How to choose ammeter and voltmeter correctly?

The measuring mechanism of ammeter and voltmeter is basically the same, but the connection in the measuring circuit is different. Therefore, the following matters should be paid attention to when selecting and using ammeter and voltmeter.

(1) Type selection. In the measurement of DC current and voltage, instruments with magnetoelectric measuring mechanisms are mainly used; in the measurement of AC current and voltage, instruments with electromagnetic and ferromagnetic electrodynamic measuring mechanisms are often used; and instruments with electrodynamic measuring mechanisms are often used for precise measurement of AC current and voltage.

(2) Selection of accuracy. The accuracy of the instrument should be selected based on the actual measurement requirements and should not be too high. Usually, 0.1 and 0.2 level instruments are selected as standard instruments; 0.5 and 1.0 level instruments are selected for laboratory measurements; instruments below 1.5 are generally selected for engineering measurements.

(3) Selection of measuring range. Correctly estimate the numerical range of the measured value and select the measuring range reasonably. Generally, the indicated value of the measured value should be greater than 2/3 of the maximum measuring range of the instrument, but not exceed the maximum measuring range.

(4) Selection of internal resistance. In order to reduce measurement errors, when measuring current, an ammeter with a small internal resistance should be used as much as possible. When measuring voltage, a voltmeter with a large internal resistance should be used as much as possible.

(5) Correct wiring. When measuring current, the ammeter should be connected in series with the circuit being measured; when measuring voltage, the voltmeter should be connected in parallel with the circuit being measured. When measuring DC current and voltage, care must be taken to ensure that the polarity of the meter is consistent with the polarity of the physical quantity being measured.

(6) Measurement of high voltage and high current. When measuring high voltage or high current, a voltage transformer or current transformer must be used. The range of the voltage meter and the current meter should be consistent with the rated value of the secondary winding of the transformer, which is generally 100V for voltage and 5A for current.

(7) Expansion of measuring range. When the measured value in the circuit exceeds the measuring range of the instrument, a shunt or voltage divider can be attached, and its accuracy level should be consistent with the accuracy level of the instrument.

(8) Before measuring, check whether the mechanical zero point of the instrument is on the zero scale. If it is not on the zero scale, it should be adjusted.

(9) The instrument’s operating environment must meet the requirements and be away from magnetic fields. When reading, the line of sight should be perpendicular to the plane of the scale.