Nine Major Principles for Selecting a Multimeter
① Functions In addition to the five basic functions for measuring AC/DC voltage, AC/DC current and resistance, digital multimeters also offer functions such as digital calculation, self-test, reading hold, error readout, diode test, word length selection, IEEE-488 interface or RS-232 interface. Select functions according to specific application requirements.
② Range and Span Digital multimeters have multiple measuring ranges, and the basic range delivers the highest accuracy. Many digital multimeters feature auto-ranging, which eliminates manual range adjustment for convenient, safe and fast measurements. Some meters have overrange capability. When the measured value exceeds the selected range but stays below the maximum display value, range switching is unnecessary, improving accuracy and resolution.
③ Accuracy The maximum permissible error of a digital multimeter includes both variable error terms and fixed error terms. When selecting a meter, check the requirements for stability error and linear error, and verify whether the resolution meets your needs. For general digital multimeters: class 0.0005 ~ 0.002 requires at least 6½-digit display; class 0.005 ~ 0.01 requires at least 5½-digit display; class 0.02 ~ 0.05 requires at least 4½-digit display; class 0.1 and below requires at least 3½-digit display.
④ Input Resistance and Zero Current Too low input resistance or excessive zero current of a digital multimeter will introduce measurement errors. The key is to check the limit permitted by the measuring setup, namely the internal resistance of the signal source. For high-impedance signal sources, select instruments with high input impedance and low zero current to minimize interference.
⑤ Series-Mode Rejection Ratio and Common-Mode Rejection Ratio Interference signals such as electric fields, magnetic fields and high-frequency noise may corrupt readings, especially for long-distance measurements. Choose instruments with high series-mode and common-mode rejection ratios according to operating conditions. For high-precision measurements in particular, pick digital multimeters equipped with the Guard terminal G to effectively suppress common-mode interference.
⑥ Display Type and Power Supply Digital multimeters can display not only numbers but also graphs, texts and symbols for convenient on-site observation, operation and management. Based on the physical size of display units, they are categorized into small, medium, large and extra-large types. Most digital multimeters use 220 V mains power. Newer models support wide input voltage ranging from 110 V to 240 V. Compact portable multimeters run on batteries. Some instruments support three power options: AC mains, built-in NiCd rechargeable batteries and external batteries.
⑦ Response Time, Measurement Speed and Frequency Range A shorter response time is preferred. Some meters have long response times and require waiting for readings to stabilize. Measurement speed matters for automated system testing; higher speed is desirable in such cases. Select the frequency range to match your application.
⑧ AC Voltage Conversion Method AC voltage measurement adopts average-value conversion, peak-value conversion or RMS conversion. Average and peak conversion produce inaccurate results under heavy waveform distortion. RMS conversion is waveform-independent and delivers more reliable measurement results.
⑨ Resistance Wiring Method Resistance measurement supports 4-wire and 2-wire connections. For low-resistance and high-precision measurements, choose a digital multimeter with the 4-wire resistance measurement function.
