Selecting Application-Appropriate Digital Multimeters
Published: June 2002
Digital multimeters (DMMs), nowadays, are smart instruments with a wide range of possible capabilities. A basic meter could suffice for an electrician to make quick checks of voltage and resistance. A higher-end, more feature-rich DMM can also perform a breadth and depth of measurements that could streamline troubleshooting and diagnostics of even fairly unusual or obscure problems in a variety of application areas. Many capabilities are software based, with the programming to perform a specific application built into the meter.
The marketplace has choices to suit most every need level. “Putting every feature into a digital multimeter, making it suitable for everybody, could wind up cluttering the unit’s user interface. That would, unfortunately, make it overly complex to access various features. Manufacturers often offer a range of instruments for particular markets and even segments within a market, with varying combinations of capabilities,” said Phil Salditt, general manager of Meterman Test Tools. “An electrician, for example, really wants an application-appropriate DMM that does everything the electrician needs to do, in one meter. He needs a meter that is right—not less and also not more.”
For best value and performance, electricians should use meters with feature sets corresponding to the type of testing they are likely to do. For example, a technician working on electronics at a bench may want test equipment that offers a full bandwidth. However, an electrician testing a large variable-speed drive needs a meter with a lower bandwidth. He will also require a greater degree of safety built into his meter—a design that provides shielding and adequate spacing to handle harmonics while working with the higher level of electricity, noted Paul Heydron, product planning manager, Fluke Corporation.
Among electrician-relevant features available in DMMs are True RMS measurements, Min/max, relative measurements and DB measurements, and, for those who need them, CAT IV capabilities. Some DMMs can use external software that runs on a PC that performs additional analysis, graphing or trending that can aid in troubleshooting.
For helping diagnose problems that are intermittent and may not occur when an electrician is present (such as shorts or harmonics), some manufacturers offer meters designed to be left on the circuit to take continuous or intermittent measurements and have the ability to record disturbances when they occur.
In some cases, sophisticated DMMs with built-in, dedicated software programs that run tests and interpret results can stand in for an electrician’s relative lack of honed, on-the-job experience in diagnosing problems, because the meters leave less to interpretation.
Safety ratings for CAT I through CAT IV are typically printed on the packaging and on the meter. The specification for the particular meter must also include the maximum voltage rating (e.g., 600V) for a line in that category for which the tool is tested.
“The CAT rating indicates the degree of separation from the utility and, essentially, tells where the unit can be operated, for example, downstream from the wall socket but not at the service panel, etc., while the voltage rating indicates the maximum allowable rated voltage for the circuit at that particular CAT level,” said Jeff Jowett, applications engineer, AVO International. Words of caution pointing out actions or conditions that might damage the tool and raise the risk of operator injury are sometimes also included and should be always be heeded.
CAT IV covers from the pole to the service entrance; CAT III covers from the service entrance to the outlet; CAT II covers PCs, TVs and appliances, with outlets more than 30 feet distant from the CAT III sources; and CAT I covers the signal level for telecommunications, electronic and other low-energy equipment with transient-limiting protection.
While most electricians’ measurements with DMMs on the job are in CAT III or less environments, occasionally some workers do need to take a CAT IV measurement. CAT IV meters, carrying a 1,000-volt rating (reflecting the higher built-in energy absorption capability to protect against transients) are used on the power line side of a service panel, typically “outdoors” or elsewhere before the circuit breaker panel, which is a fairly extreme environment and potentially a more hazardous location than indoors on the load side. As electricians are becoming more aware of CAT IV, and the availability of meters rated for that category, sales are, in fact, on the rise.
Many new meters display higher counts (e.g., 4,000, 5,000 or 6,000) than were customary with older models, providing greater levels of accuracy compared to units limited to 2,000 counts. “The higher count gives users an extra decimal place of accuracy without losing the digit of resolution,” noted Mel Hendrickson, application engineer, Amprobe. For example, if you measure a 240-volt signal with a 2,000-count meter, then you could only measure to 240 volts and not to a tenth, such as 240.0 volts. But with a 4,000-count meter, a 240-volt signal could read as 240.0 volts or 240.1 volts, etc, showing a tenth of a volt, for greater accuracy.
As John O’Brien, applications engineer at Extech Instruments, noted, an electrician would want greater accuracy when taking low-range DC measurements and when taking low-range AC voltage measurements as, for example, working on 24-volt circuit controls. The higher 5,000- or 6,000-count meters can provide desired accuracy in the millivolt ranges. An electrician would also benefit from a higher-count meter when measuring lower resistances, e.g., when measuring the winding resistance of a motor or a generator. On a 4,000-count unit, the electrician could measure, for example, to a tenth when measuring in the 400-ohm range using a 4,000-count meter, he pointed out.
Important considerations in choosing a meter should be the safety rating and the feature set. For everyday practicality, a good fit in the hand and a mode of operation that seems intuitive should be high on the list, as well.
Products below are non-inclusive representatives of manufacturer offerings.
