Update: Handheld multimeters

The five core functions of handheld multimeters have always been to measure ac and dc voltage and current, and resistance.

10/01/1998


The five core functions of handheld multimeters have always been to measure ac and dc voltage and current, and resistance. Compared to traditional analog meters, early digital models offered little more than alternative instruments for these five measurements.

The inexorable march of integrated circuit technology continues to change that scenario. The engine of a digital multimeter (DMM) is, in essence, a computer chip. Although analog instruments remain favored for certain types of measurements, DMMs have advanced to the point where a handheld unit can literally replace a whole benchtop full of test equipment. While most DMMs do not offer the range and accuracy of lab equipment, their performance is often more than adequate for maintenance and troubleshooting work.

Trends

Many electrical/electronic devices, are subject to a 1/3, 1/3, 1/3 type of trend: every 3 yr they become 1/3 smaller, 1/3 cheaper, and 1/3 more functional. Even though that depiction doesn't fully apply to DMMs, especially regarding size, it is indicative of the rate of change.

Another approach being taken is to offer modular instruments (Fig. 1 and Fig. 2). The modular setup consists of a basic instrument module plus an assortment of plug-on heads for different types of measurements.

Improved safety

A manufacturer of electric power transmission and distribution equipment made an urgent plea that this article: "Warn readers to be sure that multimeters are never used to test systems which have voltages that exceed the maximum voltage capability of the multimeter." Sounds like common sense -- use a 1000-V meter only on systems operating below 1000 V, and so on. But, the suitability involves more than just the meter's voltage rating and the operating voltage to be measured.

Multimeter manufacturers have begun to pay particular attention to two, more subtle phenomena for the system or device under test: The possibility of transient overvoltages and amount of fault energy available. Motor switching, capacitor switching, and lightning strikes are typical sources of transient overvoltages. These sources can cause spikes of thousands of volts on lines that have operating voltages of only a few hundred volts.

A large-enough voltage transient can cause arcing inside a multimeter. If sufficient fault energy is available, the meter can be blown apart. Even worse, a high-energy arc might be established between equipment terminals or between equipment terminals and ground. The fault current in such an arc may be thousands of amps, resulting in an arc blast.

IEC standard

These issues are addressed and codified in the relatively new standard 1010-1 issued by the International Electrotechnical Commission (IEC). This standard applies to low-voltage (up to 1000 V) test equipment.

The approach taken by the standard is to divide measurement applications into four categories. The categories, commonly abbreviated "CAT," are distinguished by the amount of transient energy and fault energy likely to be available at the point of measurement.

Clearly, the closer the measurement point is to the energy source, be it lightning bolt or electric utility, the more energy is available. Current flow due to a high-energy transient or a short circuit is dependent on the impedance between the point of fault and energy source. The four IEC categories are arranged according to available energy, with CAT IV the highest.

CAT IV covers utility connections and any outdoor (because of lightning hazard) conductors. Examples are service entrance, meters, primary overcurrent protection equipment, and main panels.

CAT III covers premises distribution. Examples include distribution panels, feeders, busway, switchgear, motors, and lighting systems.

CAT II covers single-phase, receptacle-connected loads more than 10 m from a CAT III source or more than 20 m from a CAT IV source. Examples are appliances and portable tools.

CAT I covers electronic equipment. Examples include high-voltage/low-energy sources, such as video display terminals.

IEC 1010-1 places stringent design and testing requirements on test leads and probes as well as multimeters. Certification, preferably by an independent lab, that they meet the desired category requirements is marked on both meters and leads.

Note that the category number of an instrument is far more important than its voltage measurement range in determining the degree of protection. For example, a CAT III 600-V meter is better protected against high-energy transients than a CAT II 1000-V meter.

Rugged dependability

A favorite attention-getting maneuver for some DMM sales personnel is to drop their product on the floor, or even throw it, and then show that it continues to work just fine. The competition is on to deliver instruments that stand up well in the toolbox and belt pouch environment. Soft, resilient outer cases are popular.

