Several principles for selecting a multimeter
1. Function
In addition to measuring AC and DC voltage, AC and DC current, resistance, and other five functions, a digital multimeter also has functions such as digital calculation, self checking, reading retention, error reading, diode detection, word length selection, IEEE-488 interface or RS-232 interface. When using it, it should be selected according to specific requirements.
2. Range and Range
A digital multimeter has many ranges, but its basic range accuracy is the highest. Many digital multimeters have an automatic range function, which eliminates the need for manual range adjustment, making measurement convenient and fast. There are also many digital multimeters with over range capability, which can improve accuracy and resolution by not changing the range when the measured value exceeds the range but has not yet reached the * large display.
3. Accuracy
The maximum allowable error of a digital multimeter depends not only on its variable error, but also on its fixed error. When choosing, it is also important to consider the amount of stability error and linearity error, as well as whether the resolution meets the requirements. If a general digital multimeter requires a level of 0.0005 to 0.002, it should have at least 61 digits displayed; 0.005 to 0.01 levels, with at least 51 digits displayed; Level 0.02 to 0.05, with at least 41 digits displayed; Below 0.1 level, there should be at least 31 digits displayed.
4. Input resistance and zero current
Digital Multimeter
Both low input resistance and high zero current can cause measurement errors. The key is to determine the allowable limit value of the measurement device, which depends on the internal resistance of the signal source. When the impedance of the signal source is high, instruments with high input impedance and low zero current should be selected so that their impact can be ignored.
5. Serial mode rejection ratio and common mode rejection ratio
When there are various interferences such as electric fields, magnetic fields, and high-frequency noise, or when conducting long-distance measurements, it is easy to mix interference signals, causing inaccurate readings. Therefore, instruments with high serial and common mode suppression ratios should be selected according to the usage environment. Especially when conducting high-precision measurements, digital multimeters with protective terminals G should be selected to effectively suppress common mode interference.
6. Display format and power supply
The display format of a digital multimeter is not limited to numbers, but can also display charts, text, and symbols for on-site observation, operation, and management. According to the external dimensions of its display devices, it can be divided into four categories: small, medium, large, and ultra large.
The power supply for digital multimeters is generally 220V, while some new types of digital multimeters have a wide power range, ranging from 1100V to 240V. Some small digital multimeters can be used with batteries, while others can be used in three forms: AC, internal nickel cadmium batteries, or external batteries.
