Copyright Goodheart-Willcox Co., Inc. 178 Essential Electrical Skills for HVACR: Theory and Labs Most meters use an automatic ranging feature, so user selection is not required. The meter analyzes the measured current and function used and then selects the range to be displayed. These meters also allow manual range selection. 10.2.1 Reading the Display Most portable digital meters display only four characters, or digits, with 1999 being the typical maximum display. Because values ranging from 0.000001 to 40,000,000 (this upper value may vary among meter models) can be measured on some meters, prefixes are used in front of a unit of measure to express these values. Prefixes are used for A, V, Ω, and F. The prefixes use scientific notation, which is used to repre- sent a large or small number. It expresses a number by a power of ten in increments of three. For example, 103 and 106 are expressed in this notation, which mean 1000 and 1,000,000 respectively. The table shown in Figure 10-11 shows various prefixes and their corresponding basic units. Often, it is necessary to convert prefix values to basic units, Figure 10-12. Multimeter Functions and Terminology The digital multimeter contains several meter functions combined in one piece of test equipment. When voltage is measured, it is common to say that the voltmeter is used. This applies for using the term ohmmeter when measuring resistance and ammeter when measuring current. The meter function selector switch, also called mode selector switch, is positioned to select the type of measurement to be taken. Pro Tip Automatic Ranging and Prefixes The user must be aware of prefixes when reading the display. Most meters are auto-ranging, which cause the device to display values using prefixes. Always take the time to read the value correctly. If a display shows 0.457 kΩ instead of 457 Ω, ensure the prefix is noted. Pro Tip Prefix Symbol Notation Basic Unit Example mega M 106 1,000,000 1 MΩ kilo K 103 1000 1 kΩ milli m 10-3 0.001 1 mA micro μ 10-6 0.000001 1 μA , 1 μF nano n 10-9 0.000000001 1 nF Figure 10-11. Prefixes denote specific unit values. Goodheart-Willcox Publisher Display Convert to Basic Unit 0.457 MΩ Move the decimal 6 places to the right to yield 457,000 Ω 0.457 kΩ Move the decimal 3 places to the right to yield 457 Ω 42.34 mA Move the decimal 3 places to the left to yield 0.042 A 4.234 μA Move the decimal 6 places to the left to yield 0.000004 A * * Rounded to the millionth place. The 234 portion of the value is insignificant. Figure 10-12. Convert a basic unit by moving the decimal point to a new place value. The prefix is then dropped. Goodheart-Willcox Publisher