Get your hands around a DMM

If you make electrical measurements, you probably use a handheld digital multimeter (DMM). With dozens of handheld DMMs on the market, how can you decide which one is right for you?

To give you a feel for what's available, I evaluated 18 models from 11 manufacturers, all of which use the true-rms (root-mean-square) method for measuring AC voltage and current. ("When do you need true rms?" at www.tmworld.com/true_rms explains why most AC measurements require true-rms meters.) In general, any meter will give you reasonably accurate measurements. The differences come down to price, features, and manufacturer's reputation.

I asked the manufacturers to provide one sample each of their two lowest-priced true-rms DMMs. The models ranged in price from $49.95 to $249. I looked at the meters for features and usability, and then I measured a 5.2-VDC signal, a 9.9-Vrms triangle wave, and an AC mains voltage with each meter. A recently calibrated HP 34401A bench DMM (Agilent, www.tm.agilent.com) served as a "golden" meter for comparison of the handheld units. A Stanford Research DS335 function generator (www.thinksrs.com) provided the 5.2-V and 9-V test signals, and a Tektronix TDS2022 oscilloscope (www.tektronix.com) supplied a view of the test waveforms. Figure 1 shows the test setup. To see the test results in detail, you can download an Excel spreadsheet with the complete data.

Table 1 lists the price of the units along with some of their features. One "feature" not listed in the table but worth noting is the color of the meters and their boots. "DMM colors: The boot isn't moot ," below, explains the issue surrounding DMM colors.

Because of the wide range in meter prices, I divided the meters into five groups. I've provided a brief description of the meters in each group and then explained what I liked and didn't like about them.

•A.W. Sperry DM-370A

This group consists of just one meter, a DM-370A from A.W. Sperry Instruments. Because the next least-expensive meter I evaluated costs twice as much, I placed this one in its own group.

The DM-370A is a no-frills meter. You get basic measurements that you'll find in just about every other handheld DMM: AC and DC voltage and current, ohms, and diode check. The display doesn't have units or a hold button. You will get a boot and a meter stand, though.

What I liked. The price; nothing else comes close. For under $50, you get a basic meter with true rms.

What I didn't. The nonstandard spacing of the input jacks makes using a standard two-prong banana jack impossible. Most instruments use 0.75-in. spacing, which corresponds to the spacing in standard banana plugs, but this meter uses 1-in. spacing. That forced me to use the meter's probes. While that may seem insignificant, consider that calibration labs use banana plugs to calibrate instruments. Not with this meter.

When you replace the meter's battery, you must open the case. That's not unusual, but when I opened this meter, its internal circuit board nearly fell out because it's not attached to the case.

•Extech 22-316
•Fluke 110
•Greenlee DM-110
•Morrow Wave DM-531T
•Wavetek Meterman 34XR

If you have about $100 to spend, you can get a rugged meter with numerous features, such as backlit displays, temperature measurement, bar graphs, auto shutoff, and overvoltage category ratings. Most meters and probes carry category-III 600-V or category-II 1000 V ratings. (To learn about overvoltage ratings, download a copy of "ABCs of Multimeter Safety," Ref. 1.)

Figure 1 The test bench included a Stanford Research DDS 355 function generateor (top), an Agilent Technologies 34401A bench DMM (center), and a Tektronix TDS2022 digital oscilloscope (right). Photo by Harry Bartlett.

When measuring a 9.9-V, 120-Hz triangle wave, the Fluke proved the most accurate, differing from the reference meter by only 0.2 mV. The Morrow Wave fared worst, differing from the 34401A by over 50 mV, although that's still within the DM-531T's specifications. When measuring line voltage, the Fluke again displayed closest to the reference meter, differing by just 0.1 V out of 118 V.

You might wonder why the Fluke 110 is in this price range. It lacks current measurements, a backlight, and alligator clips for its probe tips. Many engineers, though, choose Fluke for its reputation. If you don't need the additional features you get with the other meters, then you can't go wrong with the Fluke 110.

What I liked. For mechanical design, the Meterman 34XR wins hands down. A magnet lets you hang the meter from any steel surface. When you remove the meter, the magnet flips on its strap and attaches itself to the back of the meter. A clever triangular, rather than straight-line, configuration for input jacks prevents you from accidentally connecting the two current inputs together. The meter's blue backlight makes the display easy to read in the dark.

