How to Measure Static Electricity in a Product
1. Methods:
The commonly used methods for measuring high resistance are the direct method and the comparative method.
The direct method involves measuring the DC voltage applied to the sample and the current flowing through it to determine the sample's resistance. Direct methods mainly include the galvanometer method and the DC amplification method (high resistance meter method). Comparative methods mainly include the galvanometer method and the bridge method.
2. Accuracy:
For resistors greater than 10Ω (10 ohms), the instrument error should be within ±20%; for resistors not greater than 10Ω (10 ohms), the instrument error should be within ±10%.
3. Protection:
The insulating material used in measuring instruments generally only has properties similar to the material being measured. Test errors in the sample can be caused by the following:
① Stray currents caused by external parasitic voltages. Their magnitude is usually unknown and they exhibit drift characteristics;
② The insulating material of the measuring circuit is connected in parallel with the sample resistance, a standard resistor, or a current measuring device.
Using high-resistance insulation can improve measurement errors, but this method makes the instrument expensive and bulky, and still does not yield satisfactory results for high-resistance samples. A more satisfactory improvement method is to use protection technology, which involves placing protective conductors at all major insulation points to intercept stray currents that could cause errors; these conductors are connected together to form a protection system, creating a three-terminal network with the measuring terminals. When the wiring is properly connected, all stray currents from external parasitic voltages are diverted below the measuring circuit by the protection system, greatly reducing the possibility of errors.
The electrolytic potential, contact potential, or thermoelectric potential between the protected and protected terminals of the system can be compensated for when they are small, preventing them from causing significant errors in the measurement.
In current measurement, since the resistance between the protected and protected terminals, connected in parallel with the current measuring device, may introduce errors, the former should be at least 10 times, and preferably 100 times, the input resistance of the current measuring device. In the bridge method, the protected terminals and the measuring terminals have approximately the same potential. However, a standard resistor in the bridge is connected in parallel with the resistance between the unprotected and protected terminals; therefore, the latter should be at least 10 times, and preferably 20 times, the standard resistor.
Before starting the test, disconnect the power supply and the sample for a preliminary measurement. The equipment should indicate infinite resistance within its sensitivity range. The equipment can be checked for proper functioning using standard resistors with known values.
Volume Resistivity
To measure volume resistivity, the protection system used should be able to compensate for errors caused by surface current. For samples with negligible surface leakage, protection can be removed when measuring volume resistivity.
The gap width on the sample surface between the protected and measuring electrodes should be uniform, and the gap should be as narrow as possible without causing measurement errors due to surface leakage; in practical use, the minimum is 1 mm.
Surface Resistivity
To determine surface resistivity, the protection system used should compensate for the effects of volume resistivity as much as possible.