Handheld Digital Multimeters for Modern Industrial Troubleshooting

Industrial electrical troubleshooting has become more demanding as facilities are increasingly equipped with electronic loads, solid-state controls, switch-mode power supplies, and variable-frequency drives. These systems often draw non-sinusoidal current waveforms, which make an average-responding multimeter inaccurate because it assumes a clean sine wave. Maintenance engineers need measurements that show the True RMS value of distorted waveforms rather than an ideal AC waveform.

Safety requirements have also changed. Measurements taken near the utility grid connection have lower source impedance, which exposes test instruments to transient overvoltages. At the same time, many industrial faults are intermittent. A loose connection, voltage sag, or contact drop may appear briefly or develop over hours. Diagnosing these problems requires more than a single instantaneous reading.

The B&K Precision 390B series handheld digital multimeters are designed for professional electrical and industrial maintenance work. Across the family, the meters offer True RMS measurement, safety-related input protection, high-resolution dual displays, and connected data capture. The 394B extends the family into process control testing with current loop source, measurement, and simulation functions.

Measuring distorted waveforms

The 390B Series consists of four handheld True RMS digital multimeters: the 390B, 391B, 393B, and 394B. Across the family, the meters support core electrical measurement functions, such as DC voltage, AC voltage, AC+DC measurement, DC current, AC current, resistance, frequency, diode test, and continuity. The 390B, 391B, and 393B also support capacitance and temperature measurement with a K-type thermocouple, while the 394B adds process-multimeter functions for current-loop applications.

Figure 1: The B&K Precision 390B Series True RMS multimeter with up to 100,000-count, dual display. (Image source: B&K Precision)

The series supports broad AC and DC voltage measurements up to 1000 V, with CAT III 1000 V/CAT IV 600 V protection for use within the meter’s specified measurement-category limits. The 390B, 391B, and 393B support DC and AC current measurements up to 10 A on the ampere input, subject to a duty limit of 1 minute on the ampere input followed by a 20-minute rest period. This makes the meter suitable for direct-current checks on low-current loads and control circuits, while higher-current motors may require an appropriate current-clamp accessory.

When checking current draw, burden voltage is an important factor because the meter’s internal shunt introduces a small voltage drop into the measured circuit. For 390B, 391B, and 393B, burden voltage is specified at 2 mV/mA on the milliampere inputs and 60 mV/A on the ampere input. The same inputs are specified with overload protection of AC/DC 400 mA at the mA inputs and AC/DC 10 A at the A input.

Monitoring resistance connections is a common maintenance task in electrical panels, terminal blocks, and control wiring. The 390B, 391B, and 393B provide resistance measurements up to 40 MΩ, while the 394B extends this range to 50 MΩ. For continuity testing, the 390B, 391B, and 393B provide an adjustable threshold between 10 Ω to 50 Ω with a buzzer response time of less than 10 ms for technicians to move quickly across dense wiring points.

Diode testing is supported with a 2 V range. For the 390B, 391B, and 393B, the diode-test function uses an open-circuit voltage of 2.5 V and a short-circuit current of 1 mA, while the 394B specifies a 2.000 V diode-test range with ±1 mA test current and a maximum open-circuit voltage of ±3.5 V.

The 390B, 391B, and 393B support capacitance measurement capability up to 40 mF. Frequency measurement up to 100 kHz supports checks on generators, control signals, sensors, and low-frequency drive outputs.

For temperature diagnostics, the 390B, 391B, and 393B support external K-type thermocouple measurement from -200°C to 1200°C. This allows maintenance engineers to correlate electrical measurements with the thermal characteristics of transformer windings or power components.

Resolution, accuracy, and what display count really means

A common point of confusion in test-and-measurement is the relationship between the display count, measurement resolution, and baseline accuracy.

The 390B and 391B are 40,000-count instruments, the 393B is a 100,000-count instrument, and the 394B is a 50,000-count process multimeter. The higher count increases the number of distinct display increments available on a range. This improves visible resolution, allowing small changes to be seen without forcing a range change.

However, display count is not the same as accuracy. By definition, resolution is the smallest change the display can show, and accuracy defines how close the displayed value is expected to be to the actual value under specified conditions.

According to the 390B Series datasheet, accuracy is expressed as ± (% of reading + counts of the least significant digit) at 23°C ± 5°C, with relative humidity below 80% RH and a one-year calibration cycle. Outside the 18°C to 28°C range, the specified temperature coefficient adds 0.1 × the stated accuracy per °C.

This implies that a 100,000-count meter is not automatically more accurate than a 40,000-count meter in every situation. The higher-count display can make small deltas easier to see, but confidence in those deltas still depends on the selected range, the accuracy specification, ambient temperature, calibration status, and waveform frequency.

The addition of a dual-line display allows two measurements, or one measurement plus a math function, to appear simultaneously on the screen. This is important because it allows the field technician to monitor a primary electrical quantity while also viewing a related derived value or a secondary reading needed for interpretation.

Diagnostics through Bluetooth and isolated USB

Data collection is critical for preventative maintenance programs. The 390B Series offers two primary data interfaces: wireless Bluetooth connectivity and an optically isolated USB port.

The model 390B comes with integrated Bluetooth connectivity for direct connection to the B&K Precision Link mobile application on compatible Android or iOS devices. This wireless link provides several advantages in field environments such as enhanced operator safety, remote, multi-point monitoring, and mobile data logging.

For long-term bench logging or field monitoring near sensitive computer equipment, all models in the 390B Series include a hardware-isolated USB interface. Standard USB cables establish a direct copper connection between the multimeter’s input terminals and the computer’s chassis ground. If a fault occurs while measuring a high-energy industrial circuit, high-voltage transients can travel down the USB. The 390B Series prevents this by using an optical isolation barrier.

The series supports various data logging configurations to capture intermittent faults, monitor system changes, and track startup behavior. Field technicians can configure the multimeter to record measurements directly to its internal non-volatile memory. The 390B can store up to 40,000 readings, while models 391B and 393B can store up to 20,000 readings.

Using the provided PC application software, technicians can log up to 100,000 data points (for 394B) in graphical or tabular format. The software can display logged data in real time to make it easy to identify trend shifts, voltage sags, or thermally induced drift over hours or days.

For technicians working in noisy, electrically dense environments, the B&K Precision 390B Series shows how a handheld multimeter can move beyond basic voltage, current, and resistance checks to support more complex industrial diagnostic work. By integrating True RMS measurement, high-resolution dual-display operation, and process-control functions across the product family, the series gives maintenance teams a broader view of what is happening within electrical and control systems.

To learn more, visit B&K Precision 390B Series handheld digital multimeters.