What are the characteristics of an ammeter

Voltage and current measurement

The quality class and other measurement errors

No measuring device shows the value to be measured with absolute accuracy. The construction of the instrument causes a display error. The manufacturer takes this into account by specifying a quality class. Further measurement inaccuracies are based on the personal reading error. The position of use of the instrument, the ambient temperature, the frequency range and external electromagnetic fields can cause further measurement inaccuracies.

The classification of the quality classes indicates the error value in percent based on the full scale. Industrial measuring devices have quality class 1 - 6 or 1.5 - 2.5 or 5 percent. For precision measuring devices, the values ​​are 0.1 - 0.2 or 0.5 percent. This error value is constant over the entire range of the scale.

So that the device error cannot falsify the true measured value too much, the measurement should only be made in the upper 2/3 range of the scale. The diagram opposite shows the percentage error of the measured value depending on the scale range for a quality class of 5%. If 100 V are measured with this device at full scale, the true value can be between 95 ... 105 V. For the 50 V display, the inaccuracy of ± 5 V continues to apply. In relation to the measured value, however, this is an error of 10%. A measured value of 10 V then has an inaccuracy of 50%, which is no longer tolerable.

For more precise measurement in the upper range of the scale, the measuring device must be switched to an optimal measuring range.

Personal reading errors remain small if you align your eyes perpendicular to the pointer. If the scale is stored in a mirror, the pointer must be congruent with its mirror image. All other reading positions deliver readings that are too high or too low because of parallax, the shift on the line of the eye-pointer-scale.