What are the factors for choosing a digital multimeter?
Answer: Digital multimeters are widely used in national defense, scientific research, factories, schools, measurement and testing and other technical fields because of their high accuracy, wide measuring range, fast measuring speed, small volume, strong anti-interference ability and convenient use. However, their specifications are different, their performance indicators are varied, and their use environment and working conditions are also different. Therefore, appropriate digital multimeters should be selected according to specific conditions.
Choosing a digital multimeter is generally considered from the following aspects:
(1) Function:
In addition to measuring AC and DC voltage, AC and DC current and resistance, the digital multimeter has five functions, such as digital calculation, self-check, reading retention, error reading, diode detection, word length selection, IEE-488 interface or RS-232 interface, etc., which should be selected according to specific requirements.
(2) Range and measuring range:
Digital multimeter has many ranges, but its basic range accuracy is the highest. Many digital multimeters have automatic range function, so it is convenient, safe and rapid to measure without manual adjustment of range. There are also many digital multimeters that have over-range capability, so when the measured value exceeds this range but has not reached the maximum display, it is unnecessary to change the range, thus improving the accuracy and resolution.
(3) Accuracy:
The maximum allowable error of digital multimeter depends not only on its variable term error, but also on its fixed term error. When choosing, it depends on the requirements of stability error and linear error, and whether the resolution meets the requirements. If the general digital universal meter requires a level of 0.000 5 ~ 0.002, at least 6 and a half digits should be displayed; 0.005 ~ 0.01, at least 5 and a half digits shall be displayed; 0.02 ~ 0.05, at least 4 and a half digits shall be displayed; Below 0.1, at least 3 and a half digits shall be displayed.
(4) Input resistance and zero current:
Too low input resistance and too high zero current of digital multimeter will cause measurement errors. The key depends on the allowable limit value of the measuring device, that is, the internal resistance of the signal source. When the signal source impedance is high, the 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 field, magnetic field and various high-frequency noises or in long-distance measurement, it is easy to mix in interference signals, resulting in inaccurate readings. Therefore, instruments with high series and common-mode rejection ratio should be selected according to the use environment, especially for high-precision measurement, 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 digital multimeter is not limited to numbers, but also can display charts, words and symbols, which is convenient for field 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 digital multimeter is generally 220 V, but some new digital multimeters have a wide range of power supply, which can be between 100 and 240 V. Some small digital multimeters can be used with batteries, and some digital multimeters can use alternating current, internal nickel-cadmium batteries or external batteries.
(7) Response time, measuring speed and frequency range:
The shorter the response time, the better, but some meters have a long response time, and it will take some time for the readings to stabilize. The measurement speed should be based on whether it is combined with the system test. If it is combined, the speed is very important, and the faster the better. Frequency range, then choose it as needed.
(8) AC voltage conversion form:
Ac voltage measurement is divided into average conversion, peak conversion and effective value conversion. When the waveform distortion is large, the average value conversion and peak value conversion are inaccurate, while the effective value conversion can be unaffected by the waveform, making the measurement results more accurate.
(9) Resistance connection mode:
There are four-wire and two-wire connection methods for resistance measurement. When carrying out small resistance and high precision measurement, the four-wire resistance measurement connection mode should be selected.
With the development of large-scale integrated circuits and display technology, digital multimeters are gradually developing towards miniaturization, low power consumption and low cost, and digital multimeters are obviously divided into portable and desktop types. Portable is generally 3-and-a-half or 4-and-a-half, with small volume, light weight and low power consumption, which is suitable for production workshop or field use; The desktop can reach 6 and a half bits or 7 and a half bits, and its accuracy and discrimination are getting higher and higher. It adopts microprocessor and GPIP interface equipment, and is used as a standard meter and precision measurement in metrology, scientific research and production departments.
In short, it is not necessary to meet all the above conditions when choosing, and the most appropriate digital multimeter should be selected according to the specific requirements of use.
