Used Digital Multimeters (DMMs) for Sale

Digit count describes resolution. A 4.5-digit meter reads up to 19,999 counts — the half-digit means the leading digit is either 0 or 1. A 5.5-digit meter reads up to 199,999 counts, and a 6.5-digit bench meter reads up to 1,199,999 counts. More digits mean finer resolution for the same measurement range: a 6.5-digit meter on its 10 V DC range resolves 10 µV, while a 4.5-digit meter on the same range resolves 1 mV. For production testing of precision resistors, voltage references, or current sources, 6.5 or 7.5 digits is standard. For general bench work and field service, 4.5 to 5.5 digits is more than adequate.

A True RMS meter measures the actual root-mean-square value of an AC signal regardless of waveform shape. An average-responding meter calibrated for RMS reads correctly only for pure sine waves — apply a non-sinusoidal waveform (PWM motor drives, switch-mode power supplies, variable frequency drives) and it gives an incorrect reading. True RMS is required whenever you measure non-sinusoidal AC voltages or currents. All Fluke 87, 175, 177, 179 and above, all Keithley 2000-series, and all Keysight 34xxx bench meters are True RMS. Basic meters like the Fluke 113 are average-responding.

The Fluke 87V (and its predecessor 87) became the industry standard handheld because it combines True RMS AC measurement, a low-impedance LoZ mode to eliminate ghost voltages, 1000 V CAT III / 600 V CAT IV safety ratings, temperature measurement, and a frequency counter in a single rugged instrument. It has a 0.05% DC accuracy specification — tighter than most competitors at its price point. For industrial electricians, HVAC technicians, and field engineers, the 87V covers every common measurement without switching instruments. Calibration is straightforward: annual NIST-traceable certificate is accepted by most quality management systems.

Both are 6.5-digit precision bench DMMs that became lab staples in the 1990s and 2000s. The HP/Agilent 34401A has a 0.0015% DC voltage accuracy and 3 µV sensitivity at 100 mV range. The Keithley 2000 has 0.002% DC voltage accuracy at 1 year and 100 nA current resolution — slightly better current measurement capability. The 34401A has a faster throughput (up to 1000 readings/s at 4.5 digits) making it preferred for data acquisition applications. The Keithley 2000 has a 10-channel scanner option and is preferred in Keithley test systems. Both are excellent choices for calibration labs, production test, and R&D.

AC input circuit failures are the most frequent repair — usually caused by overvoltage on the AC voltage input when the meter is set to DC or resistance. The input rectifier or RMS converter IC fails, causing incorrect AC readings or no AC at all. Input protection fuses blow from current overloads on the milliamp or microamp ranges — this is repairable but the replacement must match the original fuse rating exactly. EEPROM corruption causes self-test failures or calibration constant loss, typically after a power surge. All of these are bench-repairable. A meter that powers on and passes self-test but has one out-of-spec range is worth calibrating — the most common cause is drift in the reference resistor network.

Annual calibration is standard for quality management systems (ISO 9001, AS9100). A 6.5-digit bench meter like the Keithley 2002 or Agilent 34470A has calibration intervals spec'd at 1 year by the manufacturer. In practice, a meter stored at room temperature and used for normal lab measurements drifts very slowly — the dominant drift mechanism is the internal voltage reference, which is specified at parts-per-million per year. NIST-traceable calibration at Aumictech covers DC voltage, DC current, AC voltage, resistance, and frequency at multiple points across each range.

A system DMM (also called a bench DMM or rack DMM) is designed for computer-controlled automated test. It has an IEEE-488 (GPIB), USB, or LAN interface, accepts trigger signals from a test controller, and outputs readings in digital format for data collection. A handheld multimeter has a local display, battery power, and manual operation — no remote control. For production test, calibration benches, and research data acquisition, a system DMM is required. For field service, maintenance, and troubleshooting, a handheld is more practical. Many labs use both: a system DMM for automated characterization and a handheld for bench work.