Copyright Goodheart-Willcox Co., Inc. 172 Essential Electrical Skills for HVACR: Theory and Labs Introduction A multimeter has been discussed and used to take measurements in prior chapter labs. This chapter will examine a meter’s internal operation and each of its meter functions. Both analog and digital meters have been used in the HVACR field. The operating charac- teristics of the original analog meter are similar to those for the dig- ital meter, but its mechanical complexities have been replaced by electronics. Today, the digital multimeter is most commonly used due to its high accuracy and usability. It contains software-driven microprocessors that allow the meter to hold multiple functions. Understanding the internal operation of these meters is criti- cal in troubleshooting electrical circuits and components as well as determining when a meter malfunctions. A technician also must be familiar with the safety features and capabilities of a meter, which can be understood by interpreting the owner’s manual specifications. 10.1 The Analog Meter An analog meter is an electromechanical device that measures current flow to determine the basic electrical quantities of volt- age, current, and resistance. The current measurement on an analog meter is displayed by a pointer, or needle, that moves due to an internal magnetic coil. The magnetic coil and pointer are known as the meter movement. The meter movement is cali- brated by the meter designer to determine voltage, current, and resistance through Ohm’s law principles. The meter designer uses calculated resistance values that are correlated to the amount the pointer travels. The amount of travel indicates the amount of current measured. Each meter function requires a specific circuit configuration to control the meter movement. A volt-ohm-milliammeter (VOM) is an ana- log meter that combines the three basic functions in one unit. See Figure 10-1. The correct range, or the maximum value to be measured, must be selected, Figure 10-2. For example, the 50 V range has a maximum value of 50 V, so it cannot be used when measuring 120 V—the 250 V range must be used. If not, the movement can become damaged. See Figure 10-3. The following sections describe how mea- sured current is translated into current, voltage, and resistance based on the mode and range set- tings in a typical VOM. 10.1.1 Current Measurement The analog meter measures ac and dc. With ac, it is first rectified to dc to prevent inductive effects. The meter itself can only handle a min- ute amount of current, so the meter is calibrated where a small amount of current measured cor- relates to the actual amount of current. To pre- vent a large amount of current to pass through Goodheart-Willcox Publisher Figure 10-1. An analog meter is known as a volt-ohm-milliammeter (VOM). Current ranges Voltage ranges Function/mode sector switch Resistance ranges Goodheart-Willcox Publisher Figure 10-2. The mode switch selects the functions and corresponding ranges.