Copyright Goodheart-Willcox Co., Inc. Chapter 10 How Electric Meters Work 185 Another useful feature is to attach one alligator clip to the neu- tral wire while using the other probe tip to check for power at various points throughout a circuit. There are many test leads rated for different voltages available today. 10.5 Specialty Meters Specialty meters, such as wattmeters and megohmmeters, are typically single-type meters. Some meters include these functions in one unit together with traditional multimeter functions. A wattmeter is used to measure true power that is con- sumed by a system. The wattmeter function is added to some multimeters, or it can serve as a standalone unit. The nonport- able standalone unit is typically panel mounted for stationary use. The wattmeter uses an inductive clamp to measure cur- rent, while source voltage is measured with the meter probes. See Figure 10-21. The meter analyzes the phase shift between voltage and current and their amplitudes to derive the true and apparent power. The display shows the true power and power factor. A megohmmeter, also known as a megger, tests the resis- tance of electrical insulators. See Figure 10-22. For HVACR applications, these meters supply 500 to 1000 Vdc to the wire under test. A high potential difference is required to test insu- lation values in the millions of ohms. The manufacturer’s instructions and the owner’s manual must be carefully fol- lowed when using this meter. Details such as the specified time that the high voltage is applied to the test circuit must not be exceeded or the circuit can become damaged. Pass/fail insula- tion resistance values depend on meter voltage and test time duration. 10.6 Specifications Every meter has specifications that can be found in the owner’s manual. It is critical that these specifications are understood. Meter specifications are used to define the following features: • Capacity limits. The amount of voltage, current, resistance, and other functions a meter can measure. • Accuracy. How close a measurement is compared to a reference point. • Resolution. The smallest increment that can be measured. • Precision. Indicates the repeatability of the same measurement. See Figure 10-23. Refresh Rate A meter’s refresh rate is a factor considered for its precision. This rate describes how often a signal is updated on a display. A typical value is three times per second. Because current is a signal, this is sampled by the A/D and processor several times per second. Pro Tip Goodheart-Willcox Publisher Figure 10-21. Wattmeter built in to a multimeter. Goodheart-Willcox Publisher Figure 10-22. The megohmmeter (megger) checks insulation integrity.