Analysis of voltage measurement errors between digital multimeter and pointer multimeter
If the measured voltage is AC power, which is 50Hz, and both meters are qualified, it can only indicate that the internal resistance of the measured voltage is too high. The biggest factor affecting the measured voltage results of a pointer multimeter and a digital multimeter at the same frequency is the difference in internal resistance, which is significant and not on the same order of magnitude. When the internal resistance of the measured voltage is small, the difference is not significant. When the internal resistance of the measured voltage is large, the measurement results will differ significantly.
In this situation, the measured voltage may not be the actual 220V live power line, or the voltage measured after the live line passes through some electrical appliance, or the voltage of the electrical leakage shell.
Excluding the above possibilities, it can only indicate that one of the two meters is inaccurate and requires maintenance and calibration.
There is an error in measuring voltage. First, you need to clarify what is the frequency of the measured AC voltage in Hz? Is this voltage a pure sine wave?
All kinds of multimeters available on the market now have their frequency response range and AC waveform marked on their manuals when measuring AC voltage. For various ordinary digital multimeters, their frequency response is generally 40-1000Hz, and it is required to be a sine wave (distortion ≤ 1%). The measured AC voltage beyond the above range does not guarantee measurement accuracy. This is because most AC/DC conversion circuits in digital multimeters are designed using low-power dual operational amplifier TL062, which has limited gain bandwidth product (GBW). Therefore, digital multimeters cannot measure high-frequency AC voltage (of course, it also depends on whether the multimeter's voltage divider resistor is compensated).
As for the general pointer multimeter (which was first invented by Americans and has been around for 100 years)
The internal structure of this old and inexpensive multimeter is quite simple, consisting of a high-sensitivity meter head, diode rectifier, and voltage divider resistor (a few pointer multimeters add an operational amplifier between the meter head and the voltage divider resistor to improve sensitivity). Therefore, the measurement accuracy of this ancient and inexpensive multimeter cannot be compared with that of a digital multimeter. The voltage divider resistor of this meter is generally not compensated by capacitance, so its frequency response is generally 40-400Hz.
Two meters measure the same AC voltage with a difference of several tens of volts. First, you need to check their voltage divider resistor networks to see if any of the resistors have changed value? If everything is normal, for a pointer multimeter, you can also check if its header pointer can point to zero? For a digital multimeter, can you check if the calibration potentiometer for its AC voltage range is loose?
By the way, if you want to accurately measure AC voltage of any waveform, it is recommended to purchase a true RMS (TRMS) multimeter. This multimeter can accurately measure AC voltage of various waveforms such as sine waves, triangular waves, rectangular waves, etc., and is independent of distortion.
