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New DMMs address specific applications: Service calls go faster if you can measure frequency, capacitance, and temperature as well as volts and amps. (DMMs).

In contrast to terms that have changed their meanings over the years, multimeter is even more appropriate today than when first coined. In addition to the basic voltage, current, and resistance measurement capabilities, modern digital multimeters (DMMs) may have 10 or more functions.

As shown in the DMM Comparison Chart that accompanies this article, the common functions are autoranging, true rms AC voltage, frequency, capacitance, diode and continuity checking, max/min/avg reading, and temperature via both thermocouples and resistance temperature devices (RTDs). Less common are readings in decibels, displaying the difference between two channels, datalogging, and power and LCR bridge measurements.

Together with backlit and dual-channel displays, auto power off, and a range of safety specifications, there's no lack of functionality. As you would expect, the greater the flexibility of an instrument, generally the higher the cost. Accuracy and resolution are two other factors that affect price, although perhaps not in an obvious way.

According to Andy Zeidler, marketing manager at GMC Instruments, "High resolution no longer is a factor for cost because the price difference between a 4 3/4- and a 5 3/4-digit processor chip now is approximately one dollar. "Accuracy is different," he explained. "A high-resolution DMM also must have long-term stability so that it maintains calibration between recertifications.

"Because DMMs often are service tools and spend time in cars at temperatures ranging from -20[degrees]F to +160[degrees]F, there is significant stress on the components. The problem can be reduced," he continued, "but requires compensation circuitry and the engineering expertise to design it. Consequently, only the more expensive DMMs offer combined high resolution and accuracy over the long term."

As an example of an application that now can be addressed by a hand-held DMM, Mr. Zeidler cited the medical instrumentation industry, where benchtop DMMs previously were required. Basic accuracy better than 0.02% is needed for this type of work.

The Measurement Environment

Capacitance

Many of the new features offered in handheld DMMs satisfy customer requests for specific capabilities. For example, maintaining motor drives involves measuring voltage and current, but it also may be necessary to determine if a starting capacitor is faulty. You may need to check the temperature rise of the output devices under load. A DMM with both temperature and capacitance measuring functions allows you to perform a more complete test than is possible with a basic volts/amps/ohms DMM.

Small size, ease of use, and low cost are important attributes of a DMM, but they also limit the performance that is attainable. According to Paul Heydron, product planning manager at Fluke, hand-held DMMs measure capacitance by solving the charge = CV equation. A constant current charges the unknown capacitor for a known time, and the resulting voltage is measured.

Alternatively, the charging continues to a set voltage, and the elapsed time is measured. Some DMMs approximate a constant current source by a DC voltage and a large resistor, especially on lower capacitance ranges.

Although you may be able to measure capacitance accurately enough to address a motor-drive application, a DMM won't tell you the capacitor's dissipation factor. Similarly, most hand-held DMMs don't measure inductance at all, so you certainly can't determine the Q of an inductor with a DMM.

Temperature

Temperature measurement presents its own set of challenges. RTDs often are made from platinum wire and are relatively expensive. On the other hand, RTDs are linear, and measuring temperature with an RTD is equivalent to measuring resistance.

Thermocouples are very low cost but they require linearization and a stable temperature reference. The socalled cold junction in a thermocouple circuit ideally is held at a constant temperature. How can that be accomplished in a small, hand-held DMM that may be in the trunk of a serviceman's car at 10[degrees]F one minute and inside a steel mill the next?

The trick is to understand the environment in which the meter will be used so that the cold-junction circuitry can be designed accordingly. A bench DMM may provide better cold-junction compensation as well as a longer thermal time constant than are possible in a portable unit. Both design features support more stable and accurate readings.

A hand-held DMM should measure the cold-junction temperature so that the difference between it and the unknown junction temperature can be correctly indicated. A very low-cost meter might be calibrated with the cold junction at 23[degrees]C, for example. Such a DMM would work well with large temperature differences in relatively stable ambient surroundings. It would be useless for measuring small temperature differences, especially if you held the meter in your hand, causing it to heat up.

Thermocouple linearization is not black magic. Well-established polynomial equations derive accurate temperature readings from millivolt thermocouple outputs. You need to make sure that your DMM has the necessary processing power to solve the equations.

