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True RMS multimeter: when the wave is not a sine

Updated: October 2026

You are measuring AC on a dimmer, an LED driver, or a variable-speed tool, and the number on the meter looks lower than the circuit should be. That is the situation a true RMS multimeter is for. An average-responding meter assumes a sine wave. Many household controls do not give you one. Peter F. updated this page on 17 October 2026. We did not measure a dimmer in a lab. A qualifying purchase can earn a commission at no extra cost to you. The [Voltcraft multimeter comparison](/hub/measuring-equipment/voltcraft-multimeter) ranks the meters. None of the twelve is a Voltcraft, even where True RMS is printed.

You are measuring AC on a dimmer, an LED driver, or a variable-speed tool, and the number on the meter looks lower than the circuit should be. That is the situation a true RMS multimeter is for.

Our top picks

VENLAB VENLAB VM-600M

Best overall

VENLAB VENLAB VM-600M
8.7/10about £26.99
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Crenova Crenova MS8233D

Preis-Tipp

Crenova Crenova MS8233D
8.9/10about £22.99
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VENLAB VENLAB VM-600A

Rank 3

VENLAB VENLAB VM-600A
9.4/10about £29.99
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On this page
  1. 1.When the wave is not a sine
  2. 2.When you can ignore it
  3. 3.Which of these twelve print True RMS
  4. 4.What True RMS does not do

When the wave is not a sine

A clean sine is what an average-responding meter was built to read. A resistive heater on an ordinary socket is close to that shape. A dimmer, many LED drivers, and inverter speed controls chop the wave. The heating effect of that wave is the RMS value, and an average-responding circuit is not calculating it. Fluke's UK note says an average-responding tool can read up to 40% low on a non-linear load. That figure is Fluke's, not a test we ran, and it is not a promise that your dimmer is exactly 40% out. The words True RMS have to be printed. A 6000-count screen does not imply them.

When you can ignore it

You can ignore True RMS on DC. A resting 12 V car battery is state of charge, and the RMS method does not change that number or prove the battery will crank. Leave hybrid packs out. You can also ignore it on a straightforward sine, such as a simple resistive heater, when a rough AC voltage is enough. Counts are a different specification. Several rows print 6000, the FNIRSI prints 9999, and the BSIDE prints 20000. Rapid's VC232, which is not an ASIN here, is 2000 counts and still prints True RMS. The how to use page still applies: red lead in the volts jack, and no claim that the reading proves a circuit dead.
VENLAB VENLAB VM-600M

Testsieger

VENLAB VENLAB VM-600M
8.7/10about £26.99
View offer

Which of these twelve print True RMS

Printed: the VM-600A, the VM-600M, the 890Z, the Proster, the Kethvoz, the UNI-T UT892, the BSIDE S500, and the FNIRSI DMT-99. Not printed: the MS8233D, the RICOKEY, the Beizkna, and the coczow. Do not copy True RMS from the 890Z onto the MS8233D. Rapid's VC191 and VC232 print it and are not rows here. The best meters page still puts the VM-600A first, because it prints True RMS and CAT III 600 V together.

What True RMS does not do

True RMS does not add a CAT voltage the listing forgot. The FNIRSI and the BSIDE print the words and no CAT rating. The 890Z prints CAT III with no voltage. The Kethvoz prints it on a clamp rated CAT III 300 V, which is not CAT III 600 V. The buying guide keeps the brand check separate, so a True RMS VENLAB is still a VENLAB. It also does not prove a circuit dead. GS38 still wants a test lamp or a two-pole detector, proved before and after. If the job is a battery or a fuse on the bench, the MS8233D is enough.

Common questions

What does True RMS change?
It changes the way the meter turns an AC wave into a number when the wave is not a sine. Fluke says an average-responding meter can read up to 40% low on a non-linear load. That figure is Fluke's, not a measurement we made. On a clean sine, such as a simple heater, the gap is much smaller and an average-responding meter can be enough.
Do I need it for a 12 V battery?
No. A resting car battery is DC. True RMS is an AC method. The reading is state of charge, not a cranking test, and the method printed on the meter does not alter a DC volts figure. Use the volts jack. A hybrid pack is outside this check.
Does the Crenova MS8233D print it?
No. The MS8233D bullets do not say True RMS. The other Crenova in the set, the 890Z, does say it in the title. You cannot transfer the words from one model to the other. The MS8233D remains the best value for a printed CAT III 600 V line, with that gap left in the open.
Is 6000 counts the same thing?
No. Counts are resolution. True RMS is the AC measurement method. Several 6000-count meters in this set print True RMS, and the MS8233D is 6000 counts without those words. Rapid's VC232 is 2000 counts and does print True RMS, and it is not an ASIN here.
Does True RMS make the meter safe on a consumer unit?
No. Safety for a place in the installation is the category plus a voltage, and even that line is the seller's claim. True RMS does not add CAT III 600 V to a meter that does not print it. It also does not replace the GS38 method for proving a circuit dead.
Where do Rapid's Voltcraft handhelds stand?
The VC191 and the VC232 both print True RMS on the Rapid pages, with CAT III 600 V, at £55.57 and £45.72 excluding VAT. They are not among the twelve Amazon files. Origin on those pages is China. If you need that brand and this method together, use those pages rather than a VENLAB row that happens to print the same words.

Related pages

Multimeter comparison

Voltcraft multimeter

Best multimeter in this amazon set: shortlist 2026

Voltcraft multimeter UK: brand, price and listings

CAT III 600V multimeter guide: category and voltage

How to use a multimeter: DC volts, continuity, stop

Sources

Public UK sources from the research digest for this comparison: Rapid Electronics for the Voltcraft brand and the VC191, VC232 and VC-86 pages, the separate amazon.co.uk VC-86 listing, HSE GS38, the Electricity at Work Regulations 1989 and Fluke. Listing prices, star lines and review themes for the 12 models are the stored amazon.co.uk figures, not a fresh page view. Updated 17 October 2026.

  1. Rapid Electronics, Voltcraft brand (retrieved 17 October 2026)
  2. Rapid Electronics, Voltcraft VC191 (retrieved 17 October 2026)
  3. Rapid Electronics, Voltcraft VC232 (retrieved 17 October 2026)
  4. Rapid Electronics, Voltcraft VC-86 (retrieved 17 October 2026)
  5. amazon.co.uk, Voltcraft VC-86 listing (retrieved 17 October 2026)
  6. HSE GS38, electrical test equipment for electricians (retrieved 17 October 2026)
  7. Electricity at Work Regulations 1989, regulation 16 (retrieved 17 October 2026)
  8. Fluke, true measurements of non-linear loads (retrieved 17 October 2026)