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Seven Questions for DMMs

Seven questions to ask before buying your next digital multimeter.

Seven Questions for DMMs

Dec 1, 2001 12:00 PM,
By Terry J. LaBrue

WITHOUT TESTING TOOLS, IT WOULD BE IMPOSSIBLE to troubleshoot electronic systems on the bench or in the field. Whether its an amplifier or video projector, every installation performed by a contractor uses some sort of electronic system that will either fail or need to be tested. The digital multimeter has made a solid entry into the test tool arsenal of the modern systems contractor. Their widespread use has almost all contractors owning at least one DMM and using them for even the simplest of measurements.

Since their invention in the early 1970s, DMMs have become as ubiquitous and as useful as the tape measure was to previous generations of workers. New shapes and styles, new features and new models for niche markets are proliferating like mushrooms after a hard rain. For many users, these indispensable tools are now professional necessities.

With the incredible growth in handheld digital multimeters estimated to be over three million units per year, the challenge is for test tool manufacturers to add utility and value to a maturing professional market. Smart manufacturers are segmenting the market with targeted products rather than taking the one-size-fits-all approach. Forward-thinking DMM designs include such perks a meter the size of a business card, a troubleshooting tool in the shape of a fat fountain pen for lighting applications, and a sleek clamp meter that combines the testing ability of a DMM with integral clamps for testing live cables and wires.

There are certainly bells and whistles to be had, but what other things should you consider before buying? Beyond conducting cursory research, comparing prices and determining where to buy a DMM, there are seven crucial factors that will affect your ultimate purchase and your eventual satisfaction with the product you select. Consider these deciding questions (on page 56), and your answers will lead you to a proper purchase decision.

GETTING THE MOST FROM YOUR DMM

The value-priced DMMs now reaching the market are full of application capabilities that were once found only in high-end industrial meters. These new products are typically smaller, lighter and more intuitive. They can include convenient extras such as data hold, special ranges to test household batteries and LED indicators. These DMMs are indispensable when working with today’s home electronic devices.

Other appealing attributes are auto-ranging and manual ranging. Some DMMs feature diode test and continuity capabilities to help find pesky electrical shorts and elusive intermittent opens. Many mid-range DMMs are designed to measure AC/DC voltage and current for appliances, timers, lighting, heating and air conditioning, doorbells, telephones, audio and video equipment and portable tools.

Basic and Extended Meter Functions

Nearly all DMMs measure electric current as volts, amps and ohms or continuity. These will be perfectly adequate for the bulk of your testing and trouble-shooting jobs around the shop. However, there are meters with extended hardware functions and features that may include diode, capacitance, temperature, transistor, frequency and logic.

Extended software features also can include hold functions, min/max analysis, relative, RS-232 serial output and others.

Doing It Right

Professionals who use a DMM daily have learned a few shortcuts, to whit:

  • When checking for voltage, start at the source (an outlet, fuse box or battery).
  • Next, check voltage across the component to see if current is flowing properly.
  • Finally, check the fuse with ohms or continuity. (Be certain to turn the power off first!)

Specific Safety Protocols

There was a time when consumers were expected to pay their money and take their chances with value-priced meters, but recently even the mid-market meters have come a long way in the safety arena. New meters with better design and manufacturing provide a meter that will be simultaneously easy and safe to use.

Working with any electricity is potentially dangerous. The user must be fully aware of the condition of the test instrument in use and of the job site. Safety is everyone’s responsibility, and ultimately it’s in your hands. Central among the safety aspects is protection from potentially dangerous electrical situations. Meters should have over-voltage protection to safeguard the user and the meter in case of surge voltages and fuses for the current ranges.

The real issue for multimeter circuit protection isn’t just the maximum, steady-state voltage range, but a combination of both that and transient, over-voltage withstand capability. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large, deadly currents. DMM buyers should look for a designation mark such as the CE symbol (Conformite Europeenne) that indicates the meter has been tested and certified to international standards to be safe when used within its rated capabilities.

