Two Key Points Should Be Noted When Measuring Large Resistances with a Multimeter
1. Stable time effect
A capacitor connected in parallel with a resistor will produce a settling time error after initial connection and range change. Modern digital multimeters insert a trigger delay, which provides the time for the measurement to reach stability. The length of the trigger delay depends on the selected function and range. When the combined capacitance of the cable and device is less than a few hundred pF, these delays are sufficient for resistance measurement, but if there is parallel capacitance on the resistor, or if you are measuring a resistance higher than 100 k Ω, the default delay may not be sufficient. Due to the influence of the RC time constant, stability may require a considerable amount of time. Some precision resistors and multifunctional calibrators use parallel capacitors (1000 pF to 100 μ F), which together with high-value resistors filter out noise currents injected by internal circuits. Due to the dielectric absorption (wetting) effect in cables and other devices, it is possible to increase the RC time constant and require a longer stabilization time. In this case, you may need to increase the trigger delay before conducting the test.
Bias compensation in the presence of capacitors
If there is a parallel capacitor on the resistor, it may be necessary to turn off the bias compensation. When bias compensation takes the second reading without a current source, it will measure any voltage bias. But if the device has a long stable time, it will cause biased measurements with errors. A digital multimeter will use the same trigger delay for bias measurement in an attempt to avoid settling time issues. Increasing trigger delay is another solution to make the device completely stable.
2. Connection in high resistance measurement
When measuring high resistance, insulation resistance and surface contamination can cause significant errors. Various preventive measures need to be taken to maintain the cleanliness of the high resistance system. The test wire and clamp are very sensitive to leakage caused by moisture absorption of insulating material and "dirty" surface facial mask layer. Compared to PTFE Teflon insulation (109 Ω), nylon and PVC are relatively poor insulators (1013 G Ω). If you measure a 1 M Ω resistance under humid conditions, the contribution of nylon or PVC insulation leakage to the error can easily reach 0.1%.
