Today’s DMMs can help you make measurements quickly and easily because they offer the features needed to check even the newest electronic equipment. They provide DC voltage resolution in the microvolt range and accuracy as low as 0.025% of reading to meet the most stringent requirements.

But before you purchase your next hand-held DMM, a look at the fundamentals will help you make the right choice. Important specifications include counts, digits, accuracy, resolution, AC frequency response and average or true rms readings.

Counts and digits are measures of resolution. Counts are an indication of the range of numbers a display can show, said Rob Baumgartner, product marketing manager at Tektronix. For example, 40,000 counts mean the display can range from 00,000 to 39,999.

Meters with more counts offer better resolution. For instance, a 1,999-count meter will not measure down to one-tenth of a volt if you are measuring 200 V or more. However, a 3,200-count meter will display one-tenth of a volt up to but not including 320 V.

The digits specification is a bit anachronistic; it dates back to when segmented displays determined the resolution of the meter, said David Pereles, senior product specialist at Fluke. A meter that used three eight-segment displays and a fraction of a fourth eight-segment display was a 3 1/2-digit meter. The more digits it had, the better its resolution.

Although the terms *digits* and *counts* are related, industry experts disagree about how to convert one to the other. When digital displays were introduced, the normal practice was to put a blank or a one in the most significant digit, said John O’Brien, applications engineer at Extech. Usually, this digit was called one-half. A typical display of 1,999 became 3 1/2-digits.

One way to convert between the two numbers is to count the zeros in the display, put the leading number in the denominator of the fraction, and then to subtract one from that number for the numerator, said Mr. Baumgartner. For example, 20,000 counts become 4 1/2 digits because it has four zeros, the two becomes the denominator, then one is subtracted from the two for a result of one in the numerator. With this method, a 40,000-count DMM becomes a 4 3/4-count display.

This method is not as straightforward when describing a 3,000-, a 3,200-, a 5,000- or a 7,000-count meter. Because of the variety of counts available, the relevance of digits has shrunk, said Mr. Pereles.

Accuracy and Resolution

More important than the number of digits and counts is the accuracy of the meter, said John Kinsey of RCC Electronics. This is the tolerance that the meter is guaranteed to read within.

What we really specify with the term accuracy is uncertainty, said Mr. Pereles. It indicates how close an observed value is to the one actually present at the test point. Uncertainty also expresses the width of a probability distribution. It answers the question: What is the most likely range of values around V that a measurement of V might have? Uncertainty should always be tacked to important measurements.

Another way to describe accuracy is to say that it is the largest allowable error that will occur under specific operating conditions. It is an indication of how close the displayed measurement is to the actual value of what is being measured.

The tolerances are indicated as plus or minus a percent, followed by a number of digits. For example, an accuracy of ± 1.0% of the reading + three digits means that a reading of 10 VAC on a meter with a resolution of 0.01 V can indicate 9.87 VAC to 10.13 VAC and still be within the tolerance of the meter.

Resolution expresses the smallest change that a meter can detect or resolve, said Mr. Pereles. It is directly related to the resolution of the analog-to-digital converter in the meter and the number of measurement ranges.

Imagine a 3,000-count meter with only a 0 to 10-V range. This hypothetical meter would resolve 10 V/3,000 or 3.3 mV. The same meter with an additional 0 to 1-V range would resolve 0.33 mV.

It is important to look at accuracy and resolution together because the combination of these attributes is the precision you need, said Mr. Baumgartner. A good analogy to help understand precision is timing a 100-meter race. If the stopwatches are electronically timed and have good accuracy but a resolution of only seconds, you will not know who was fastest by looking at the times.

Conversely, if the stopwatches have a resolution down to 0.01 but have inferior accuracy because they are hand timed, you still cannot be sure who won the race. Only the combination of resolution and accuracy provides the precision to determine who was fastest.

Average and True RMS Meters

Both average and true rms techniques are used to measure AC waveforms. They produce similar results for pure sinusoidal waves. As a signal’s harmonic content increases, average and true rms methods yield increasingly dissimilar results.

The average sensing meter calculates an average of the peaks of the waveform to arrive at an estimated current or voltage value. It multiplies by a scale factor to determine the rms value. But it assumes that the shape of the voltage waveform is perfectly sinusoidal. It works reasonably well for line voltage but not at all for complex waveforms, said Mr. Pereles.

Most rms meters calculate current and voltage by measuring the instantaneous peak of a waveform and multiplying it by 0.707. A true rms meter typically converts the AC by measuring the heating effect on a resistance. It is needed to ensure accurate readings in systems where there is distortion (harmonics) and noise.

True rms meters give correct readings for any wave shape within the instrument’s crest factor and bandwidth specifications. The crest factor is the ratio of the peak value to the rms value. For a sine wave, the crest factor is 1.414. A true rms meter will have a crest-factor specification. It relates to the peak level that can be measured without error.

A typical true rms hand-held DMM has a crest factor of 3.0 at full scale, which is usually adequate for most power-distribution measurements. At half-scale, the crest factor is double or 6.0. For example, in the 300-VAC range, a meter has a crest factor of 3.0. When measuring 150 VAC, it has a crest factor of 6.0.

AC Frequency Response

Most DMMs have a bandwidth specification for AC measurements. This specification indicates the frequency range for which the accuracy guidelines are valid. The AC measurement accuracy degrades or rolls off as the frequency increases. At the roll-off point, the signal-conditioning circuitry in the meter attenuates the incoming signal and the display shown is lower than the actual signal.

What’s New

While DMMs already offer a wide array of measurement functions, manufacturers continue to improve on existing capabilities as well as add new ones. For example, typical hand-held meters measure both AC and DC components of a signal simultaneously. Some companies, such as Tektronix, now offer meters that measure the total true rms value of the waveform, including the AC and DC portions of the signal.

The total true rms value helps if you are interested in the amount of power dissipated in a circuit. For example, if you are trying to determine why a 1/8-W resistor burned up, you can now measure the DC offset causing the problem.

A total harmonic distortion (THD) measurement is another capability offered on new DMMs. It allows you to determine if there is distortion on the AC voltage. On the RCC Electronics DMM, you can estimate the THD by pressing a button. This function is important because of the increased use of nonlinear loads such as switching power supplies.

Some of the new features include better accuracy. For example, the Fluke 860 Series DMM provides a basic DC uncertainty of 0.025% and an input impedance of 1,000 MW . It allows the meter to measure low-level voltages or check transducers with millivolt outputs.

If you want a meter that doesn’t use batteries you can look for a solar-charge type or one with a different twist offered by Extech. The company’s Model 380230 uses a quick-charge capacitor instead of a battery, providing several hours of operation from a one-minute charge.

Many other hand-held models are available with capabilities such as temperature, conductance and decibel measurements. Graphical displays for showing pictures of voltage waveforms and measurement trends also are offered.

For example, the Fluke 867 has a component test feature that shows a voltage-current curve. The instrument checks the integrity of transistors and diodes by putting a sinusoidal voltage across the component and measuring the resulting current.

The right DMM is available for you, whether it’s a highly specialized unit or an all-around meter for a variety of measurement tasks. Before you buy your next hand-held DMM, check the accompanying chart in this article to compare the many features.

**Copyright 1997 Nelson Publishing Inc.**

February 1997