There is no substitute, however, for following the safety procedures in the owner’s manual. Such procedures may include remembering to test a known live circuit before proceeding to test an unknown source when checking voltage, keeping the meter and test leads dry and clean, and never working with electricity where water is present.

DMM Accessories

To really explore the full capability of a DMM, buying a basic selection of test probes and leads is recommended. These are:

  • Test-lead insulated tip extenders to help you check in narrow, deep places.
  • Alligator clips with rubber covers that prevent leads from shorting out when attached to the circuit.

Other useful DMM additions and accessories include temperature converters (which allow your meter to test for temperature) and current clamps (which permit you to check current flow without breaking the circuit in test).

CONCLUSIONS

Whether your DMM spends more time inside the office or out in the field, it is an indispensable tool once you understand its capabilities, proper use and safety procedures. When following these guidelines, your digital multimeter can provide years of handy measurements. Just remember to be cautious and use the right tools.

Terry J. LaBrue is an associate at Wavetek Meterman Test Tools, based in Everett, WA.

Which Meter Is Right for You?

There are two kinds of meters now commonly available as DMMs: average responding and true RMS. Average-responding units are widely used, are usually lower in cost, and work well for linear electrical loads, such as residential electrical systems with standard induction motors, resistance heaters and incandescent lights. But when the loads are nonlinear, include adjustable-frequency drives, or contain semiconductors (as in photocopiers, commercial lighting, elevators and commercial air conditioning units), then the average responding meters typically read low, perhaps as much as 40% in error. These cases require the best possible DMM for all applications: a true-RMS meter.

THE SEVEN DECIDING QUESTIONS

Be sure to ask yourself these before you purchase a DMM.

For what purpose will you use your DMM?

There are hundreds of types of DMMs on store shelves. Without considering how you will use your DMM, you could end up with a meter with the wrong feature set or improper ranges for the tasks at hand. Determine early in the process what you want to do with it, and frame your search along the lines of your specific needs.

  • Will you measure voltage, current, ohms or a combination of all three?
  • Do you need frequency, temperature and/or capacitance?
  • What range is necessary? Some models are specifically designed for electronics troubleshooting (CAT I and II). Others are right for home and automotive (CAT I to III). While still others are offered for high-voltage use (CAT IV). Make certain your new meter fulfills your basic electrical requirements.

In what environment will you use the test tool?

  • Do you work in a shop or in the field? Will your meter need to be heavy-duty, waterproof and rugged, or will you mainly perform delicate, precise measurements indoors?
  • Will you be working in daylight or inside a dark electrical closet? Back-lit LCD displays are a necessity where lights and visibility are low.
  • What electrical power range do you need? Each current level is described by category such as CAT I for electronics and CAT IV for power lines that run from a building to utility lines. Select the right meter for safety rating (including over-voltage protection).

How do you handle your tools? What is your work style?

Will your meter stay indoors? Will it accompany you on a tool belt? Will you toss it into a toolbox? Some models incorporate handy prop-up stands, and others feature rugged holsters, tough enough for the most demanding situations. If your style is rough and tumble, a few dollars spent for a carrying case or holster to protect your investment is well worth it.

Is this purchase going to be a long- or short-term investment?

Will this meter be your primary tool, or will it be a backup for another, more high-end meter? Many workers like to be prepared and have a backup DMM always on hand. Others have only occasional need for their DMM. If this tool will be your best friend for years to come, make an investment in a sturdy, state-of-the-art meter.

Do you need a complete set of accessories, or do you prefer to buy each oneà la carte?

Many suppliers offer complete lines of test leads, probes, thermocouples and more. Several companies also have task-related kits that employ everything you’ll need to undertake specific measurements. Make a list of everything you’ll need, and see if there is one model that includes it all, or one maker that offers a complete solution.

What are the factors of after-purchase service and warranties?

Many meters and test tools are manufactured in other countries, so the basic meter warranty is an important consideration. Who will repair it? Where will you send it for warranty work? Does the manufacturer offer an on-the-spot replacement program that saves time, money and hassle?

What is the hidden cost of buying “too much meter?”

