True RMS Digital Multimeter

[Sahara]

how can i sure the multimeter is a true rms. what is the method to check an true rms multimeter.

 

[nguyennam]

That multimeter must be labeled with "true rms" on its case.

 

[Sahara]

hi, my question is not that true rms labled. i m intersted to know, what does true rms mean?. how it differ from simple multimeter

 

[laktronics]

Hi,
Normal multimeters assume the input to be a sine wave and calculates RMS value based on the peak voltage. Such meters show the same RMS value for a sine wave and a nonsine wave (say square wave), if both have the same peak value. True RMS meters use special ICs to calculate the true RMS value.

Regards,
Laktronics

 

[Sahara]

hi,

is there any calculation or equation for that. suppose if someone shows us demosntraion that see, this is my true rms multimeter. how can i crosscheck that he is right in his right in calculating rms voltage. i think multimeter will not show me sinewave vaule. so if my sine wave peak voltage is of 10 V. does it mean my multimeter will show me rms value of 10 V??

 

[subharpe]

True rms meters work on the principle of heating of a junction. Beacause the joule heatng is proportional to the rms value of the current, be it any type of waveform.

 

[Kripton2035]

more explainations here :
**broken link removed**

 

[laktronics]

Hi,
I meant normal meters assume the input to be sine wave and computes the RMS value from the average or peak value. So a sine wave of +/- 10V amplitude or a 50% duty cycle square wave of +/-10V amplitude, both will show 7.07 volts.
Even most of the true RMS meters do not take into account the DC componenet of the AC input. They measure the input AC after capacitive coupling. so that the DC componenet is eliminated and does not show up in the results. So even those meters are really not true RMS.

Regards,
Laktronics

 

[Kral]

Sahara,
Here's a simple test: Feed a symmetrical square wave into the meter. The RMS, average, and peak values of a symmetrical square are equal. If the meter indication is equal to the peak value of the square wave, then it's a true RMS meter. If the meter indicates .707 X the square wave peak or , then its a peak sensing, RMS indicatiing meter (not True RMS). If the meter indicates 1.118 X the square wave peak, then its an average sensing RMS indicationg meter (not true RMS).
regards,
Kral