Using True RMS Meters in
HVAC/R Applications
Many HVAC/R equipment have a more complex operation, rather than just switching on and off. Due to the multiple functions, you will need a True RMS meter to measure these loads. True RMS means "root mean square". This is essentially the measurement of AC voltage or current in a way that reflects the amount of work that can be accomplished by an equal amount of DC voltage or current. This is important when measuring voltage or current in variable speed motors and other switching equipment. The mathematical formula is:
(RMS reading) x (1.414) = Peak to Peak reading
DC voltage and current values dont change if there is a change in the load or supply. The formula for power is very simple.
Power (W) = voltage (V) X current (A)
However, AC voltage and current change very quickly, even if the load isnt changing. The formula for power can be very complex. The RMS values are used to simplify the relationships of current and voltage to power on AC lines. AC RMS values are used in the same basic formula as for DC.
The AC voltage for a 120 VAC power line varies from +169V to -169V. It does this cycle 60 times a second. If the load is a resistive load, like a heater, the current will go up and down exactly the same as the voltage. The "waveforms" (the plot of voltage or current vs. time) will look the same. They will be sine waves. Nice smooth waveforms that go, in the case of a 120VAC line, from 0 to 169V to 0 to 169V and back to zero, 60 times a second.
Digital multimeters and clamp meters are designed to display the value thats "equivalent" to the DC value. By "equivalent", they mean the value that will work in the simple power calculations for DC. This is the "RMS" (root mean square) value. Most meters display the RMS value for a sine wave. They call this "average responding, RMS indicating".
In HVAC/R, most voltage and current signals are sine waves because the loads are from AC motors. For these sine waves, an "average" measuring meter will give you the same exact reading as a True RMS meter. However, variable speed fan motors, heaters and some control signals distort the signal to a non-sine waveform. For these types of equipment, a True RMS meter is needed to measure an equivalent DC value that can be used in the power formula.