Digital multimeters are like any electronics purchase, from audio equipment to computers. You can spend plenty and get a multifunction, multitask, multifeature unit. But be aware that as the options and features mount, so do the potential problems. Add to that the cost of learning to use all the extra buttons and buzzers and familiarizing yourself with the proper measurement protocols, and it’s easy to see how purchasing too much meter can be a waste of time and money.

High-end meters often include features that you will seldom, if ever, use. Conversely, you can select a quality basic meter that offers straightforward features and is easy to use and read. This option saves money and avoids potential problems while decreasing the learning curve.

Walking through a meter’s manual before you walk down the dealer’s aisle can greatly simplify your next test tool purchase. Treat the test tool as a major investment worthy of your time and effort.

KNOW YOUR OVER-VOLTAGE CATEGORIES

CAT I: Electronics

  • Protected electronics equipment
  • Equipment connected to source circuits in which measures are taken to limit transient over-voltages to an appropriately low level
  • Any high-voltage, low-energy source derived from a high-winding resistance transformer, such as the high-voltage section of a photocopier

CAT II: Single-phase receptacle-connected loads

  • Appliances, portable tools and other household and similar loads
  • Outlet and long brand circuits
  • Outlets at more than 30 feet (10 meters) from CAT III source
  • Outlets at more than 60 feet (20 meters) from CAT IV source

CAT III: Three-phase distribution

  • Single-phase commercial lighting
  • Equipment in fixed installations, such as switch-gear and polyphase motors
  • Bus and feeder lines in industrial plants
  • Feeders and short branch circuits
  • Distribution panel devices
  • Lighting systems in larger buildings
  • Appliance outlets with short connections to a service entrance

CAT IV: Three-phase at utility connection and any outdoor conductor

  • Origin of installation, i.e., where a low-voltage connection is made to utility power
  • Electricity meters
  • Primary over-current protection equipment
  • Outside and service entrance
  • Service drop from pole to building
  • Run between meter and panel
  • Over power line to a detached building
  • Underground line to a well pump

A Closer Look at Clamp Meters

If you ever need to measure electrical current without shutting down your power system, you will appreciate the utility of a current clamp or clamp meter. These indispensable tools combine current and voltage measurements with an integrated digital display in a handy piece of test equipment.

Clamp meters make detection of AC and DC electrical problems fast and easy, even where it is difficult, or impossible, to turn off the system. You use a clamp meter by simply clamping the jaws around the conductor of a circuit and reading the appropriate values on the built-in digital display. There is no need to turn off the circuit, alter the operation or remove any insulation. It’s simple.

Basic Types

Clamp meters, like DMMs, are generally either average-responding or true-RMS. Because of potentially dangerous overheating where current can burn out circuit breakers, electrical equipment and wiring, the best clamp meter for all applications is one that measures true RMS (even though it’s slightly more expensive). At the outset, decide where and how you will be using the meter. These two factors should drive your purchase decision.

Features

With the large number of clamp meters on the market, it pays to compare features, both in terms of function and design. For most clamp-meter applications, you need to perform basic voltage and current measurements in AC and sometimes DC. Additional features on new clamp meters can make your life easier and readings more accurate. Max hold is one such option, designed to capture start-up and peak-circuit loads. The current that is drawn by a device when it is initially turned on is higher than the nominal steady-state value. An electric motor, for instance, commonly draws start-up currents of three to eight times the nominal operating value.

Another handy option, peak hold, can measure instantaneous peak values of the current waveform. It can be used to determine the crest factor that indicates the presence of harmonics on the electrical line. Also keep in mind product features that are related to the meter’s physical design:

  • Tapered clamp jaws allow easier test access in crowded junction boxes and can still accept cables up to 1.4 inches around.
  • Hand guards prevent accidental contact with live conductors.
  • Manual ranging is easier to read, especially when taking repetitive measurements.
  • Safety-shrouded input jacks for voltage and resistance inputs using external test-leads increase safety.

Most clamp meters come with a warranty, but those manufactured overseas may not include repair options. Some manufacturers may offer an on-the-spot product replacement policy that eliminates the hassle involved in shipping and making-do without your unit while the meter is away for service. It’s a warranty to look for.

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