It’s Not Your Father’s DMM

Modern micro-circuitry has enabled the addition of many functions that were unheard-of in the original digital multimeter.

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Advances in multimeter technology prove valuable

By Jeff Jowett

There is nothing new under the sun. You can’t teach an old dog new tricks. A rose is a rose is a rose. These familiar aphorisms aimed mainly at defending the status quo are widely heard. But are they true? Not always, and especially not in the electrical world, where innovation has now achieved a break-neck pace. Keeping up with new technologies can save the electrical worker valuable time, reduce or eliminate error, and above all, save a life!

Possibly no test instrument is more familiar, generic, and taken for granted than the digital multimeter, or DMM. It’s the most ubiquitous piece of test equipment. You can buy one for $35 and they’re frequently given away as premiums (just remember the old adage “you get what you pay for”).

Original DMMs typically measured voltage, current and resistance, with possibly a few other conveniences like data hold and MIN/MAX. This article will examine some advanced functions and their usefulness.

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Dual screen testers show both AC and DC components simultaneously so the operator knows if there is any AC on a DC measurement and vice versa, plus allowing √AC2 + DC2 calculation. This is particularly useful in solar applications.

Convenience

Modern micro-circuitry has enabled the addition of many functions that were unheard-of in the original DMMs. The primary advantage is to save time. The operator no longer has to carry multiple instruments or spend additional time disconnecting one, putting it aside, and going to the tool box to retrieve, connect and set up another.

One such addition is that of a two-wire phase rotation indicator. The electrician no longer has to fumble with the connection of three leads to a three-phase system. The tester remembers the relationship of the first two phases for a brief interval (to prevent error) and then one lead is moved to the third phase and rotation indicated as OK or NOT OK.

Pass/fail buzzer responses are now immediate. The short pause that occurs with minimalist meters may seem inconsequential when performing just one test. But it’s like lifting a five-pound weight once versus a hundred times in a row. When proofing a large panel, the elimination of these electronic pauses will be appreciated at the end of the day.

Another convenience is the microelectronic digital/analog arc display that combines the advantages of both types of measurement. The progress of the test is followed with an electronic pointer while the final reading is rendered with digital accuracy rather than guessing the pointer position. A smart meter can even search and freeze stable values, a plus for inexperienced operators testing insulation resistance.

Sophisticated LCDs now show two or more corresponding measurements at once, such as resistance and test voltage, while always keeping the main measurement highlighted. This saves time and prevents confusion in not having to switch back and forth. Automatic range switching also reduces time and error.

Built-in temperature measurement may seem like an extravagance but it’s vital to a complete understanding of many test results. Resistance, for instance, is highly dependent upon temperature and its inclusion in a test result or report can make the difference between a correct or errant conclusion.

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With the addition of a two-wire phase rotation indicator on new DMMs, the electrician no longer has to fumble with the connection of three leads to a three-phase system.

Accuracy

Dual screen testers can also show both AC and DC components simultaneously so that the operator knows if there is any AC on a DC measurement and vice versa, plus allowing √AC2 + DC2 calculation. This is particularly useful in solar applications and is also a built-in safety feature as it prevents the operator presuming to be on a dead line when in fact the tester is merely on the wrong AC/DC setting.

Generic multimeters typically measure only to 0.1 Ω. This may be adequate for simple pass/fail testing, but more rigorous demands and record keeping typically call for 0.01 resolution. Remember also that there is always a specified uncertainty with the last digit of digital measurements. Having resolution to the hundredth fixes the certainty of the tenth, valuable in assuring the quality of components, frequency of electrical supply and low circuit resistances. Resolution is based on number of counts, the higher the better. So a high-quality tester may afford 10,000 counts as opposed to a lower quality/lower resolution meter at 2,000 counts.

Inexpensive average-responding multimeters may be fine for lab work in an electrically quiet environment or for measurements that need only be approximate. But for demanding work, a true root mean square (tRMS) meter is a must. TRMS means that the instrument includes all deviation from the pure sine wave (“noise”) in the measurement, thereby providing a truly accurate reading rather than an adjusted approximation. If noise is an interference, the presence of a low-pass filter can be a vital tool in suppressing its effect on accuracy.

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Sophisticated LCDs now show two or more corresponding measurements at once, such as resistance and test voltage, while always keeping the main measurement highlighted. This saves time and prevents confusion in not having to switch back and forth.

Safety

When doing electrical work, it’s easy to forget that you may be in close proximity to enough power to destroy the building, let alone the operator. Always be aware of the IEC 61010 rating (CAT IV if outside the building, CAT III if between panel and outlet) plus the operating voltage of the circuit under test. And be sure the DMM is adequately rated for that environment in order to prevent arc flash/blast. But going farther, full-featured DMMs now can offer non-contact live detection, audible and visual. This not only saves the trouble of acquiring and carrying an extra piece of equipment but also protects the pre­occupied worker who hasn’t even taken out the voltage detector. Hi/lo settings enable initial voltage detection from a distance and then switching to low for pinpointing the particular circuit.

A quality meter can automatically shutter unused terminals with the turns of the selector switch. This prevents dangling leads that may be energized or an unwanted current path that can damage the instrument or injure the operator.

Although small capacitors are unfortunately employed in practical jokes, capacitors can store lethal charges and should always be treated in accordance with safety practices. A smart meter will detect any stored charge and automatically switch to low impedance to discharge it before a capacitance measurement test is performed.

Of course, a similar protection should be in place against accidental connection to a live circuit. On a resistance test, which is performed on a presumably dead circuit, an operator could be caught unaware and the instrument damaged, fuses blown, or the operator injured. A safe tester will automatically prevent this occurrence by issuing audible and visual warnings that the test item is live.

A vital modern function relating to all three categories is impedance switching. The operator can select high or low input impedance. A DMM’s input impedance is normally high (~10 MΩ) so as not to affect the tested circuit. Switching to low impedance helps the electrician to distinguish capacitively coupled stray voltages on “dead” conductors in closely bundled cables.

Conclusion

While simple, economic, basic-function DMMs are still valuable tools, the electrical worker can now avail himself or herself of a plethora of vital functions that will make the job more thorough, accurate, easier, quicker, less expensive and, above all, safer. UP

The Author: Jeff Jowett is a senior applications engineer with Megger, makers of electrical test equipment to help you install, improve efficiency and extend the life of assets and cable networks at high, medium and low voltage. Learn more at https://us.megger.com.

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