Selection of ammeter and voltmeter
When measuring current and voltage, correctly selecting and using an 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. Make a comprehensive and focused selection.
(1) Select instrument type
To measure DC current and voltage, a magneto electric instrument should be selected. When measuring AC current and voltage, electromagnetic or rectifier instruments should be selected. When high accuracy is required, electric instruments can be chosen. If both AC and DC are required, electromagnetic instruments that can be used for both AC and DC can be selected; In situations where high accuracy is required, electric instruments can be chosen.
(2) Select instrument accuracy
When used as a standard meter or for precision measurement, instruments of level 0.1 or 0.2 can be selected; Laboratories can use instruments of level 0.5 or 1.0; General engineering surveying can use instruments below level 1.5.
Additional devices used in conjunction with instruments, such as shunt resistors, voltage divider resistors, instrument transformers, etc., should have an accuracy level 2-3 levels higher than the accuracy level of the instrument itself to ensure the accuracy of measurement results.
(3) Select instrument internal resistance
After connecting the instrument to the tested circuit, the power consumption of the instrument itself should be minimized as much as possible to avoid affecting the original working state of the circuit. Therefore, when selecting the internal resistance of the instrument, it is necessary to meet the following requirements: 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) Select instrument range
In actual measurement, in order to minimize measurement errors and ensure the safety of the instrument, the range of the ammeter and voltmeter should be selected according to the following principles: ① The selected range should be larger than the measured range; ② Select the measured range within two-thirds or more of the full scale of the instrument scale; ③ When it is impossible to estimate the measured size, the maximum range of the instrument should be selected for testing first, and then gradually replaced with a suitable range.
(5) Select the working conditions of the instrument
Portable instruments are generally selected for laboratory use; Instruments on switch boards or electrical equipment panels should be installed. When there are specific requirements for environmental temperature, humidity, external electromagnetic fields, etc., the selection should be made according to their requirements to minimize the additional error of the instrument.
(6) When selecting an instrument for insulation strength, it is also necessary to determine the insulation strength of the instrument and additional devices based on the voltage of the tested circuit, in order to avoid accidents that may endanger personal safety and damage the instrument.
