UNI-T UT139S 6000-Count True RMS Digital Multimeter: The Ultimate Guide

Overview

The UNI-T UT139S is a professional-grade, 6000-count true RMS digital multimeter from the advanced UT139 series, designed specifically for electricians, HVAC technicians, and maintenance professionals who demand exceptional visibility and advanced diagnostic features . This compact, auto-ranging instrument stands out with its unique black EBTN (Enhanced Brightness Twisted Nematic) display, which provides unparalleled contrast and readability in all lighting conditions—especially in dark environments .

Engineered with specialized functions like LoZ (Low Impedance) for eliminating ghost voltages and LPF (Low Pass Filter) for accurate variable frequency drive measurements, the UT139S delivers professional-grade diagnostics typically found in much more expensive meters . With its rugged construction, comprehensive safety certifications, and full range of measurement capabilities including temperature, it represents the pinnacle of the UT139 series alongside the UT139E .

Key Features at a Glance

• 6000-Count True RMS Display: High-resolution backlit EBTN (black) display with 31-segment analog bar graph for precise readings and trend observation—provides exceptional contrast and visibility in dark environments .

• LoZ (Low Impedance) Mode: Exclusive function that eliminates "ghost voltages" (capacitively coupled voltages) by presenting a low impedance load to the circuit, ensuring you only measure real voltages .

• LPF (Low Pass Filter) for VFDs: Built-in filter removes high-frequency noise for accurate voltage and frequency measurements on variable frequency drives, inverters, and motor controls .

• Wide Measurement Range: Measures DC/AC voltage up to 600V, DC/AC current up to 10A, resistance up to 60MΩ, and capacitance up to 99.99mF .

• Temperature Measurement: Built-in K-type thermocouple support with included probe for readings from -40°C to 1000°C (-40°F to 1832°F)—essential for HVAC diagnostics .

• Frequency & Duty Cycle: Measures frequency from 10Hz up to 10MHz and duty cycle from 0.1% to 99.9% for advanced signal analysis .

• Non-Contact Voltage (NCV) Detection: Safely identify live AC circuits with a multi-level audio and visual alarm that indicates field strength .

• 31-Segment Analog Bar Graph: Provides real-time visual indication of changing values and trends, complementing the digital readout .

• Comprehensive Testing: Includes diode test (around 3V open circuit), continuity test with acoustic indication (<50Ω), MAX/MIN/REL modes, data hold, and auto power off .

• User-Friendly Design: Auto and manual ranging, adjustable backlight brightness, 1-meter drop protection with rubber holster, and low battery indication .

• Safety Certified: CAT III 600V rating with full-range protection, compliant with CE, UKCA, and cETLus standards .

Detailed Specifications

The UNI-T UT139S delivers professional-grade accuracy across an extensive range of electrical and environmental parameters .

Measurement Ranges & Accuracy

General & Mechanical Specifications

Special Functions

What's in the Box

UT139 Series Comparison: UT139S vs. UT139A/B/C/E

The UT139 series offers several models with varying capabilities, with the UT139S serving as the most advanced model alongside the UT139E :

Note: The main differences between the UT139S and UT139E are the display type (black EBTN vs. standard TN) and battery consumption (higher on the S due to the EBTN display) . Both include LoZ and LPF functions.

Key Functions & Operation

1. LoZ (Low Impedance) Mode for Ghost Voltage Elimination

• Set the rotary switch to the LoZ position .

• This mode presents a low impedance load (via a PTC) to the circuit, bleeding off capacitively coupled "ghost voltages" .

• Application: Essential for verifying whether a circuit is truly de-energized before working on it, especially in long cable runs or high-capacitance systems .

2. LPF (Low Pass Filter) for VFD Measurements

• In AC voltage mode, press the blue SELECT/LPF button to activate the low pass filter .

• This filters out high-frequency carrier signals and noise, allowing accurate measurement of fundamental voltage and frequency on variable frequency drives (VFDs), inverters, and motor controls .

3. EBTN Display & Adjustable Backlight

• The high-contrast black EBTN display provides exceptional readability in all lighting conditions .

• Press and hold the yellow HOLD/light button to cycle through backlight brightness levels .

4. NCV (Non-Contact Voltage) Detection

• Set the rotary switch to the NCV position .

• Bring the front of the meter near a live conductor. The LCD displays 1-4 bars, a red LED lights up, and the buzzer sounds—all increasing with field strength .

5. Temperature Measurement

• Set the rotary switch to the °C position .

• Connect the included K-type thermocouple to the meter .

• Place the probe on the object to measure temperature from -40°C to 1000°C .

• Application: Essential for HVAC diagnostics, furnace troubleshooting, and appliance repair.

6. DC/AC Voltage Measurement (Up to 600V)

• Set the rotary switch to the V position (or mV for millivolt ranges) .

• Press SELECT to choose between DC and AC measurement .

• Connect test leads in parallel to the circuit under test.

• Input impedance is ~10-11MΩ on standard ranges, 5MΩ on mV AC range .

7. DC/AC Current Measurement (Up to 10A)

• Set the rotary switch to the appropriate μA, mA, or A position based on expected current .

• Press SELECT to choose between DC and AC current.

• Connect test leads in series with the circuit under test.

• Warning: The 10A range can only withstand 10+ amps for a short time; an audible alarm sounds above 19A .

• Fuse protection: 0.6A/600V fast-acting fuse (μA/mA); 10A/600V fuse (A) .

8. Frequency & Duty Cycle

• In any AC voltage or current range, press the Hz% button to display frequency and duty cycle .