Greenlee's DM-110 is the lowest-priced meter I tried with a dual display. It lets you measure AC voltage and frequency at the same time. Although you must open the case to change the battery, you'll find the circuit board attached to the case. You'll also find a shield on the inside of the back cover. Several other meters in this and other categories also use shielded cases. The shields help suppress outside EMI from interfering with the meter, and they reduce EMI emissions eminating from the meter's digital circuits.

Extech's 22-316 fits nicely in your hand. So does the Fluke 110. Both meters are small enough for you to grip easily. The Fluke meter, with its 6600-count display, leads this group in resolution.

What I didn't. Opening the stand on the Greenlee meter required a screwdriver. You shouldn't have to work that hard to stand a meter on a table. Extech's boot lacked a place to hold probes, something you'll find on most meters. If you need a bar graph, then skip the Extech meter in this price range.

•Brighton Electronics Summit 610
•Brighton Electronics Summit 620
•Extech MT330
•Fluke 111
•Global Specialties Pro-80

At this price range, features such as serial ports, measurement limits, and pulse outputs begin to appear. Fluke adds current measurements to the Model 110 (in Group 2) with the Model 111. If you need resolution, then go for the Global Specialties Pro-80 or the Fluke 111, although the 80,000 counts you get with the Pro-80 seems like overkill in a handheld meter.

The Global Specialties Pro-80 had the best DC accuracy, differing from the 34401A's 9.9-V reading by 3.8 mV. The rest were equal performers, differing by 11.3 mV or 11.4 mV. For the 5.2-Vrms, 120-Hz triangle wave, the Fluke came out on top, differing by 1.2 mV. The Brighton Electronics 620 and Global Specialties Pro-80 followed closely behind. The Brighton 610 and the Extech meter differed from the reference by 42 mV and 52 mV, respectively, although both still met their accuracy specifications. The Excel spreadsheet, available with the online version of this article, covers the specific measurements. You can get the accuracy specs from the manufacturers' Web sites.

What I liked: Global Specialties Pro-80 gives you pulse outputs that range in frequency from 0.5 Hz to 5 kHz. You also can change the duty cycle from 1% to 99%. The Pro-80—the lowest-priced meter I tested that has a full dual display—lets you measure voltage and current at the same time. You can't do that with the lower-priced Greenlee meter's dual display (see Group 2).

The Global Specialties Pro-80 and the Brighton 620 provide high and low test limits. The meters will beep and display the pass-fail conditions, a useful feature for technicians who just need to know if a reading is good or bad but don't need the value. The beep also alerts you to a fail condition without requiring your eye.

The Extech, Brighton, and Global Specialties meters have a "relative" feature. Think of it like a tare feature on a scale. When you press the button, you get a zero reading. The meter then displays the difference of a reading from that point.

What I didn't: The backlit displays of the Extech and Global Specialties meters were difficult to read in low ambient light. If you need a visible backlit display, stay away from both meters.

Although the Global Specialties and the Brighton 620 meters let you set range limits, don't try to use this feature on either meter without reading the manual. That's always a problem with handheld instruments. The more features you get, the less intuitive the instruments become. If you don't need advanced features such as limits, then look for a simpler, lower-priced meter.

•B&K Precision 391A
•B&K Precision 2880A
•Protek 506
•Global Specialties Pro-60

As DMM prices approach $200, the meters deliver even more bells and whistles. You won't get better accuracy from meters in this group than you get with meters in Group 3. It's the features that drive up the price. For example, the Global Specialties Pro-60 and B&K Precision 2880A provide isolated serial ports. Isolation lets you connect your meter to a high-voltage source and download data to a PC without risking damage to the PC from high-voltage spikes or accidental ground loops.

The Protek 506 also provides a serial port, but not an isolated one. This meter is also the lowest-priced meter I tested that measures inductance. Most meters measure capacitance only. (Eds. note– Protek has since provided documentation proving that the Model 506 serial port is indeed isolated.)

What I liked. The Protek 506 and Global Specialties Pro-60 come with carrying cases, and the Global Specialties case is a real winner. You get a soft case with a shoulder strap and compartments for carrying probes, clips, a serial cable, and serial-port software. The Protek meter comes with a hard carrying case that lets you store everything you need.

The dual displays on the Protek 506, Global Specialties Pro-60, and B&K 2880A give you lots of information. For example, the instruments display AC signals in both volts and decibels; the meters also include relative measurements. The Pro-60 has a large dual display that, when lighted, is easy to read in the dark.

