These modern tape measures combine the functions of a voltmeter, ammeter and ohmmeter into a single unit.
One of the most widely used electronic test instruments, the digital multimeter (DMM) is more commonly known as the modern tape measure. However, DMMs cannot handle a single function (i.e. linear measurements), but instead combine the functions of multiple instruments such as voltmeters, ammeters, and ohmmeters into a single unit.
Digital multimeters are used in applications requiring electrical engineering, maintenance, quality control, inspection and/or electrical service and diagnostic testing. Basic digital multimeters work on energized and non-energized circuits, and measure alternating current (AC), alternating voltage, direct current (DC), direct voltage, resistance, continuity, and diodes. Advanced models measure capacitance, frequency, temperature, pressure and vacuum (some of these measurements require special accessories).
Choose a multimeter
There are three things to consider when choosing a DMM for a specific application:
Resolution refers to how finely a meter can measure. By knowing the resolution of the meter, you can determine whether you can see small changes in the measurement signal. The terms digital and count are used to describe the resolution of a meter. DMMs are grouped by the counts or digits they display. A 3 ½ digit meter can display 3 full digits from 0 to 9, as well as 0.5 digits that can be displayed as a single "1" or left blank. The 3 ½ digit meter can display up to 1,999 counts resolution and the 4 ½ digit meter can display up to 19,999 count resolution. Some DMMs may have enhanced resolutions that provide orders of magnitude higher counts.
Accuracy is the maximum allowable error in the reading. This shows how close the measured value displayed by the DMM is to the actual value of the signal being measured. Accuracy is usually expressed as a percentage of reading. An accuracy of 1% of the reading means that for a reading of 100 volts, the actual value of the voltage may be between 99 and 101 volts.
Autorange (or "dial") is a useful feature that allows the user to quickly move between different measurement scales on the instrument while maintaining accurate readings.
Another important consideration when choosing a DMM is whether it is a true RMS (root mean square) meter. A true RMS meter can accurately measure non-sinusoidal AC signals. A typical DMM is not a true RMS meter, and can give misleading voltage readings when used to measure anything other than DC signals or pure sinusoidal AC signals.
Regardless of the multimeter chosen, the electrical test equipment should be used and maintained carefully in accordance with the manufacturer's instructions. As with all electrical test equipment, personnel using the DMM should be trained in proper operation.
It is important to be extremely careful when working with current (AC or DC) and voltage. To avoid catastrophic consequences, work involving electricity should always establish and follow an electrical safety work plan.
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