• Measures frequencies up to 10MHz and duty cycles from 0.1% to 99.9% .

• Application: Signal analysis, PWM troubleshooting, and motor drive diagnostics.

9. Capacitance Measurement (Up to 99.99mF)

• Set the rotary switch to the ohm position, then press SELECT to cycle to capacitance .

• Fully discharge capacitors before testing.

• Connect test leads in parallel with the capacitor.

• 70,000μF (70mF) takes approximately 7 seconds to measure .

10. Resistance, Continuity & Diode Test

• Set the rotary switch to the ohm position .

• Press SELECT to cycle through functions.

• Resistance: Up to 60MΩ, takes about 2.5 seconds to measure 100Ω .

• Continuity: Very fast (~10ms response), beeps below 50Ω .

• Diode: ~3.3V open circuit voltage, max display 3.000V .

11. MAX/MIN & Relative Mode

• Press MAX/MIN to record the highest and lowest readings .

• Press REL to set a reference value for comparative measurements .

12. Data Hold & Auto Power Off

• Press HOLD to freeze the current reading .

• Meter automatically shuts down after approximately 15 minutes .

• Hold SELECT while turning on to disable auto power off .

Safety Features

The UT139S incorporates multiple advanced safety features to protect users :

• CAT III 600V Rating: Certified for measurements on distribution circuits and permanently installed equipment.

• Full-Range Overload Protection: Comprehensive protection on all measurement ranges.

• Fuse Protection: Ceramic fuses on current ranges (0.6A/600V and 10A/600V) with high breaking capacity.

• PTC and MOV Protection: Multiple protection components on voltage input paths.

• NCV Detection: Allows safe, non-contact verification of live circuits.

• Low Battery Indication: Prevents false readings due to insufficient power.

• Double-Insulated Housing: Rubber holster provides 1-meter drop protection.

Applications

The UNI-T UT139S is an ideal all-in-one tool for :

⚡ Electrical Installation & Commercial Electricians

• LoZ mode to eliminate ghost voltages and verify de-energized circuits .

• Safe voltage measurements up to CAT III 600V.

• NCV detection for non-contact live wire identification.

• Continuity testing for wiring and connections.

🔧 HVAC/R Technicians

• Temperature measurement with included probe (-40°C to 1000°C) .

• LPF mode for accurate measurements on variable frequency drives and motor controls .

• Testing start/run capacitors up to 99.99mF.

• Motor and compressor current measurement.

🏭 Industrial Maintenance & VFD Troubleshooting

• LPF mode for diagnosing variable frequency drives and inverters .

• Frequency analysis up to 10MHz for signal integrity testing.

• Capacitance measurement for power supply filtering.

🔧 Electronics Repair & Diagnostics

• Precision component testing with 6000-count resolution.

• Diode and continuity testing for PCB troubleshooting.

• Frequency and duty cycle analysis for PWM signals.

🏠 DIY & Home Workshop

• General electrical troubleshooting.

• Appliance repair and maintenance.

• Battery testing and continuity checks.

• Temperature measurement for ovens, dryers, and HVAC systems.

Customer Reviews & Real-World Performance

Users consistently praise the UT139S for its excellent build quality, unique EBTN display, and advanced features .

Positive Feedback

• EBTN Display: "The Enhanced Background Twisted Nematic (EBTN) display is a HUGE extra bonus" — Verified eBay reviewer .

• LoZ and LPF Functions: "I LOVE THE LoZ and LPF functions! ... the UT139S can operate in any electrical environment" — Verified eBay reviewer .

• Build Quality: "I felt the usual UNI-T sense of reliability and confidence" — Verified eBay reviewer .

• Protection: A detailed teardown review noted "With large ceramic fuses, MOV's, PTC's and multiple resistors in series the protection looks fairly good" .

• Continuity Speed: "Continuity is very fast (About 10ms)" .

• Value: Reviewers consistently note that the UT139S offers exceptional value for professionals who need advanced features like LoZ and LPF .

Considerations

• Battery Life: The EBTN display consumes more power—current consumption is 21mA with normal backlight vs. 11mA on lower settings . This is a trade-off for the superior display.

• Frequency Sensitivity: Requires higher input voltage (∼1.2V) to measure frequencies above 8MHz .

• Continuity Threshold: Beeps below 50Ω, which may be slightly different from some professionals' expectations.

• High Resistance Stability: Meter is "not very stable or precise at very high ohm values (50Mohm+)" .

UT139S vs. Competitors

Why Choose the UNI-T UT139S?

Final Verdict

The UNI-T UT139S is the definitive choice for professionals who need superior display visibility combined with advanced diagnostic features in a rugged, reliable multimeter. Its unique EBTN black display, exclusive LoZ and LPF functions, and comprehensive measurement capabilities—including temperature and 99.99mF capacitance—make it exceptionally well-suited for electricians, HVAC technicians, and industrial maintenance professionals who work in varied lighting conditions and with modern electronic equipment .

A detailed reviewer concluded: "This meter has all the common ranges and functions for a good multimeter... With large ceramic fuses, MOV's, PTC's and multiple resistors in series the protection looks fairly good. I will call it a good basic meter, both for low volt and mains usage" .

While the EBTN display consumes more battery power than standard LCDs, the trade-off is exceptional readability in dark environments—a feature that professionals working in dimly lit panels and electrical rooms will greatly appreciate .

Whether you're an electrician verifying de-energized circuits with LoZ, an HVAC technician measuring temperatures and capacitors, or an industrial maintenance professional troubleshooting VFDs with LPF, the UT139S delivers professional-grade performance with the visibility you need to work confidently in any environment.