Some DMMs only measure a thermocouple's output voltage and leave the linearization to you. DMMs that quote temperature accuracy use an integral microcontroller to perform the required mathematics. Some meters allow you to choose the type of thermocouple you wish to use. Others support only one type, such as the common K-type.

Frequency

Counting input pulses for a period of time provides a measure of frequency. Complications arise because input signals may not have perfectly square edges. This means that some amount of signal conditioning is needed between the DMM input and the actual digital counter.

The simplest approach is to limit or clip the input signal above a set level. This method works well with many sizes of signals. It fails when the input is too small and may give false results when the signal is too large. Some clipping circuits fold back at high levels of overdrive, producing false signals equivalent to extra edges.

A more sophisticated design supports switching between ranges so that large signals can be attenuated or small ones amplified before the clipping stage. Additional cost and complexity accompany the higher performance solution. And greater complexity is required not just in the circuitry, but also in the user interface.

How many hand-held DMM users need the flexibility afforded by a frequency mode with switchable input coupling? How many users prefer the simplicity of a single range accompanied by fewer control settings? DMM manufacturers are seeking customer feedback to answer questions like these and guide the development of new, ever more capable DMMs.

The Future

You can look for measurement performance improvements in many areas of new hand-held DMMs, but these may not be dramatic. Instead, manufacturers are concentrating on providing the best tool for a particular job, and greater measurement accuracy, for example, may be irrelevant.

Problems of safety, ruggedness, reliability, and access are being aggressively addressed. As an example, Fluke provides a means of hanging probes to allow hands-free operation, a display mode with large readout digits for viewing at a distance, and big controls on some meters that can be operated by users wearing gloves. You may be attracted to these features if you work on live motor drives or other applications involving potentially lethal voltages and currents.

At the other end of the scale, the Wavetek Meterman DM73B is a very small, pen-probe-style DMM that fits in a shirt pocket. The DM73B is popular among servicemen who require confirmation of gross operating parameters in a very portable package.

In summary, the diversity of products being offered is the real story in hand-held DMMs. This means that when selecting a DMM you no longer need to make as many compromises as in the past. The bottom line is affordable, easy-to-obtain, comprehensive field measurements that should lead to more satisfactory equipment installation, commissioning, and service.
DMM Comparison Chart

                                                      DC Voltage
                                                             Accuracy
                                                           (% + Counts)

                                 Model       Full Scale     Resolution
Company                         (Counts)      (V to V)      ([micro]V)

A. W. Sperry Instruments        DM-360A         0.2          1.2% + 3
631-231-7050                                     to
www.awsperry.com                (2,000)         600            100

ATP-Amprobe                     AM-91RS         0.04        0.08% + 1
800-327-5060, ext. 7598                          to
www.amprobe.com              (4,000/40,000)     600             10

B+K Precision                    2880A          0.04        0.06% + 3
800-462-9832                                     to
www.bkprecision.com             (40,000)       1,000            1

                                  391A          0.2         0.05% + 3
                                                 to
                                (20,000)       1,000            10

Extech Instruments              MultiLog        0.05        0.08% + 2
781-890-7440                     ML720           to
www.extech.com                                 1,000            10
                                (5,000)

Fluke                           187 and         0.05        0.025% + 5
800-443-5853                      189            to
www.fluke.com                                  1,000            1
                                (50,000)

                                  179           0.6         0.09% + 2
                                                 to
                                (6,000)        1,000           100

GMC Instruments                 METRAHit        0.3         0.02% + 5
800-462-4040                      29S            to
www.gmcinc.com                                 1,000            1
                               (310,000)

Hioki USA                       3256-51         0.42         0.5% + 2
609-409-9109                                     to
www.hiokiusa.com                (4,200)        1,000           100

Omega Engineering              Supermeter       0.43        0.25% + 1
203-359-1660                     HHM290          to
www.omega.com                                  1,000            1
                               (430,000)

                                Talking         0.4         0.25% + 2
                               Multimeter        to
                                  HHM2         1,000           100

                                (1,999)

Protek Test & Measurement         608           0.5         0.05% + 50
201-760-9898                                     to
www.protektest.com              (50,000)       1,000            10

Test Products International       440           0.1          0.3% + 2
800-368-5719                                     to
www.tpi-thevalueleader.com      (4,000)        1,000           100

Wavetek Meterman                 DM73B          0.34         0.5% + 2
877-596-2680                                     to
www.metermantesttools.com       (3,400)         600            100

