Electrical measuring instruments are simple in structure, highly reliable, and easy to maintain. They can be categorized based on their operating principles into types such as magnetoelectric, electromagnetic, electrodynamic, and induction instruments. Generally, the dial of these instruments has a black dot near the starting scale. Why is this black dot placed there? To answer this question, we first need to understand the concept of accuracy class for measuring instruments.

The accuracy of measuring instruments is commonly referred to as precision, while the accuracy class is often referred to as the precision class. This is usually indicated on the instrument's scale or nameplate. The accuracy class of direct-reading instruments is determined based on the absolute value of their maximum permissible error. Denoted as "K," the relationship between K and the maximum permissible error is:
K ≥ maximum permissible error.

If the maximum permissible error falls between two specified class values, the instrument is classified as belonging to the lower accuracy class. For example, if the maximum permissible error of an ammeter is 0.7%, it would be classified as a 1.0 class instrument rather than a 0.5 class instrument. Lower class numbers indicate higher accuracy. Common accuracy classes for electrical instruments are 0.1, 0.2, 0.5, 1, 1.5, 2.5, and 5, with 0.1 being the highest accuracy and 5 being the lowest.

Most measuring instruments indicate their accuracy class on the scale. The black dot near the starting point marks the range from this point to the full scale where the instrument's accuracy meets its stated class standard. Typically, the black dot is positioned at 20% of the instrument's maximum scale value. For instance, on a 5A ammeter, the black dot would be located at the 1A mark.

This design implies that if the pointer indicates a value below the black dot, the measurement error may be significant, falling outside the instrument's accuracy range. In such cases, the instrument or its associated transformer should be replaced to ensure the pointer reads within the 20%-100% range.

Selecting Instruments

When selecting an instrument, both the accuracy class and range should be considered. It's not sufficient to focus solely on the accuracy class. Additionally, to minimize relative measurement error, the measured value should be as close as possible to the instrument's full scale and, at the very least, above half of the full scale.

Reasons for the Black Dot:

1.Calibration and Reference Point:
The black dot serves as a calibration or reference point, assisting users in reading measurements more accurately. Aligning the pointer with the black dot ensures a reliable starting point.

2.Error and Precision Indication:
The black dot signifies the starting point of the instrument's specified accuracy range, reminding users to take measurements within this range for reliable results.

3.Visual Cue:
The black dot draws the user's attention to the starting scale area, helping them focus on the accurate range and improving reading accuracy.

In summary, the black dot near the starting scale of measuring instruments provides practical and visual guidance to ensure the precision and reliability of measurements.