To explain how a true rms multimeter works we will start off with explaining what RMS means. RMS stands for Root Mean Square. This means that it explains the effectiveness of a waveform.
This is a term that comes from the meter world. The term is created to make a difference in measurement principles. Unfortunately TRMS has no clear definition, it is just created as a marketing term. This is why some of the suppliers add some other values to a device description. Those values can look like: AC+DC, this particular example says that it is able to measure voltage and alternating current.
Back in the days there was also a term called: the ideal sinus. This was a formula that was used to measure the same value that is measured with a TRMS multimeter. But in this time we only use TRMS multimeters because the technique is a little more advanced, but more precise. And the devices are more professional because of the TRMS calculation system.
The differences in TRMS multimeters
There are multiple multimeters that have the TRMS option. But if you are looking for such a device then we need to look a little bit closer to the specifications. We also have to look at the possible measurement range. If we look closer at the range that we see that some of the devices only have the value AC on them, while there is no combined AC+DC measurement range. You will see that with an Fluke 179 for example.
The other thing you have to pay attention to is the bandwidth of the (T)RMS device. You have to look if the device matches the requirements for the object that you are going to measure. With other words, the device can be sold as a True RMS multimeter but it has a range of 45 up until 1000 kHz, or for example 20 till 400 Hz.
Here we have a list with more advanced TRMS multimeters and their ranges:
- 5 kHz (Fluke 83V)
- 10 kHz (Metrahit PRO)
- 20 kHz (Fluke 87V, Metrahit X-tra)
- 50 kHz (Metrahit X-tra, Metrix MTX 3281)
- 100 kHz (Fluke 287, Fluke 289)
- 200 kHz (Metrix MTX 3283)
When to use an True RMS multimeter?
We advise when you have to measure for professional purposes to use an TRMS device. For example, if you have to measure industrial installations very often it is much better to use an TRMS device to get the best results.