Technical specifications of the digital multimeter
1. Display digits and display characteristics
The number of display digits of digital multimeter is usually 31/2 digits to 81/2 digits. There are two principles for determining the number of display digits of a digital instrument:
One is that the number of digits that can display all the numbers from 0-9 is the whole number of digits;
The second is that the value of the fraction is to * large display value in the * high digit as the numerator, with the full scale when the count value of 2000, which indicates that the instrument has three integer bits, and the fraction of the numerator is 1, the denominator is 2, so it is called the 31/2 bit, read as "three and a half", and its * high bit can only display 0 or 1 (0 is usually not displayed). The *higher bit can only display 0 or 1 (0 is usually not displayed).
32/3 bit (read as "three and two-thirds bit") digital multimeter * high bit can only display 0 ~ 2 of the number, so the * large display value of ± 2999. In the same case, it is to be higher than the 31/2 bit digital multimeter limit of 50%, especially in the measurement of 380V AC voltage is very valuable.
For example, in the digital multimeter to measure the grid voltage, ordinary 31/2 digital multimeter * high bit can only be 0 or 1, if you want to measure the 220V or 380V grid voltage, can only be displayed in three places, the resolution of the file is only 1V.
In contrast, with 33/4-bit digital multimeter to measure the grid voltage, the * high bit can be displayed 0 to 3, so that can be displayed in four places, the resolution of 0.1V, which is the same as the 41/2-bit digital multimeter resolution.
Popular digital multimeter generally belongs to the 31/2-bit display of the handheld multimeter, 41/2, 51/2-bit (6-bit or less) digital multimeter is divided into handheld, desktop two kinds. 61/2-bit or more mostly belong to the desktop digital multimeter.
Digital multimeter using advanced digital display technology, display clear and intuitive, accurate reading. It can ensure the objectivity of the reading, but also in line with people's reading habits, can shorten the reading or recording time. These advantages are the traditional analogue (i.e. pointer) multimeter does not have.
2. Accuracy (precision)
The accuracy of the digital multimeter is a combination of systematic and random errors in the measurement results. It indicates the degree of consistency between the measured value and the true value, and also reflects the size of the measurement error. Generally speaking, the higher the accuracy, the smaller the measurement error, and vice versa.
There are three ways of expressing accuracy, as follows:
Accuracy = ± (a% RDG + b% FS) (2.2.1)
Accuracy = ± (a % RDG + n words) (2.2.2)
Accuracy = ± (a % RDG + b % FS + n words) (2.2.3)
In equation (2.2.1), RDG is the reading value (i.e., display value), FS indicates the fullness value, the former item in the brackets represents the combined error of the A/D converter and the functional converters (e.g., voltage dividers, shunts, and true RMS converters), and the latter item is the error due to the digitisation process.
