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Understanding Terms - True RMS

When looking at multimeter specifications TRMS, True RMS or True Root Mean Square is often quoted.  But what is it… without the maths!

AC in electrical terms means ‘alternating current’.  This means the current has a cycle that goes from zero to a peak value in one direction, back to zero, then to a peak value in the opposite direction, returning to zero. It does this repeatedly.  

Depending on what point in time during the cycle you look at, you could measure a value anywhere between these two positive and negative peaks. 

So how can we take a measurement of an AC current that makes sense?

Well, let’s first get some sort of comparison to the AC current.  Let’s compare it with a DC current which is unchanging and continuous in one direction.  What values in the AC current can we find that will match what the equivalent DC current can do? 

Multimeter showing True RMS function

By doing a little bit of fancy maths (which we won’t) on the AC signal you can calculate a special type of average value, called a root mean square (RMS).

It turns out that this calculated current represents a figure from the AC supply that allows us to directly compare the AC and DC currents.  That is, a value where the same AC current will cause the same dissipated power in a resistor as the equivalent DC current. 

 

Some digital multimeters do not use a true RMS calculation to obtain a value, instead they take an average value and multiply it by a special factor that gives the equivalent RMS value. This is known as the averaging or mean method. (Here the negative values have to be flipped to positives or the average would be zero.)

With a pure sine wave, the two calculation methods, whether TRMS or averaging, will give the same result.  However, if there is any impurity or ‘noise’ in the sine wave, or the wave you are measuring is not a sine wave, then the averaging calculation method will have errors.  The True RMS method however, will still be accurate.  

This is why DMM brands make sure they advertise TRMS in their specifications if they use that method of calculation. 

Although current is used to explain the RMS value above, RMS value is also used to measure the alternating voltage. 

Note: a grid connected supply voltage is given as the RMS value of the supply, and a quoted current on apparatus designed for this supply is deemed to be an RMS value.

 

AC+DC

It is possible to have a wave form that is not symmetrical about the x axis (zero point). It may have more of the wave in the positive or negative region. This is known as a DC off set. 

It turns out that some digital multimeters that advertise True RMS cannot give accurate values for this situation because of the way RMS is derived.

There is great YouTube video here that explains True RMS and AC+DC in detail.

If any of your measuring is likely to fall into this category, then ensure that the meter has AC+DC function if accuracy is critical. The function will be mentioned in the specifications sheet and will include the accuracy margins.