Cases sealed with o-rings and gaskets are becoming more common. These units are likely to offer battery replacement without disturbing case seals or calibration. Some are even designed for calibration without opening their sealed cases.

Accessories

Multimeter manufacturers offer a wide array of accessories that extend both measurement ranges and types of measurements. Some measurement accessories have standardized output signals and connections, analogous to traditional 1-mA/A current clamps.

These are the major capabilities offered via accessories:

- Current

- Power

- Energy (kWh)

- Power factor

- Insulation resistance

- Harmonics

- Temperature, contact (single and dual probe for differential) and IR

- Relative humidity

- Airflow

- Carbon monoxide

- Light intensity.

Some multimeter and independent accessory makers have beefed up their test leads, making them longer, stronger, and more pliable. Likewise, probes, clips, and other attachments tend to show a more industrial-duty design approach (Fig. 3).

Data handling

Since DMMs are built around microprocessors, the ability to store and manipulate measurement data comes naturally. In addition to simple storage of readings, features include capture of minimum, maximum, and average readings over a period of time (some offer time stamping as well) and storage of sequential values for trending.

Some DMMs multiply their power as measurement tools by including the ability to communicate with PCs. The link is typically accomplished by means of a cable that uses an optical coupler (for isolation) at the DMM end and a serial port connector at the PC end (Fig. 4). USB connectors for PC communication haven't yet invaded the DMM market.

In addition to connection hardware, the PC-friendly DMM is usually sold with a software package. The software greatly leverages DMM capabilities through additional data manipulation. Analysis of readings, plotting trends, generating reports, and exporting to spreadsheets are examples.

The PC software may be able to search the DMM's memory locations and may also be able to take control of the DMM and program measurements over time. One manufacturer includes a "virtual DMM" PC display, showing the instrument with its settings and real-time readings. Some DMMs can also be connected directly to a printer to produce hard copies of stored data.

Displays and ergonomics

Bigger and better displays are typical of the current generation of DMMs. "Bigger" occurs in two ways. Not only are characters larger, often discernible from a considerable distance, but some display panels have grown large enough to show two or more items of information simultaneously. For example, both voltage and frequency may be displayed.

Another dual-display approach augments the numerical reading of a measured quantity with a quasi-analog/real-time bar graph display of the same quantity. The moving bar graph (visible on the DMM in Fig. 3) provides the kind of intuitive information about slowly changing signals that users might otherwise seek from a true analog (pointer type) meter. The most advanced processing/display option is found in the "graphical multimeter," an instrument configured like a multimeter but capable of displaying waveforms like a scope.

Two features help when readings must be taken in poorly lighted areas. One is lighted displays, and another is "hold" buttons. When a hold button is pressed, the measurement reading is frozen so that the meter can be taken to a more convenient area for viewing. This feature can also be of use in other difficult situations. One manufacturer offers a multimeter having a head-mounted projection display. Readings are viewed on a miniature transparent screen suspended in front of one eye, for the ultimate in visibility.

In a manner reminiscent of the competition for maximum number of cupholders per vehicle in the auto industry, multimeter manufacturers are striving for cleverly thought out ergonomic designs. The most common result seems to be cases having combination clip/stand/hanger features.

Tips for buyers

Multimeter prices range from as little as $20 for an extreme bottom-end unit to well over $1000 for a high-end model with accessories. A little homework goes a long way toward getting the best return on a multimeter investment.

The trend toward all-in-one test tools means that the buyer is confronted with a bewildering array of choices. Multimeter manufacturers have a tough job helping the buyer distinguish between their own models, and comparison charts abound. Nonetheless, a real value of the current generation of troubleshooting tools is that they allow the technician to focus more attention on the problem, rather than on test equipment.

Define needs

A good strategy is to begin by defining where and how the instrument will be used. If it will travel around the plant on a cart or in a tool pouch, a tough, gasketed case is important. Basic electrical protection features such as guarding against reverse polarity and transients help, too.

What are the safety concerns? For maximum protection, determine the highest IEC category level for any measurements likely to be made with the instrument. (CAT III, 1000 V is the highest rating offered for multimeters currently on the market.)

