True RMS Performances of MP730655

What means RMS? RMS = root mean square. I am not going to discuss the details of the math formula for RMS here. The basic idea of RMS is to calculate the equivalent DC(direct current) value from an AC(alternative current) signals. I have a bench DMM, Tenma 72-1012, which is an average-responding DMM. I found that Tenma72-1012 is good at measuring Sine waveform signal, which having similar performance as a true RMS DMM. However, it is not accurate to measure square waveform and other non-Sine waveform signals.

Here is a rule of thumb for true RMS DMM vs average responding DMM AC regarding measurement performance.

I used 4 different  digital meters to measure AC voltage as comparison:

1. Fluke 177 Trum RMS DMM
2. MP730655, Trum RMS DMM
3. Tenma 72-1012 bench DMM (Average responding)
4. Rigol MSO5074 DMM feature

Sine waveform and square waveform are evaluated here.

A.Sine wave

Agilent 33120A function is used to generate sine wave from 50 Hz to 2K Hz as below for the bench testing. The Vpp_max is configured at 1V, which you can check from the oscilloscope.

As I mentioned before, Rigol oscilloscope is the gold reference reading for RMS measurement. A short testing spreadsheet for Vrms is presented in the table below.

The comparison performance is easier to check via visual line charts.

From this short testing, not all true RMS DMM perform in similar patterns.

Fluke 177 can give us the very good Vrms reading when the frequency of Sine waveform is below 600Hz among these 3 DMM equipment. Staring from 1300~1400 Hz, MP730655 performs better than other 2 DMM as the frequency of Sine wave increased. MP730655 Vrms reading error is generally flat for Sine waveform up to 2Khz during the bench testing, which is between 1~2%. If the frequency of Sine wave is 100Hz, MP730655 seems perform same as Fluke 177 and Tenma 72-1012(average responding).

B. Square wave

Agilent 33120A function is used to generate square wave from 50 Hz to 2K Hz as below.

The VAC voltage dial points to 2V setting for MP730655 and Tenmar 72-1012. A short testing for Vrms at Vpp=1V is presented in the table below.

The comparison performance is illustrated as line charts.

For non-Sine wave RMS measurement, MP730655 constantly gave 10% reading error. Fluke 177 can give me correct reading when the frequency of the square wave is lower than 100 Hz. Interestingly, Tenma 72-1012 can give me better reading when the frequency of the square wave is higher and higher. However for frequency lower than 200~300 Hz, Tenma 72-1012 has around 9% error.

I am more interested in square wave measurement for MP730655. So I conducted another  bench testing for square wave at Vpp=5V as below today. Then I can set the voltage dial at 20V for VAC of both MP730655 and Tenma 72-1012, which can give me more information about the correlation between Vrms measurement with the voltage range setting from DMM.

Here are the Vrms data from DMMs.

The comparison performance is illustrated as line charts.

The overall Vrms performance from all DMMs is quite surprised to me at the end of the testing. Fluke 177 can do correct job when square wave frequency is below 100Hz. MP730655 measurement can be trust with 10% known reading error when square wave frequency is blow 100Hz. Similar methodology is applied to Tenma 72-1012.  RMS measurement of non-Sine wave looks pretty challenging for all DMMs in general.

Conclusions

MP730655 is a very good low cost DMM for students or hobbyists. It can measure AC/DC voltage, resistance, capacitance and battery voltage. I did not run full review testing. However, from what I tested regarding true RMS for Sine and square waveform, MP730655 can do RMS for low frequency AC waveform measurement. 10% of the reading value might be subtracted from the reading value to get a better accuracy for AC waveform measurement. As a basic DMM in my basement EE lab, I am glad to have MP730655 join the family.