Q: What does the precision of various instruments mean?
A: The precision of an instrument refers to the degree of accuracy it can achieve in measurements. It is usually expressed as a percentage of the full scale value. For example, an instrument with a precision of 0.5% means that its maximum error will not exceed 0.5% of the full scale. For a first-class instrument, the maximum error is typically no more than ±1%. So, if the range is 100A, the maximum allowable error would be 1A.
Q: What are absolute, relative, and folding errors in meters?
A: Absolute error is the difference between the measured value and the true value. Relative error is the ratio of the absolute error to the true value, expressed as a percentage. Folding error, also known as percentage of full scale error, is the ratio of the absolute error to the full scale of the meter. These errors help determine how accurate a meter is under different conditions.
Q: I often see symbols like ↑→∠45° on meters. What do they mean?
A: The symbols indicate the correct orientation for using the meter. The arrow ↑ means the meter should be placed vertically, → means it should be placed horizontally, and ∠45° means it should be tilted at a 45-degree angle. If the meter is used incorrectly, the readings may become inaccurate or even exceed acceptable limits.
Q: What do the symbols on the dashboard represent?
A: The symbols indicate the proper placement and usage of the meter. A horizontal line means it should be used horizontally, a vertical line means it should be used vertically, a single line represents DC use only, a double line indicates AC use, and a combination symbol means it can be used for both DC and AC applications.
Q: Why do some meters have a lens on the dial?
A: The lens helps reduce parallax error. Since there's a small gap between the pointer and the dial, when you look from an angle, the reading might appear different. By using a lens, you can align the pointer with its reflection in the lens, ensuring a more accurate reading. This is especially important in high-precision instruments.
Q: Why do some meters have a metal film inside the casing?
A: The metal film acts as a shield against external magnetic fields. Instruments like electromagnetic meters are sensitive to external interference. The metal film, which has high magnetic permeability, diverts the external magnetic field away from the internal components, protecting the accuracy of the meter. High-precision meters often use two layers of such shielding.
Q: Why are capacitors connected in parallel with electrical instruments in high-current equipment?
A: Capacitors are used to filter out high-frequency noise or interference. When connected in parallel, they allow high-frequency currents to bypass the meter, preventing them from affecting the readings. This ensures more stable and accurate measurements in environments with strong electrical signals.
Q: Why shouldn’t the measuring wires be tangled when using a megohmmeter to test insulation resistance?
A: If the wires are tangled, especially if the insulation is poor, it creates a low-resistance path that affects the measurement. This can lead to incorrect results and distort the capacitance of the circuit being tested. It’s especially problematic during absorption ratio tests, where accuracy is crucial. Keeping the wires separate ensures reliable and precise readings.
4.2mm Pitch
4.2mm Pitch
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