Q: What does the precision of various instruments mean?
A: The percentage of the maximum error that represents the meter. For example, the maximum measurement error of 0.5 is not more than 0.5% of the full scale value; for the first-class instrument, the maximum measurement error is not more than ±1%. For example, if the maximum range is 100A, the maximum error is 1A.
Q: What does it mean to indicate the absolute, relative, and folding errors of the meter?
A: The difference between the indicated value of the test form and the indicated value of the standard table is the absolute error of the test form. The percentage of the "absolute error" value compared to the value indicated by the standard table is the "relative error". The "folding error" is the percentage of the "absolute error" value compared to the full scale value of the test table.
Q: I often see signs of ↑→∠45° on the meter. What do they mean?
A: ↑ indicates that the surface should be placed vertically when the meter is used; → indicates that the surface should be placed horizontally when the meter is used; ∠45° means that the surface should be placed at an angle of 45° to the horizontal line when the meter is used. If the meter is not used in accordance with these prescribed placement methods, the reading error of the meter will increase and may exceed the allowable value.
Q: What are the meanings of the symbols on the dashboard?
A: Indicates that the instrument is required to be placed horizontally at work; the above indicates that it should be placed vertically; one indicates that it is only used for DC; one indicates that it is only for communication; all means that it is suitable for dual use. 
Q: Why do some instruments have a lens on the dial?
Answer: Because there is a certain gap between the pointer and the surface of the scale, the larger the gap, the parallax is likely to occur. If a lens is placed on the dial and the reading is taken when the pointer coincides with the projection of the pointer in the lens, the parallax can be reduced. Therefore, the role of the lens is to reduce the parallax of the reading and improve its accuracy.
Q: Why do some meters have a metal film on the inner wall of the casing?
A: Because the magnetic fields of the electric and electromagnetic meters are weak, the accuracy of the meter will be seriously affected by the external magnetic field. When the inner wall of the instrument is attached with a magnetically permeable metal film, the external magnetic field can be passed through the metal film with high magnetic permeability, so as not to enter the inside of the instrument, thereby ensuring the accuracy of the meter. This is what we often call shielding. Some instruments with higher precision are usually attached with a two-layer metal film.
Q: Why do electrical instruments in high-intensity equipment have a capacitor connected in parallel?
Answer: This is to prevent the interference of high-intensity current to the meter. Parallel a capacitor can bypass the high-frequency current through the capacitor to eliminate interference.
Q: Why do the measuring wires not be entangled when measuring the insulation resistance of electrical equipment with a megohmmeter?
A: If two measuring wires are entangled for measurement, when the wire insulation is not good, it is equivalent to making a low resistance in parallel with the electrical equipment under test, which makes the measurement inaccurate and also changes the capacitance of the circuit under test. It is not accurate when doing the absorption ratio test.
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