Digital multimeters, also known as DMMs, are among the most widely used electronic testing instruments. DMMs are often referred to as the tape measure of the new millennium. DMMs consolidate the functions of multiple instruments such as voltmeters, ammeters and ohmmeters into one unit.
A basic DMM works with energized and de-energized electrical circuits and measures alternating current (AC), AC voltage, direct current (DC), DC voltage, resistance, continuity and diodes. Advanced models also measure capacitance, frequency, temperature, pressure and vacuum (special accessories are required for some of these measurements). It’s important to note that anytime you work with current (AC or DC) and voltage you need to use extreme caution. To avoid catastrophic consequences, safe practices must be followed whenever working with electricity.
There are broad applications for the use of DMMs particularly for processes involving electrical engineering design, maintenance, quality control, inspection and any application requiring electrical servicing and diagnostic testing.
To choose a DMM that is best for your application, first consider resolution, digit and accuracy.
Resolution refers to how fine a measurement a meter can make. By knowing the resolution of a meter you can determine if it is possible to see a small change in the measured signal. The terms digits and counts are used to describe a meter’s resolution. DMMs are grouped by the number of counts or digits they display. A 3 ½-digit meter can display three full digits ranging from 0-9 and one half digit which displays only a “1” or is left blank. A 3 ½ digit meter will display up to 1,999 counts of resolution and a 4 ½ digit meter can display up to 19,999 counts of resolution. Some DMMs may have enhanced resolution offering higher counts within their digit range.
Accuracy is the largest allowable error in the readings. It’s an indication of how close the DMM’s displayed measurement is to the actual value of the signal being measured. Accuracy is generally expressed as a percent of the reading. An accuracy of one percent of the reading means that for a display of 100 volts, the actual value of the voltage could be somewhere between 99 and 101 volts.
Autoranging, or dialing, is also a useful multimeter feature. It allows the user to quickly move from the instruments various measuring scales while maintaining accurate readings.
Another important consideration to take into account when selecting a DMM is whether it is a True RMS (root mean square) meter. The typical DMM is not a True RMS meter and as a result it will produce misleading voltage readings when used to measure anything other than DC signals or pure sine wave AC signals. True RMS meters allow non-sinusoidal AC signals to be accurately measured.
DMMS are designed with different levels of protection against common electrical hazards. Therefore, when selecting a DMM you must thoroughly understand the test equipment and test requirements. Three key characteristics to consider are the maximum voltage rating of the circuit, transient voltage rating of the circuit and the energy capacity. Practice safety when using a DMM:
- Use meters within their rating
- Use replacement fuses approved by the manufacturer
- Use high quality safety related testing leads
- Whenever possible work on de-energized circuits and follow proper lockout/tagout procedures
Refer to the Working Safely with Digital Multimeters resource below for a comprehensive overview of DMM safety.
The most important factor to keep in mind is that electrical test equipment should be used with caution, respect and maintained in accordance with the manufacturer's guidelines. As with all electrical test equipment, individuals using a DMM should receive training in its proper operation.
CAT II - This category is applied to all equipment connected from the wall socket up to the equipment’s first level of power conversion. Measurements at the wall socket itself might not be limited to CAT II levels.
CAT III - This category is applied to building circuit installations that are completely within the building, including parts of the service panel and the branch circuits. It also applies to many of the building’s fixed equipment, which is connected directly to the building mains instead of being connected through cords and plugs.
CAT IV - This category is applied to the source of the building’s electrical installation: the entrance service panel, the primary mains meter, or perhaps the secondary side of the building distribution transformer, if the transformer is located within the building.
Find even more information you can use to help make informed decisions about the regulatory issues you face in your workplace every day. View all Quick Tips Technical Resources at www.grainger.com/quicktips.
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The content in this newsletter is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific compliance questions should refer to the cited regulation or consult with an attorney.
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