What I didn't. To my surprise, I found no voltage rating on the Protek 506 probes, making it the only meter I tested that lacked those ratings. Therefore, I wouldn't recommend using the meter for high-voltage applications because of the probes and the lack of an isolated serial port. But if you need a full-featured meter for an electronics bench, this meter is fine.

If you plan to use the 506's serial port, beware of the software. You get a simple application written in Basic and some programming examples, but there's no Windows application to run this meter. Expect to write your own datalogging software. (Eds. note–according to Protek, every Model 506 meter includes a diskette that contains a Windows datalogging application. You can download the Windows application from the company's Web site, http://www.hcprotek.com/ProdDnLd.asp?type=SW.)

The Global Specialties Pro-60 lacked a stand. It's also the only one besides the $49.95 Sperry meter that used nonstandard spacing between its inputs. The inputs for current measurements are spaced 0.5-in. apart, although the meter uses the standard 0.75-in. spacing between the voltage input and the common input. The nonstandard spacing will, however, prevent you from accidentally connecting a two-prong banana plug into the wrong inputs.

The B&K 391A looks and feels like an old meter. It has no light, no serial port, no autoranging, and no min/max readings. The boot is an integrated part of the meter's case, and you must remove four screws to change the battery. It seems overpriced for what you get. For an additional $12, B&K's 2880A is a better deal.

The 2880A has two unmarked buttons, which I found confusing. A light blue button matched some colors on the meter's dial, which I quickly figured out would set the meter to the function of that color. For example, the button changes the meter from DC to AC measurements. An unmarked small white button changed the meter from 4 digits to 5 digits. The manufacturer should add markings to indicate the functions of those buttons.

•AEMC CA 5240
•Omega Engineering HHM 290
•Protek 608

The three meters in this group vary widely in functionality. The AEMC CA 5240 offers relatively few features for a meter in this price range, but it's designed for rugged, high-voltage applications. It comes with the most impressive probes of the lot. It also bears the double-insulation marking. You get safety with this meter at the expense of features.

The Omega Engineering HHM 290 tries to be all things to all people. It succeeds, but you pay for it with a large, bulky package. I almost didn't review this meter because Omega didn't send its lowest-priced true-rms DMM, but since I had other meters of comparable price, I included it. The meter was originally priced at $345, but Omega cut it by $100 soon after introducing the meter. I can see why. Although you get a lot of features in this meter, the $345 price would have scared away many users.

The Omega meter accepts two thermocouple inputs, and it has a noncontact temperature measurement feature. The laser beam shines a red pattern at the target, and the extra-large display lets you simultaneously see the temperature in both Celsius and Fahrenheit.

The Protek 608 doesn't have dual thermocouples or noncontact temperature measurements, but is has an isolated serial port and Windows software. The box says that the serial port cable and software are options, but they were included with the meter I received.

What I liked. AEMC's probes, which look and feel rugged. Use this meter for high-voltage measurements. The meter's small size and unique boot make it easy to handle.

The Omega meter's dual temperature probes let you make differential measurements, making it the only one in my evaluation that can do so. The noncontact temperature feature is unique, but don't point the laser at people because it can hurt their eyes. You'll find Omega's display easy to read—even in the dark—because of its bright backlight. You can easily read the meter's numeric display and bar graph from across the room.

Although most meters in this evaluation contain a bar graph, the bar graph in the Protek 608 is unique. Most bar graphs use linear-weighted markers; but Protek 608 features a binary-weighting system. It's confusing at first, but it has advantages. The weighting lets the graph cover a wider range than a linearly weighted graph. Electrical engineers will quickly figure out the binary weighting, but it might confuse others. The Protek 608 is also the only meter in my evaluations that can store readings without a computer. It stores up to 10 measurements.

What I didn't. As handheld meters go, the Omega HMM 290 is big and heavy. You get a bundle of features, but if you don't need them, buy a smaller, less-expensive meter. The laser is partly to blame; the meter requires six AA batteries to get enough power.

• Best mechanical design: Wavetek Meterman 34XR. Whoever designed this meter's package deserves a raise.
• Safest look and feel: AEMC CA 5240. Double insulation and rugged probes make this a good choice for high-voltage measurements.
• Best sense of humor: Brighton Electronics (both models). Instead of displaying the usual "OL" to indicate an overrange condition, these meters say "OUCH."