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A true root mean square (RMS) multimeter is a measurement device that measures the effective level of alternating current (AC) or voltage. Root mean square is a mathematical term that suggests effective level. Other multimeters may be designed to measure just the average level. For power computations, the true RMS multimeter provides the correct voltage or current level.
Several units can be used when measuring voltage. Direct current (DC) is simple because there the only unit is the volts direct current (VDC). A 12 VDC power supply will provide 12 VDC. AC refers to voltage levels of sinusoidal waveforms commonly referred to as sine waves. A 60 cycle per second (cps) or hertz (Hz) sine wave takes a 0.0167 second to make one cycle, which means the polarity peaks at opposite every 0.0083 second.
The true RMS voltage of electrical utility gives about 110 volts alternating current (VAC). This is equivalent to about 155.6 volts peak (V-Pk). The true RMS level is defined as 0.707 of the peak level for a pure sine wave. When the waveform is not a pure sine wave, the relationship between peak and RMS will not be a scale of 1:0.707.
There is a simple relationship between the VDC and the volts root mean square (Vrms). If 1 VDC and 1 Vrms were fed into a resistive load, the amount of power being delivered is the same. It should be noted that 1 VDC is steady instantaneous voltage, while 1 Vrms is referenced to a time-changing voltage. The words multimeter and multitester are used interchangeably, but a multimeter is used for metering or measurement, while a multitester is used for testing. Some manufacturers may argue that testers may be used just to affirm the presence or absence of voltage or current, while meters are used to measure the actual levels.
Besides power applications, a true RMS multimeter may be used in frequencies much higher than 60 Hz. Audio RMS meters measure sound voltages at various test frequencies in sound laboratories. Even in frequencies higher than audio, in ultrasonics, a true RMS multimeter is used for the calibration and maintenance of high-accuracy equipment, such as ultrasonic ranging used in submarines.
There are numerous radio frequency (RF) applications of RMS meters. In component-level troubleshooting, technicians use the RF RMS meter to measure voltages that are more than about 500 million cps. The readings provide performance indications or help isolate a faulty component. The true RMS multimeter is useful at a work site, at the repair workshop, and in research and development (R&D) laboratories.