Make a list of the specific types of troubleshooting to be performed in order to define what instrument capabilities are important. When a technician heads out with tools and a work order, he or she isn't likely to know in advance what caused the problem.

What parameters may need to be measured? In what magnitudes? How much accuracy is actually needed? Might it be necessary to monitor a signal over a period of time? An approach sometimes suggested is to ask the questions "who, what, when, where, and how."

High-accuracy measurements are not often needed for troubleshooting work. But if accuracy is important, the multimeter manufacturer's provisions for calibration should be a matter of concern.

What are the recommended calibration intervals? Who can perform calibration services? The accuracy of measurements made with an out-of-calibration instrument cannot be relied upon.

Rms vs average

"True rms" measurement capability has been the hottest specification item in recent years. Here's why.

The physical principle by which traditional multimeters, both analog and digital, measure a signal results in a measurement of the average value of the signal. For a pure, clean sine wave such as shown in Fig. 5, the average value is 63.6% of the peak value. Unfortunately, the average value of a signal is seldom of interest to anyone.

The measurement value most often sought is the rms (root-mean-square) value. When ac voltage or current values are stated, they are understood to be rms values unless indicated otherwise.

For sinusoidal ac, the rms value is equivalent to a dc reading of the same magnitude. That is, the rms value of ac is that which produces the same amount of heat in a resistor as an equal value of dc. Hence the term effective value is sometimes used in place of rms value. For the sine wave in Fig. 5, the rms value is 70.7% of the peak value.

Because rms and average values are both related to the peak value by constants, they are related to each other by a constant: rms value = (0.707/0.636)(average value) = 1.11(average value). Meter makers simply calibrate analog meter faces or digital displays to indicate a value that is 1.11 times the average value sensed by the meter.

That approach works for pure sine waves, but for other waveshapes, including distorted sine waves, the 1.11 relationship does not hold. The result is that in this era of "polluted" power, an average-responding meter may give an incorrect reading.

Ideally, an instrument designed to measure "true rms" values gives correct readings for any waveshape. Real-world true rms DMMs do a good job up to a certain level of harmonic distortion, indicated by a crest factor rating.

Manufacturers guide

The manufacturers guide at left provides a rough indication of the product offerings from 18 multimeter makers. It is important to note that parameters listed indicate product line capabilities, but do not necessarily apply to any single instrument. Additionally, the measurement range for a given parameter can vary considerably from manufacturer-to-manufacturer and model-to-model. That's why it helps to define your specific needs before wading into a sea of specifications.

Plant Engineering magazine extends its appreciation to Fieldpiece Instruments, Inc.; Fluke Corp.; and Tektronix, Inc., for information used in this article. The cover photo was furnished by Fluke Corp.

-- Gary Weidner, Senior Editor, 847-390-2689, gweidner@cahners.com

Key concepts

New generations of multimeters deliver more "bang for the buck." This trend will continue, resulting in increased worker productivity.

Safety and rugged dependability are two growing concerns.

Data handling capabilities are gaining in popularity.

Accuracy

The accuracy of an analog meter is specified as a

Resolution indicates the smallest change in a signal that can be measured. A resolution of 1 mV means that a signal change of 1/1000 V can be discerned.

Digits indicate the number of DMM display digits. Often, a fractional digit is specified. A fractional digit is only partially functional, such as having only the values 0 or 1.

Counts indicate resolution in terms of the number of digital steps into which the maximum signal range is divided. For example, a 3 1/2-digit DMM can display up to 1999 counts of resolution. A 4 1/2-digit DMM can display up to 19,999 counts of resolution.

Accuracy of a DMM is commonly specified as a percent of reading plus a range of the least significant digit. For example, an accuracy might be specified as 0.5%+2. At this accuracy, a display reading of 100.0 V means that the actual voltage lies between 99.3 V and 100.7 V.

More info

The complete text of all our articles is available on our website at www.plantengineering.com. Copies of articles published in Plant Engineering can be purchased by calling 847-390-2694.





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