                                 HD160B          1          0.25% + 6
                                                 to
                                (10,000)       1,500           100

Yokogawa Corp. of America        732-01         0.4          0.2% + 1
800-888-6400                                     to
www.yca.com                     (4,300)        1,000           100


                                 734-02         0.05        0.03% + 2
                                                 to
                                (50,000)       1,000            1


                                           AC Voltage
                                             Accuracy
                                           (% + Counts)
                                                         Frequency
                             Full Scale     Resolution     Range
Company                       (V to V)      ([micro]V)     (Hz)

A. W. Sperry Instruments        0.2             2%          50
631-231-7050                     to                         to
www.awsperry.com                600           100 mV        500

ATP-Amprobe                     0.4          0.5% + 3       50
800-327-5060, ext. 7598          to                         to
www.amprobe.com                 600            100          50k

B+K Precision                   0.04         0.7% + 5       50
800-462-9832                     to                         to
www.bkprecision.com             750             1           20k

                                0.2          1% + 10        50
                                 to                         to
                                750             10          2k

Extech Instruments              0.05        0.08% + 2       40
781-890-7440                     to                         to
www.extech.com                 1,000            10          20k


Fluke                           0.05        0.4% + 40       20
800-443-5853                     to                         to
www.fluke.com                  1,000            1          100k


                                0.6           1% + 3        45
                                 to                         to
                               1,000           100          1k

GMC Instruments                 0.3         0.5% + 30       15
800-462-4040                     to                         to
www.gmcinc.com                 1,000            10         100k


Hioki USA                       0.42         1.2% + 3       50
609-409-9109                     to                         to
www.hiokiusa.com               1,000           100          500

Omega Engineering               0.43          1% + 2        50
203-359-1660                     to                         to
www.omega.com                   750             1           60


                                0.4         0.25% + 2       50
                                 to                         to
                                750            100          5k



Protek Test & Measurement       0.5         0.75% + 20      50
201-760-9898                     to                         to
www.protektest.com              750             10          20k

Test Products International     0.1         1.50% + 10      20
800-368-5719                     to                         to
www.tpi-thevalueleader.com     1,000           100         200k

Wavetek Meterman                3.4          1.5% + 8       50
877-596-2680                     to                         to
www.metermantesttools.com       600           1,000         500

                                 10          1.5% +5        45
                                 to                         to
                               1,000          1,000         30k

Yokogawa Corp. of America       0.4           1% + 5        50
800-888-6400                     to                         to
www.yca.com                    1,000           100          1k


                                0.5         0.4% + 30       10
                                 to                         to
                               1,000            10         100k


                             AC Voltage           DC Current
                                                       Accuracy
                                                     (% + Counts)

                             True        Full Scale   Resolution
Company                      rms         (mA to A)    ([micro]V)

A. W. Sperry Instruments      No              0.2      1.5% + 3
631-231-7050                                   to
www.awsperry.com                               10        0.1

ATP-Amprobe                  Yes              0.4      0.2% + 2
800-327-5060, ext. 7598                        to
www.amprobe.com                                10        0.1

B+K Precision                Yes              0.4      0.2% + 3
800-462-9832                                   to
www.bkprecision.com                            10        0.01

                             Yes              0.2      0.5% + 5
                                               to
                                               20        0.01

Extech Instruments           Yes              0.5      0.2% + 4
781-890-7440                                   to
www.extech.com                                 10         0.1


Fluke                        Yes              0.5     0.15% + 2
800-443-5853                                   to
www.fluke.com                                  20        0.01


                             Yes               60       1% + 3
                                               to
                                               10         10

GMC Instruments              Yes              0.3     0.05% + 5
800-462-4040                                   to
www.gmcinc.com                                 10       0.001


Hioki USA                     No            0.056      1.5% +4
609-409-9109                                   to
www.hiokiusa.com                               10         10

Omega Engineering            Yes             0.43      0.5% + 1
203-359-1660                                   to
www.omega.com                                  10       0.001


                             Yes              400      0.5% + 2
                                               to
                                                4        100



Protek Test & Measurement    Yes                5      0.2% + 5
201-760-9898                                   to
www.protektest.com                             10        0.1

Test Products International  Yes              0.1      0.5% + 5
800-368-5719                                   to
www.tpi-thevalueleader.com                     10        0.1

