We expect a hand-held digital multimeter (DMM) to measure DC voltage and current, AC voltage and current, resistance, and sometimes frequency and capacitance. Year by year, we see improvements in these versatile instruments. There are more digits of resolution, the displays are bigger, and you can make more readings per second. Possibly the bottom ranges are lower than before and the AC frequency coverage is broader. Even the housings may have been redesigned to be smaller, more rugged, or easier to hold.
This year is no exception. Basic capabilities are expanding in several areas. You have a broad selection from which to choose, even before you look at the remarkable offerings in innovative features.
Do you need a DMM with a count of 50,000 or greater? Look at Extech Instruments, Fluke, GMC Instruments, Protek Test & Measurement, and Yokogawa. What about a VDC and VAC resolution of 10 µV or less? Try the same companies. You can measure VAC at frequencies from below 50 Hz up to 20 kHz or higher with a DMM from one of these companies, too.
Several instruments can measure AC and DC current with a resolution of 0.1 µA or better, and you can resolve 0.001 µA with the MetraHit® 29S from GMC Instruments. Review the resistance and capacitance measurements, too. You get a resolution of 0.01 W or better from Extech Instruments, Fluke, and GMC Instruments.
Finally, you may be surprised to see the low prices. A DMM for less than $100 is available from B+K Precision, Extech Instruments, Hioki USA, or Wavetek Meterman.
Innovative Features
Some users have unusual applications for their portable, multipurpose meters. To meet these needs, designers keep expanding the capability of the little meter far beyond that of the basic unit. Recent innovative features include the following:
AC + DC Mode—This measurement mode responds to both AC and DC. When the AC + DC reading is greater than the DC reading, there is a DC bias on the point being measured.
Bar Graph—B+K Precision is one company that offers a small bar-graph display of the measured value so you can easily grasp the relative magnitude of a parameter.
Continuity—Some meters emit a beep when continuity is sensed, leaving the operator free to look at test points rather than the display.
Credit-Card Size—The Amprobe CDM-1 DMM, a 2.1² × 4.3² × 0.5² instrument, has a 3¾-digit LCD and a 32-segment bar-graph readout. In addition to DC voltage, AC voltage, and resistance measurements, it incorporates an audible continuity check.
Data-Hold Capability—This safety feature freezes the data display when a reading is made so you can look at the test point first and observe the measurement later.
Datalogging—Acquired measurement data from one to as many as 10 channels is displayed in tabular format.
Data-Presentation Software—The GMC Instruments METRAHit 29S has an interface adapter and METRAwin® 10 Data Presentation Software. With a Windows-based PC, the meter becomes a full-function datalogger/processor. Information is displayed in Y(time) or XY format. Alternatively, it can be presented on an analog scale or in a tabular digital layout. Math functions offer online or off-line analysis.
dB/dBm Measurement—Several DMMs have the capability to measure and display dB or dBm. The A.W. Sperry DM-9500, for example, does this. You can select the proper reference impedance for the measurement.
Diode Test Feature—This is a low-voltage, current-limited mode to avoid damaging the diode under test. Typical operation is 2 V with current limited to 2 mA.
Graphical Display—The Extech Instruments Mini MultiScope™ II includes a graphical data display, allowing you to see the measured waveform as well as the numeric data.
Harmonic Distortion Check—The Hioki 3805 measures the relationship of a nonsinusoidal wave to a sine wave and displays the proportion as a percentage between 0.0% and 99.9%. This is a direct indication of harmonic distortion.
Instrument Maintenance Mode—Some DMMs measure the 4-mA to 20-mA output of an industrial instrument and convert it into a 0% to 100% reading so you can grasp the value intuitively. One of these meters is the Hioki 3805.
Min/Max/Avg Reading—Several instruments have the capability to hold the minimum and maximum values and compute the average for a specified time. The Fluke 80 Series III has 250-µs peak min/max for capturing transients. The Fluke 179 emits an audible tone whenever a new minimum or maximum reading is sensed and stored.
Phase Rotation Measurement—The Fieldpiece HB74 Heavy-Duty DMM measures and displays phase rotation on a three-phase power line to enable you to connect equipment properly.
Power Measurements—The GMC Instruments METRAHit 29S measures and displays W, VAR, VA, PF, and Whr.
Pulse Generation—The GMC Instruments METRAHit 26M DMM generates 3-V pulses at rates from 1 Hz to 1,000 Hz for simulation of counter/timer functions and test of transmission paths.
Recording Mode—The capability to store measurements in the DMM is especially helpful in transient detection. For example, “A railroad company had a series of electronic equipment failures on a train and suspected that its electrical system had damaging spikes,” reported Andy Zeidler, GMC Instruments marketing manager. “Using the METRAHit 29S DMM, which stores 60,000 measurements at rates up to 2,000/s, engineers monitored the power source and found transients in excess of 1,000 V.
“Armed with this proof of the defective power source,” he continued, “the engineers discovered that some capacitors on the train were not functioning. After replacing them, there was no more trouble.”
Removable Memory—GMC Instruments offers an optional Memory Adapter for several of its DMMs. This allows on-site storage of measurement data and subsequent playback to a PC. Data points are synchronized with an integrated clock. The sampling rate can be 50 ms to 1 min.
Smart Auto-Power Off—This is a power shut-off circuit that functions only if there is no signal on the measurement leads. The Extech MultiPro™ DMM includes this feature for customers who can’t tolerate having the standard power-control circuit turn off the instrument during a critical measurement sequence, yet don’t want to lose the automatic shut-off capability.
Temperature Measurement—Several companies offer some form of temperature measurement capability, generally with a choice of °F or °C readout. This can be used in many applications, including HVAC, automotive, electrical, process control, and plant maintenance. For example, the Omega HHM29-IR has a built-in infrared (IR) sensor with laser sighting so it is not necessary for the test leads to touch the temperature measurement point.
Temperatures From Two Points—The Hioki 3805 DMM measures and displays temperatures from two points. Using this feature, you could evaluate the performance of an air conditioner by observing its cooling unit and air-outlet temperatures.
Triple Display—The GMC Instruments METRAHit 28S and 29S DMMs have three displays. The current value and two other readings—the minimum and maximum, for example—are shown, and it is not necessary to switch back and forth between them when troubleshooting a problem.
Voice Annotation—Omega’s OMEGASAYS® HHM2 Talking Digital Multimeter/Thermometer announces each measurement value in English, French, Spanish, German, or Japanese.
Comparison Chart
The comparison chart that accompanies this article shows basic performance, special features, and prices for several new hand-held DMMs. Also new are two recent realignments in the list of suppliers: Tektronix sold its hand-held DMM product line to Fluke, and the Wavetek Wandel Goltermann line of hand-held DMMs now is known as Wavetek Meterman. (Please see chart in the February 2001 issue of Evaluation Engineering.)
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February 2001