The choice of digital multimeter is generally considered from the following aspects
1. Function
In addition to the five functions of measuring AC and DC voltage, AC and DC current, and resistance, the current digital multimeter also has digital calculation, self-test, reading hold, error readout, detection, word length selection, IEEE-488 interface or RS-323 interface and other functions should be selected according to specific requirements.
2. Range and range
A digital multimeter has many ranges, but its basic range has the highest accuracy. Many digital multimeters have automatic range function, no need to manually adjust the range, making the measurement convenient, safe and fast. There are also many digital multimeters with over-range capability. When the measured value exceeds the range but has not reached the maximum display, it is not necessary to change the range, thereby improving the accuracy and resolution.
3. Accuracy
The maximum error allowed by a digital multimeter depends not only on its variable error, but also on its fixed error. When choosing, it also depends on the requirements of stability error and linearity error, and whether the resolution meets the requirements. If the general digital multimeter is required to be 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 measurement 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 disturbances such as electric fields, magnetic fields and various high-frequency noises or long-distance measurements, it is easy to mix in interference signals and cause inaccurate readings. Therefore, instruments with high string and common-mode rejection ratios should be selected according to the use environment, especially When performing high-precision measurement, a digital multimeter with protection terminal G should be selected, which can well suppress common-mode interference.
6. Display form and power supply
The display form of the digital multimeter is not limited to numbers, but can also display charts, text and symbols, so as to facilitate on-site observation, operation and management. According to the dimensions of its display devices, it can be divided into four categories: small, medium, large and super large.
The power supply of a digital multimeter 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 in three forms: alternating current, internal nickel-cadmium batteries, or external batteries.
7. Response time, measurement speed, frequency range
The shorter the response time, the better, but the response time of some meters is relatively long, and the reading can be stabilized after a period of time. The measurement speed should be based on whether it is used in combination with the system test. If it is used in combination, the speed is very important, and the faster the better. The frequency range can be appropriately selected according to the needs.
8. AC voltage conversion form
AC voltage measurement is divided into average value conversion, peak value conversion and effective value conversion. When the waveform distortion is large, the average conversion and peak conversion are inaccurate, but the effective value conversion can not be affected by the waveform, so that the measurement results are more accurate.
9. Resistor wiring
There are four-wire system and two-wire system for resistance measurement wiring. When performing small resistance and high-precision measurement, the resistance measurement wiring method with four-wire system should be selected.