Wavetek Meterman              No              N/A        N/A
877-596-2680
www.metermantesttools.com

                             Yes               10      0.75% + 5
                                               to
                                               10         1

Yokogawa Corp. of America    Yes              0.4       1% + 2
800-888-6400                                   to
www.yca.com                                    10        0.1


                             Yes              0.5      0.2% + 2
                                               to
                                               10        0.1


                                      AC Current
                                             Accuracy
                                           (% + Counts)

                              Full Scale    Resolution
Company                       (mA to A)     ([micro]V)

A. W. Sperry Instruments          NA            NA
631-231-7050
www.awsperry.com

ATP-Amprobe                      0.4          1% + 4
800-327-5060, ext. 7598           to
www.amprobe.com                   10           0.1

B+K Precision                    0.4          1% + 5
800-462-9832                      to
www.bkprecision.com               10           0.01

                                 0.2        1.2% + 10
                                  to
                                  20           0.01

Extech Instruments               0.5         0.2% + 4
781-890-7440                      to
www.extech.com                    10           0.1


Fluke                            0.5        0.75% + 5
800-443-5853                      to
www.fluke.com                     20           0.01


                                  60         1.5% + 3

                                  10            10

GMC Instruments                  0.3        0.5% + 30
800-462-4040                      to
www.gmcinc.com                    10           0.01


Hioki USA                       0.056        2.5% + 4
609-409-9109                      to
www.hiokiusa.com                  10            10

Omega Engineering                0.43         1% + 2
203-359-1660                      to
www.omega.com                     10          0.001


                                 0.4          2% + 2
                                  to
                                  4            100



Protek Test & Measurement         5         0.75% + 20
201-760-9898                      to
www.protektest.com                10           0.1

Test Products International      0.1        0.75% + 10
800-368-5719                      to
www.tpi-thevalueleader.com        10           0.1

Wavetek Meterman                 N/A           N/A
877-596-2680
www.metermantesttools.com

                                  10        1.75% + 5
                                  to
                                  10            1

Yokogawa Corp. of America        0.4          1% + 5
800-888-6400                      to
www.yca.com                       10           0.1


                                 0.5        0.75% + 20
                                  to
                                  10            10


                                        Resistance
                                                  Accuracy
                                                (% + Counts)

                                Full Scale       Resolution
Company                      ([ohm] to M[ohm])    ([ohm])

A. W. Sperry Instruments            200             1.5%
631-231-7050                        to
www.awsperry.com                    20              0.1

ATP-Amprobe                         40           0.15% + 2
800-327-5060, ext. 7598             to
www.amprobe.com                     40              0.01

B+K Precision                       400           0.2% + 3
800-462-9832                        to
www.bkprecision.com                 40              0.01

                                    200          0.15% + 3
                                    to
                                    20              0.01

Extech Instruments                  50            0.1% + 2
781-890-7440                        to
www.extech.com                      50              0.01


Fluke                               500          0.05% + 2
800-443-5853                        to
www.fluke.com                       500             0.01


                                    600           0.9% + 1
                                    to
                                    50              0.1

GMC Instruments                     300          0.05% + 5
800-462-4040                        to
www.gmcinc.com                      30             0.001


Hioki USA                           560           0.5% + 4
609-409-9109                        to
www.hiokiusa.com                    56              0.1

Omega Engineering                   430           0.3% + 3
203-359-1660                        to
www.omega.com                       43              0.01


                                    400          0.25% + 2
                                    to
                                    40              0.1



Protek Test & Measurement           50            0.2% + 5
201-760-9898                        to
www.protektest.com                  50              0.01

Test Products International         400           0.3% + 2
800-368-5719                        to
www.tpi-thevalueleader.com          30              0.1

Wavetek Meterman                    340            1% + 4
877-596-2680                        to
www.metermantesttools.com           34              0.1

                                   1,000          0.5% + 1
                                    to
                                    40              0.1

Yokogawa Corp. of America           400           0.4% + 1
800-888-6400                        to
www.yca.com                         40              0.1


                                    500          0.05% + 2
                                    to
                                    50              0.01


                              Frequency     Capacitance



                             Full Scale      Full Scale
Company                      (Hz to MHz)  (nF to [micro]F)

