materials decreases as the ambient temperature and humidity increase. In contrast, surface resistance (rate) is more sensitive to ambient humidity, while bulk resistance (rate) is more sensitive to temperature. As the humidity increases, the surface leakage increases and the electrical conductivity flow increases. As the temperature rises, the carrier's movement speed increases, and the absorption current and the electric conduction current of the dielectric material increase accordingly. According to relevant data, the resistance value of the general medium at 70C is only 10% at 20C. Therefore, when measuring the resistance of a material, it is necessary to specify the temperature and humidity at which the sample and the environment reach equilibrium.

b. Test voltage (electric field strength)

The resistance (rate) value of a dielectric material generally cannot be kept constant over a wide voltage range, ie Ohm's law does not apply to this. Under the normal temperature condition, in the lower voltage range, the electric conduction current increases linearly with the increase of the applied voltage, and the resistance value of the material remains unchanged. After a certain voltage is exceeded, as the ionization motion increases, the increase of the electric conduction current is much faster than the increase of the test voltage, and the resistance value exhibited by the material is rapidly reduced. It can be seen that the higher the external test voltage is, the lower the resistance value of the material is, so that the resistance value of the material obtained by testing under different voltages may have a large difference.

It is worth noting that the decisive factor that causes the change in the material resistance value is the electric field strength at the time of the test, not the test voltage. For the same test voltage, if the distance between the test electrodes is different, the test results for the material resistivity will also be different. The smaller the distance between the positive and negative electrodes, the smaller the test value.

c. Test time

When the measured material is pressurized with a certain DC voltage, the current on the measured material does not reach the stable value instantaneously, but there is an attenuation process. At the same time as the pressurization, a large charging current flows, followed by a relatively long and slowly decreasing absorption current, and finally a relatively stable electric conduction current is reached. The higher the measured resistance value, the longer the time to reach equilibrium. Therefore, in order to read the measured resistance value correctly during measurement, the value should be read after stabilization or the reading after one minute of pressure is taken.

In addition, the resistance of high insulation materials is also related to its history of electrification. In order to accurately evaluate the electrostatic properties of materials, in the resistance (rate) test of the material, it should first be subjected to a power-discharge treatment and allowed to stand for a certain period of time. The standing time can be taken for 5 minutes. Then, the measurement procedure is repeated. In general, for a material test, at least 3 to 5 samples shall be randomly selected for testing, and the average value shall be used as the test result.

d. Leakage of test equipment

In the test, the connection line with low insulation resistance in the circuit is often unfavorably connected in parallel with the tested sample, sampling resistance, etc., which may have a greater impact on the measurement result. to this end:

In order to reduce the measurement error, protection technology should be used to install a protective conductor on the line with large leakage current to basically eliminate the influence of stray current on the test results;

High-voltage line surface due to ionization, there is a certain degree of leakage to the ground, so try to use high-insulation, large-diameter high-voltage wire as a high-voltage output line and minimize the connection, reduce the tip to prevent corona discharge;

Test stands and supports are made of polyethylene, PTFE, and other insulating materials to avoid low test values ​​due to such causes.

e. External interference

After the high insulation material plus the DC voltage, the current through the sample is very small, and it is easily affected by external interference, resulting in a large test error. Thermoelectric potential and contact potential are generally small and can be neglected. The electrolysis potential is mainly caused by the contact of wet specimens with different metals. It is only about 20mV. Moreover, the relative humidity is lower in the electrostatic test. When testing in a dry environment, Eliminate the electrolytic potential. Therefore, the external interference is mainly the coupling of stray currents or the potential generated by electrostatic induction. When the test current is less than 10-10A or the measured resistance exceeds 1011 ohms; the test sample, test electrode and test system should adopt strict shielding measures to eliminate the influence of external interference.