Extech Instrument’s MultiPro auto-ranging digital multimeter Model MP530 provides a DC voltage accuracy of 0.08 percent of measurements on a backlit 5,000-count LCD with high resolution. The unit measures AC/DC voltage and current, resistance, frequency, capacitance, diode and continuity and provides True RMS measurements for AC voltage and current. Features include a wide AC voltage bandwidth (40Hz to 20kHz), auto power off, and high- and low-resistance auto-lead zero. It also measures selectable degrees Fahrenheit/degrees Celsius temperature and features Relative, Min, Max, Max-Min and 5ms Peak Hold. The UL Listed model meets IEC 1010 CAT III 600V, CAT II 1,000V. An RS-232 PC interface with Windows 95/98-compatible software (optional) allows the user to collect, display, plot, save or export data or graphs.
The shirt-pocket-sized Meterman Test Tools DM73B pen-style digital multimeter is well suited to ladder use because it can be operated with one hand. The 7½-inch-long slender instrument fits into tight spaces, small recessed areas or other areas, such as overhead stage lighting, where two-handed operation is not convenient. An attached probe point enables easy access to hard-to-reach measurement spots. The unit, which features 0.5 percent accuracy, auto power off, a 3,400 count 3¾-digit LCD display and an analog bar graph, conforms to Cat II-600V and Cat III-300V and measures AC and DC voltage, resistance and continuity. A touch-hold function freezes the measurement value on the display.
Complying with the CAT III standards, the UL-listed Greenlee Textron DM-820 S-Series Voltage Tester general-purpose digital multimeter features record max, min, max-min and average readings; and data hold, auto-ranging and crest capture of 5ms peak hold for max, min, and max-min readings. The battery-operated unit measures up to 1,000V AC and DC and up to 10Amps AC and DC and sports a temperature probe that measures up to 1,832 degrees Fahrenheit. The instrument, which comes with a backlit 5,000-count display and a 52-segment analog bar graph capability, has True RMS for measurements when harmonics are present. The meter works with optional Windows software and RS 232 cable for retrieval and display of stored data and real-time data logging and graphing.
Fluke Corporation’s handheld Digital Multimeter Model 179, featuring a True-RMS engine, gives 0.09 percent accuracy with 6,000 counts of resolution on a bright, backlit display. The meter, which provides V ac range from 0.1 mV to 1,000V, frequency range from 2 Hz to 100 kHz, and capacitance range from nF (nanofarad) to 9,999 mF (millifarad), offers a Min/Max/Avg mode for enhanced troubleshooting. The unit measures temperature with sensor leads plugged into front panel sockets. A battery-access door facilitates battery change with a standard screwdriver without breaking the calibration seal. A closed-case calibration feature allows adjustments directly from the front panel. An optional ToolPak provides strap and magnet hangers for safe hands-free operation.
Amprobe AM-91RS Digital Multimeter features True RMS measurement and measures AC/DC volts, AC/DC amps, dBm, frequency, duty cycle, resistance, conductance, continuity, capacitance and diode test. The model, which is equipped with a backlit LCD with bargraph, sports a selectable 3¾ digit, 4,000 count or 4¾ digit, 40,000-count and is auto- or manual-ranging selectable. The unit, which gives accurate readings regardless of power quality and allows viewing of two parameters simultaneously, also checks for waveform distortions and provides for event tracking. Readings are downloadable to a PC through standard RS 232 connection.
Gardner Bender 10-function, 32-range digital multimeter GDT-292A measures AC/DC voltage and current, resistance, continuity and diodes. The UL-Listed model, which comes with heavy-duty test leads, a battery and an instruction manual, includes a rubber holster with bench stand and built-in lead storage. Fuse protected up to 10 A, the unit has an audible continuity beep, a low-battery indicator and auto power off.
AVO International M8037 Dual-Display Multimeter measures TRMS dc and ac, and features choice of operation in manual- or auto-ranging mode. The unit, offering accuracy to 0.08 percent showing on a backlit display, features six dual-display functions: AC volts/frequency, AC amps/ frequency, conductance/resistance, duty cycle/frequency, dbm/frequency, and values/events. Other capabilities include record max, min, min/max and average, capacitance, frequency, conductance and diode test facilities, audible and visible input warnings and, along with data logging, the ability to sort and compare results. Users can select a 4,000-count mode or the higher resolution 40,000-count mode. The unit, rated to Cat III 1,000 V, also sports an analog bar graph that displays trending characteristics.
The Hioki Model 3257-50 DIGITAL HiTESTER Handheld DMM makes TRMS measurements that includes harmonic components. Accurate measurements are guaranteed for components in the range from 50 to 500 Hz. The 4,000-count tester, overload protected up to CAT III 1,000 V, features a Terminal Shutter Function interlock mechanism that exposes only the correct terminals for connection in the currently selected function, preventing the operator from connecting the test leads incorrectly. The DMM also features temperature measurement, a Hold-auto function, auto power save and auto/manual range switching. The LCD display includes a 56-segment bar graph.
The AEMC Instruments Multifunction clamp-on meter Model F05 auto detects, measures and displays AC/DC volts to 600V (900V peak), AC/DC amps to 400A (600A peak), resistance to 400kohms (kilo-ohms), continuity to 400 ohms and watts to 240kW (kilowatts). The compact unit also determines power factor and phase rotation. The meter has the capability to measure in-rush currents at frequencies between 15 and 70HZ by measuring successive RMS values calculated on ½-, 1-, 2½-, 5- and 10-period increments. Sampling time down to 500 microseconds is achieved. The unit, which has automatic power-off mode, is well balanced for one-handed operation and function selection. EC
THE FELDMANS write for various magazines and Web sites. They can be reached at email@example.com or 914.238.6272.