Why buy a True RMS multimeter?

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Not every electrical technician knows that electrical measurements are not what they appear to be. Direct Current ( DC ) is straightforward, but when we use a multimeter to measure Alternating Current ( AC ) the plot thickens because as most technicians realise is that AC is transmitted using a sine wave and therefore is constantly changing in voltage.

The diagram below paints the picture with respect to AC and DC : voltage :











The term “RMS” stands for “Root-Mean-Squared”. Most books define this as the “amount of AC power that produces the same heating effect as an equivalent DC power”, Note that the RMS value is not the same as the average of all the instantaneous values.  Most multi-meters, either voltmeters or ammeters, measure RMS value assuming a pure sinusoidal waveform.

One method to measure AC current would be take current measurements at increments across one complete cycle and average them together This would give us an average value of the current. If the current is a perfect sine wave, mathematically, the average value is always 0.636 times the value of the peak amplitude:














This is commonly done in an "average responding multimeter" these are widely used and are usually lower cost. They give correct readings for linear loads such as heaters, standard induction motors and incandescent lights. 

For AC currents, they are graphically represented by a sine wave, the RMS current will always be 0.707 times the peak Voltage. With that said, we can calculate current by multiplying peak measurements by 0.707 provided the current is a pure sine wave. However, pure sine waves are rare in most commercial and industrial applications and this is because resistive loads in commercial applications are not linear which results in unpredictable or variable current requirements.

Although Meter Shop Australia sells multimeters that can calculate current using any of the above methods, the most accurate way to calculate current in my opinion is a True RMS method. Average  values often are 40% less than True RMS values and that could mean the difference between blown circuit breakers, malfunctioning motors, or worst case, potential fire hazards. True RMS multimeters only cost about 20-30% more than the alternative. How much is an accurate current reading worth to you?


But why buy a TRUE RMS MULTIMETER ?

Electrical power system components such as fuses, bus bars, conductors, and thermal elements of circuit breakers are rated in rms current because their main limitation has to do with heat dissipation. If we want to check an electrical circuit for  overloading, we need to measure the rms current and compare the measured value to the rated value for the component in question. The table below shows the comparision of average responding vs a true RMS unit:

 

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