# What is my multimeter measuring when in AC mode?

My multimeter says "True RMS".

Nowhere in the manual do they say that when in AC mode, the value displayed will be the RMS. Appearently it is so obvious for everybody that they just don't say it.

Is it always assumed by default that a multimeter will measure RMS in AC mode? Just making sure.

• In general, I tend to assume that if a meter is cheap and analog then it does an AC average because of the meter movement. If the meter is cheap and digital, I will probably assume no RMS and that it also uses some kind of averaging, but probably different than an analog meter movement. If it is an expensive meter and digital then I tend to expect that it uses RMS measurement. But I don't assume. I'd read the manual, at the very least even then. Expensive analog meters are a ?? Usually, there, I expect a really really sensitive d'arsonval movement (like a 0.2 uA movement.) – jonk Apr 17 at 0:39
• If you tell us the model number, maybe we can help you decide. – Mattman944 Apr 17 at 1:27
• All meters that I’ve come across at least try to indicate RMS, I think that’s what you’re asking. In other words they don’t indicate peak-to-peak or zero-to-peak (unless that’s available as a special function). So-called True RMS is calculated by sampling the voltage at high speed, squaring each sample, adding them together, dividing by the number of samples and then taking the square root. That takes a moderate amount of computational effort and so some meters use different methods to approximate to this. – Frog Apr 17 at 1:42
• The multimeter is an OW16B the brand is OWON – Daniel Melo Avila Apr 17 at 1:50
• Daniel - OWON OW16B claims True RMS for AC measurements. Most any AC multimeter will measure RMS volts or RMS current for sine waves. Only a true RMS meter will properly measure RMS volts for waveforms other than sine shape. – glen_geek Apr 17 at 2:14

Since this is a cheap DMM, I suspect that it is not true RMS.

1. This is an expensive feature to do properly.
2. Most people won't care.
3. The specification doesn't list a crest factor for which the specification is valid.

If you have a function generator, you can run tests to determine if it is true RMS. Measure various waveform types and compare with the table below. Use 100 Hz, a typical meter is only accurate from about 50 Hz to a few hundred Hz. I used a peak voltage of 5 Volts, you can adjust the table if you can't get this from your function generator, I have provided the formulas.

Next, realize what an average reading meter will do. First, it will calculate the average of the absolute value of the signal. Then, it will assume that the signal is a sine and multiply the average by a correction factor. With show formulas enabled: My Fluke 111 reads 3.544 Vrms, 2.894 Vrms, 4.997 Vrms for sine, triangle, square respectively. All are within 0.3%.

You can find the average and RMS factors on the internet, or you can derive them if you are good with calculus.

Many (most?) meters will AC couple the signal with a cap if you are on the AC setting. My Fluke 111 AC couples, removing any DC offset first.

A good quality true RMS meter will always specify the crest factor that the measurements are valid for. https://en.wikipedia.org/wiki/Crest_factor