Several principles for choosing a multimeter
1. Function
In addition to the five functions of measuring AC and DC voltage, AC and DC current, and resistance, the digital multimeter also has digital calculation, self-test, reading retention, error readout, diode detection, word length selection, IEEE-488 interface or RS-232 interface and other functions should be selected according to specific requirements when using them.
2. Range and range
Digital multimeters have many ranges, but their basic range has the highest accuracy. Many digital multimeters have an auto-ranging function, eliminating the need to manually adjust the range, making measurements convenient, accurate, and fast. There are also many digital multimeters with over-range capabilities. When the measured value exceeds the range but has not yet reached the maximum display, there is no need to change the range, thereby improving accuracy and resolution.
3. Accuracy
The maximum error allowed by a digital multimeter depends not only on its variable term error, but also on its fixed term error. When choosing, it also depends on how accurate the stability error and linear error are, and whether the resolution meets the requirements. For general digital multimeters, if the requirements are 0.0005 to 0.002, at least 61 digits should be displayed; 0.005 to 0.01, at least 51 digits should be displayed; 0.02 to 0.05, at least 41 digits should be displayed; 0.1 Below, at least 31 digits should be displayed.
4. Input resistance and zero current
If the input resistance of the digital multimeter is too low and the zero current is too high, it will cause measurement errors. The key depends on the limit value allowed by the measuring device, that is, the internal resistance of the signal source. When the signal source impedance is high, an instrument with high input impedance and low zero current should be selected so that its influence can be ignored.
5. Series mode rejection ratio and common mode rejection ratio
In the presence of various interferences such as electric fields, magnetic fields and various high-frequency noises or when long-distance measurements are made, interference signals are easily mixed in, causing inaccurate readings. Therefore, instruments with high string and common mode rejection ratios should be selected according to the use environment, especially When making high-precision measurements, you should choose a digital multimeter with protective terminal G, which can suppress common-mode interference well.
6. Display form and power supply
The display form of the digital multimeter is not limited to numbers. It can also display charts, text and symbols to facilitate on-site observation, operation and management. According to the external dimensions of its display device, it can be divided into four categories: small, medium, large and ultra-large.
The power supply of digital multimeters is generally 220V, and some new digital multimeters have a wide power supply range, which can be between 1100V and 240V. Some small digital multimeters can be used with batteries, and some digital multimeters can be used with AC power, internal nickel-cadmium batteries, or external batteries.
