Analysis of the main factors about resistance or resistivity testing

The main factors affecting resistance or resistivity testing are:

a. Ambient temperature and humidity

The resistance value of general materials decreases with the increase of ambient temperature and humidity. Relatively speaking, surface resistivity is more sensitive to ambient humidity, while volume resistivity is more sensitive to temperature. As humidity increases, surface leakage increases, and volume conductance current also increases. As temperature rises, the movement rate of carriers increases, and the absorption current and conductance current of dielectric materials will increase accordingly. According to relevant data, the resistance value of general dielectrics at 70°C is only 10% of that at 20°C. Therefore, when measuring the resistance of a material, it is necessary to specify the temperature and humidity at which the sample reaches equilibrium with the environment.

b. Test Voltage

(Electric field strength) The resistance (rate) value of dielectric materials generally cannot remain constant over a wide voltage range, that is, Ohm's law does not apply to this. Under normal temperature conditions, within a lower voltage range, the conductance current increases linearly with the increase of the applied voltage, and the resistance value of the material remains unchanged. After exceeding a certain voltage, due to the intensification of ionization movement, the increase in conductance current is much faster than the increase in the test voltage, and the resistance value presented by the material decreases rapidly. It can be seen that the higher the applied test voltage, the lower the resistance value of the material, so that the resistance value of the material tested at different voltages may be greatly different. It is worth noting that the determining factor that causes the change in the resistance value of the material is the electric field strength during the test, not the test voltage. For the same test voltage, if the distance between the test electrodes is different, the test results of the material resistivity will also be different. The smaller the distance between the positive and negative electrodes, the smaller the test value.

c.Testing time

When a certain DC voltage is applied to the material to be tested, the current on the material to be tested does not reach a stable value instantaneously, but has a decay process. While the pressure is applied, a large charging current flows, followed by an absorption current that slowly decreases over a relatively long period of time, and finally reaches a relatively stable conductance current. The higher the measured resistance value, the longer it takes to reach equilibrium. Therefore, in order to correctly read the measured resistance value during measurement, the value should be read after stabilization or the reading value should be taken 1 minute after the pressure is applied. In addition, the resistance value of high-insulation materials is also related to their history of being charged. In order to accurately evaluate the electrostatic properties of the material, when testing the material for resistance (rate), it should first be de-charged and left to stand for a certain period of time, which can be 5 minutes, and then tested according to the measurement procedure. Generally speaking, for the test of a material, at least 3 to 5 samples should be randomly selected for testing, and the average value should be used as the test result.

d. Leakage of test equipment

During the test, the connection line with low insulation resistance in the circuit is often improperly connected in parallel with the tested sample, sampling resistor, etc. , which may have a greater impact on the measurement results. Therefore: In order to reduce the measurement error, protection technology should be used to install protective conductors on the line with large leakage current to basically eliminate the influence of stray current on the test results; high-voltage lines have a certain leakage to the ground due to surface ionization, so high-insulation, large-diameter high-voltage wires should be used as high-voltage output lines as much as possible, and the connection lines should be shortened as much as possible to reduce the tip and eliminate corona discharge; polyethylene, polytetrafluoroethylene and other insulating materials should be used to make test benches and supports to avoid low test values ​​due to such reasons.

e. External interference

When a DC voltage is applied to a high-insulation material, the current passing through the sample is very small and is easily affected by external interference, resulting in large test errors. Thermoelectric potential and contact potential are generally very small and can be ignored; the electrolytic potential is mainly generated by the contact between the wet sample and different metals, which is only about 20mV. Moreover, the relative humidity is required to be low in electrostatic testing. When testing in a dry environment, the electrolytic potential can be eliminated. Therefore, external interference is mainly the potential generated by the coupling of stray currents or electrostatic induction. When the test current is less than 10-10A or the measured resistance exceeds 1011 ohms; the tested sample, test electrode and test system should all take strict shielding measures to eliminate the impact of external interference.