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Selection of ammeter and voltmeter

When measuring current and voltage, the correct selection and use of ammeter and voltmeter not only directly affects the accuracy of the measurement results, but also relates to the safety of the operator and the service life of the instrument. That is, comprehensive and focused selection.
(1) Selecting the instrument type To measure DC current and voltage, a magnetoelectric instrument should be selected. When measuring AC current and voltage, an electromagnetic system or a rectifier system instrument should be selected. When higher accuracy is required, the electric system instrument can be selected. If it is required to use both AC and current, the electromagnetic system instrument for both AC and DC can be selected; in the occasions requiring higher accuracy, the electric system instrument can be selected.
(2) When selecting the accuracy of the instrument as the standard meter or precision measurement, the instrument of class 0.1 or 0.2 can be selected; the instrument of class 0.5 or 1.0 can be selected in the laboratory; the instrument of class 1.5 or less can be selected for general range measurement. For additional devices used in conjunction with the meter, such as shunt resistors, voltage divider resistors, instrument transformers, etc., the accuracy level should be 2 to 3 stops higher than the accuracy level of the meter itself to ensure the accuracy of the measurement results.
(3) After selecting the internal resistance of the meter and connecting the meter to the circuit under test, the power consumption of the meter itself should be minimized to avoid affecting the original working state of the circuit. Therefore, when selecting the internal resistance of the instrument, it should be satisfied: the internal resistance of the voltmeter should be as large as possible; the internal resistance of the ammeter should be as small as possible.
(4) Selecting the range of the meter In the actual measurement, in order to minimize the measurement error and ensure the safety of the meter, the range of the ammeter and the voltmeter should be selected according to the following principles:
①The selected range should be larger than the measured range.
②Select the range to be measured within more than two-thirds of the full scale of the instrument scale.
③ When the size of the measurand cannot be estimated, the maximum range of the instrument should be selected for testing, and then gradually changed to a suitable range.
(5) Selecting the working conditions of the instrument Portable instruments are generally selected for instruments used in the laboratory; installed instruments should be selected for instruments on switch panels or electrical equipment panels. When there are specific requirements for ambient temperature, humidity, external electromagnetic field and other conditions, the selection should be made according to their requirements to minimize the additional error of the instrument.
(6) Select the insulation strength of the instrument When selecting the instrument, the insulation strength of the instrument and additional devices should be determined according to the voltage of the circuit under test, so as to avoid accidents that endanger personal safety and damage the instrument.