Analysis of Measurement Errors of Digital Multimeter

1. Multimeter range selection and error analysis

1.1 Human Error

Human reading error is one of the reasons that affect measurement accuracy. Pay special attention to the following points during use:

(1) Before measuring, place the multimeter horizontally and perform mechanical zero adjustment.

(2) Keep your eyes perpendicular to the pointer when reading.

(3) When measuring resistance, zero adjustment must be performed each time the gear is changed. If zero adjustment cannot be achieved, replace the battery with a new one. Do not hold the metal part of the test lead with your hands to avoid shunting the human body resistance and increasing the measurement error.

(4) When measuring the resistance in a circuit, cut off the power supply in the circuit and discharge the capacitor completely before measuring.

1.2 Multimeter voltage and current range selection and measurement error

The accuracy level of a multimeter is generally divided into 0.1, 0.5, 1.5, 2.5, 5, etc. The accuracy level of each level of DC voltage, current and AC voltage, current, etc. is calibrated by the percentage of its maximum absolute allowable error △x to the full scale value of the selected range.

The error caused by measuring voltage with a multimeter is different from the error caused by measuring the same voltage with multimeters of different accuracy. When choosing a multimeter, the higher the accuracy, the better. With a multimeter with high accuracy, you must also choose the right range to give full play to the potential accuracy of the multimeter. The error caused by measuring the same voltage with different ranges of a multimeter is also different. When the value of the measured signal is met, you should try to choose a small range, which can improve the accuracy of the measurement. Therefore, when measuring voltage, the measured voltage should be indicated at more than 2/3 of the multimeter range, so as to reduce the measurement error.

1.3. Resistance range selection and measurement error

When using a multimeter to measure the same resistance, different ranges will produce different errors, and the measurement errors will vary greatly. When selecting the range, try to make the measured resistance value in the center of the arc length of the range scale, and the measurement accuracy will be higher.

2. Analysis of multimeter measuring non-sinusoidal AC voltage

The rectifier type meter composed of the magnetoelectric measuring mechanism and the rectifier circuit of the multimeter indicates the average value of the AC voltage. In engineering technology, it is usually necessary to measure the effective value of the AC voltage or current. In order to meet this need, the AC voltage scale of the multimeter is calibrated according to the effective value of the sinusoidal AC voltage.

2.1. Determination coefficient

The AC voltage range of the multimeter belongs to the average voltage meter. Although the dial is scaled according to the effective value when measuring AC voltage, the rectifier circuit actually detects the average voltage. The ratio of the effective value U of the voltage to the average value/U is called the meter's coefficient of determination, expressed as K, which reflects the proportional relationship between the AC voltage range reading of the multimeter and the average value of the measured voltage.

The indication when measuring the sine wave voltage at the rated frequency with a multimeter is:

a=U

Where:

U is the reading of the AC voltage range of the multimeter;

U is the effective value of the sinusoidal voltage.

It can be seen that when a multimeter is used to measure a sine wave voltage in the AC voltage range, the reading a is the effective value of the voltage being measured; when measuring a non-sinusoidal voltage, the reading has no direct physical meaning, and we only know that 0.9a is equal to the average value of the voltage being measured. If the waveform factor of the voltage being measured is known, the effective value of the voltage being measured can be obtained by conversion.

2.2 Form Factor KF

The form factor Kf is defined as the ratio of the effective value to the average value of the AC voltage, that is: