By Sean Anderson, Fluke Application Specialist
For many buyers, the choice between a True-RMS multimeter and a more affordable, average-responding meter can be confusing. While True-RMS meters excel at measuring distorted or non-sinusoidal waveforms, they do cost more. So, when is that extra expense worth it?
What is true-RMS? Watch this video to learn about true-RMS and average-responding meters.
Understanding True-RMS vs. Average-Responding Meters
True-RMS meters measure root mean square (RMS) by sampling the waveform, squaring each value, averaging the squares, and then taking the square root of that average. This method accounts for any distortion or harmonics in the signal, providing accurate measurements even for complex waveforms.
On the other hand, average-responding meters measure the average value of the waveform, then multiply by a constant to estimate RMS — assuming a pure sine wave. If the waveform is indeed a clean sine wave, this approach works well. However, it can lead to errors when distorted or complex signals are involved.
When True-RMS Is Essential
There are some scenarios where having a True-RMS multimeter is non-negotiable. Here are a few common examples:
- Industrial environments with variable frequency drives (VFDs). VFDs create waveforms filled with harmonic distortion. True-RMS meters accurately measure the total voltage and current in such scenarios, where average-responding meters often fail.
- Modern electronics with switching power supplies. Devices like computers, LED lighting, and chargers draw current in short bursts, producing non-sinusoidal waveforms. A True-RMS meter captures these peaks accurately.
- HVAC systems using variable speed motors. High-frequency components from variable-speed drives require precise measurement, which only True-RMS meters can provide.
- Power quality analysis. Diagnosing harmonics, flicker, or voltage irregularities demands accurate readings. True-RMS meters ensure reliable power quality data.
- Renewable energy systems. Solar inverters and other renewable sources often produce waveforms that aren’t purely sinusoidal. Only True-RMS measurements can give you an accurate picture.
- Automotive systems. There are several systems in vehicles where True-RMS meters are necessary for accurate measurements:
- PWM signals: Fast-switching signals (e.g., in EVs, hybrids, or throttle controls) are often distorted, making True-RMS meters necessary for capturing precise data.
- Ripple testing: Alternators generate DC with an AC ripple. If diodes fail, a True-RMS meter can precisely measure that ripple.
- EV/hybrid power electronics: High-frequency switching for inverters and converters necessitates True-RMS accuracy.
When an Average-Responding Meter Is Sufficient
An average-responding meter can be a suitable tool for many troubleshooting scenarios. Some common use cases include:
- Residential electrical work. Most homes have near-sinusoidal AC (though modern loads like LED bulbs and smart devices can add some distortion). For basic troubleshooting — outlets, fixtures, wiring — an average-responding meter often suffices.
- Basic appliance repair. Appliances such as refrigerators, ovens, and washing machines typically run on fairly standard AC waveforms. Unless you’re troubleshooting their internal power electronics, precision beyond average response isn’t usually required.
- Education and training. For beginners learning electrical measurements, a low-cost, average-responding meter is a good starting tool.
- Standard automotive diagnostics. Checking battery voltage (DC) or minor AC ripple doesn’t necessarily need the added accuracy of a True-RMS meter.
- Marine electrical systems. Boats often use 12V or 24V DC systems. Most simple checks won’t require True-RMS capability unless diagnosing alternator output or complex onboard electronics.
Deciding Factors
When choosing between a True-RMS or average-responding meter, it can help to weigh your options based on specific features and capabilities. Consider the following factors for your use case:
Waveform Type
Are the signals you measure primarily clean sine waves or heavily distorted? If distorted, you need a True-RMS meter.
Budget and Brand
For a reliable and accurate multimeter, choose a brand like Fluke, which has a strong reputation for producing high-quality measurement equipment. You don’t want to jeopardize your measurements or personal safety by purchasing electrical tools from sellers that lack the experience or reliability of an established brand.
For standard waveforms and a reliable average-responding option, consider the Fluke 15B+ Digital Multimeter, an affordable option under $200 for basic troubleshooting. It’s a well-regarded meter with a solid reputation for durability and accuracy under typical conditions.
If you’re regularly dealing with complex or distorted waveforms, look into a True-RMS model like the Fluke 117 Electrician’s Multimeter with Non-Contact Voltage, which offers enhanced measurement accuracy and additional measurement capabilities.
Application
Consider your use case when weighing multimeter options.
Industrial, HVAC, renewable energy, automotive (EV/hybrid), and telecom: These typically demand True-RMS for accurate measurements.
Residential, simple DC circuits, and routine troubleshooting: An average-responding meter is often sufficient.
Future-Proofing
If you expect your work might eventually involve more advanced measurements (e.g., harmonics, switching supplies), investing in a True-RMS meter now could save you from upgrading later.
Conclusion
A True-RMS multimeter gathers the most accurate measurements when working with non-sinusoidal waveforms, which especially matters in industries or applications with harmonic distortion, switching power supplies, and variable-frequency drives. However, for residential, light commercial, and standard DC measurements, an average-responding meter is both cost-effective and practical.
By weighing factors like waveform type, budget, and application, you can choose the right meter for your needs. If you’re still uncertain, consult a trusted professional or distributor who can help steer you in the right direction. Whether you go with a reliable average-responding model like the Fluke 15B+ or a higher-end True-RMS tool like the Fluke 117, the key is selecting a meter that matches your measurement requirements.
About the Author
Sean Anderson has been a product application specialist at Fluke since 2024, drawing on his AD01 Electrical Administrator background and power systems knowledge to deliver comprehensive support and expert guidance for users of the multimeter, clamp meter, and oscilloscope product lines. Since joining Fluke, Sean has focused on customer-facing research to assist in developing innovative new products and educational resources for core electrical test and measurement tools. With a commitment to customer satisfaction, great user experience, and clear communication, he continues to empower professionals in the field through hands-on service and support. Connect with Sean on LinkedIn.