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​Every electronics engineer and technician should have a digital multimeter (DMM). Here are some things to consider when purchasing a DMM:

The most basic piece of test equipment in the engineer's or technician's toolbox is the digital multimeter, or DMM. With a DMM, you can make voltage, current, and resistance measurements. Today's DMMs, though, can make other measurements, such as continuity and capacitance, and have advanced features that can help you make the measurements you need quickly and easily.

Types of Measurements

First, think about the kind of measurements you'd like to make. Almost all DMMs make voltage, current, and resistance measurements, but say, for example, that you will be making AC measurements: most DMMs make accurate AC measurements when measuring sinusoidal signals, but when AC signals are not simple sine waves, their accuracy suffers.

If you need to make AC measurements of signals that have a lot of harmonic distortion, you may want to purchase a DMM that makes true RMS AC measurements. These DMMs will cost more, but they will make more accurate measurements.

Many new DMMs can do much more than measure voltage, current, and resistance.. For example, many DMMs can now make capacitance measurements. One application where this feature comes in handy is troubleshooting equipment that uses single-phase motors.

Many DMMs can also record measurements over some period of time. This data logging capability can be useful in a number of applications. For example, you might want to record the output voltage of a power supply over a long period of time. When selecting a DMM to do data logging, the important specifications include the speed at which the DMM can make measurements and the amount of memory. These two specifications will determine the amount of time over which the DMM can record data.


How Accurate Does Your DMM Need to Be?

For most common measurements, even the most inexpensive DMMs will be accurate enough. The accuracy of most DMMs is between ± (0.7 % + 1 count) and ± (0.1 % + 1 count) of reading, with the better meters being at the low end of that range. If, for example, you were measuring a 5 VDC signal with a 3-1/2 digit multimeter, the uncertainty of that measurement would be:

5 V * .001 + 10 mV = 15 mV

For most applications, that's plenty of accuracy.

[The Fluke 115 handheld DMM can make true RMS measurements and has a CATIII 600 V safety rating.]

The Fluke 115 handheld DMM can make true RMS measurements and has a CATIII 600 V safety rating.


Handheld or Benchtop?

The question of whether to purchase a handheld DMM or a benchtop DMM rests entirely on how you plan to use it. Benchtops, in general, offer higher resolutions, i.e. more digits, than handhelds, and are generally more accurate. They may also offer more features, such as the ability to make four-wire resistance measurements.

The downside, of course, is that they are generally more expensive than handhelds, and they are much more difficult to use out in the field. If you spend most of your time at an engineering workbench, a benchtop DMM is probably the way to go. On the other hand, if you spend a lot of your time on the factory floor, or traveling to troubleshoot field problems, a handheld is the right choice for you.


Safety and Reliability

Perhaps even more important than accuracy is safety, especially when measuring high voltages and currents. IEC 61010-1, 2nd Edition, and UL Standard 61010B-1, “Electrical Measuring and Test Equipment; Part 1: General Requirements,” describes general safety requirements for electrical equipment intended for professional, industrial process, and educational use, including equipment and computing devices for measurement and test. It categorizes test and measurement equipment into four different categories:

  • CATI applies to test equipment used to make measurements on protected electronic equipment and equipment that has transient overvoltage protection.
  • CATII applies to test equipment used to test appliances, portable tools, and similar loads.
  • CATIII applies to test equipment used to make measurements on industrial equipment, including switchgear, polyphase motors, and busses and feeder lines in industrial plants.
  • CATIV applies to test equipment used to make measurements on three-phase utility lines and any outdoor connections.

The higher the CAT rating, the more resistant the instrument is to an overvoltage spike, which could not only damage the meter, but injure personnel using the meter.

Choose a meter with a CAT rating that corresponds to the voltage levels that you'll be measuring and the type of equipment that you'll be making measurements on. One benefit of choosing a DMMs with a higher CAT rating is that they are generally more reliable than DMMs with lower ratings. They'll not only be able to handle electrical stresses, but they'll be protected from mechanical stresses as well. Even if you're only purchasing a CATI handheld, though, look for one with an over-molded enclosure that protects it from accidental drops or other mechanical incidents.

By carefully considering the features you need, you'll be better able to choose the DMM that's right for you. You'll get the features you need and have more confidence in the measurements that you make.