A. W. Sperry Instruments         NA              NA
631-231-7050
www.awsperry.com

ATP-Amprobe                      100             4
800-327-5060, ext. 7598          to              to
www.amprobe.com                   4            4,000

B+K Precision                    100             4
800-462-9832                     to              to
www.bkprecision.com              0.2           10,000

                                 2k             N/A
                                 to
                                 0.2

Extech Instruments              0.001           N/A
781-890-7440                     to
www.extech.com                  0.125


Fluke                            500             1
800-443-5853                     to              to
www.fluke.com                     1            50,000


                                 100           1,000
                                 to              to
                                 0.1           10,000

GMC Instruments                  300             3
800-462-4040                     to              to
www.gmcinc.com                   0.3           30,000


Hioki USA                        50             N/A
609-409-9109                     to
www.hiokiusa.com               0.0005

Omega Engineering                100            4.3
203-359-1660                     to              to
www.omega.com                    1.8            430


                                 N/A            N/A





Protek Test & Measurement        50              5
201-760-9898                     to              to
www.protektest.com               5k            50,000

Test Products International      100        400 [micro]F
800-368-5719                     to
www.tpi-thevalueleader.com        2

Wavetek Meterman                 NA              NA
877-596-2680
www.metermantesttools.com

                                 N/A            N/A



Yokogawa Corp. of America        100             10
800-888-6400                     to              to
www.yca.com                      0.1           1,000


                                  2              5
                                 to              to
                                 0.1           50,000







Company                      Special Features

A. W. Sperry Instruments     CE, UL, CUL, designed to CAT III
631-231-7050                 600 V, diode test, microwave
www.awsperry.com             diode test, continuity buzzer

ATP-Amprobe                  CE, IEC, 1010-1, CAT IV 600 V, UL,
800-327-5060, ext. 7598      dB, crest, duty cycle, min/max/avg,
www.amprobe.com              dual display, backlit, PC interface

B+K Precision                Selectable 4,000 or 40,000 count,
800-462-9832                 data hold, min/max/avg, diode/
www.bkprecision.com          continuity measurement, auto-off

                             CE, IEC 1010-1 UL, CAT III 1,000 V, diode/
                             continuity measurement, auto-power off,
                             logic probe, data hold, temperature

Extech Instruments           CE, UL, IEC 1010-1, CAT III 600 V,
781-890-7440                 temperature, RS-232, bar graph, data
www.extech.com               hold, backlit, 43,000-point datalogger


Fluke                        CE, CSA, UL IEC 1010-1, min/max,
800-443-5853                 conductance, temperature, diode
www.fluke.com                test, auto-hold, logging (189 only)


                             CE, CSA, UL, IEC 1010-1, CAT IV
                             600 V, min/max/avg, temperature,
                             auto hold, sleep mode

GMC Instruments              CE, CSA, IEC 1010-1, CAT IV 600 V,
800-462-4040                 power (W, VAR, VA, PF, W-h)
www.gmcinc.com               datalogging up to 2,000 S/s


Hioki USA                    CE, IEC 1010-1, CAT III 600 V, live
609-409-9109                 AC indication without direct connection,
www.hiokiusa.com             fail-safe shuttered probe sockets

Omega Engineering            CE, IEC 1010-1, CAT II 600 V,
203-359-1660                 thermocouple and infrared
www.omega.com                temperature, digital and analog
                             display

                             Readings spoken in English,
                             French, German, Japanese, or
                             Spanish; two thermocouple inputs
                             or their difference


Protek Test & Measurement    CE, RS-232, bar graph, backlit, duty
201-760-9898                 cycle, pulse width, go, no-go, low-V
www.protektest.com           Ohms, min/max/avg, peak detect, memory

Test Products International  IEC 1010-1, diode test, in-circuit
800-368-5719                 comonent test, logic test, RS-232
www.tpi-thevalueleader.com   output

Wavetek Meterman             CE, UL, IEC 1010-1, CAT III 300 V,
877-596-2680                 diode/continuity measurement,
www.metermantesttools.com    very small pen-style meter

                             CE, IEC 1010-1, CAT III, backlit, min/max,
                             relative, diode/continuity test, safety
                             tester to 480 V, IP67 water and dust

Yokogawa Corp. of America    CE, IEC 1010-1, CAT III 600 V, diode test,
800-888-6400                 continuity buzzer, select mean or RMS,
www.yca.com                  percent, relative, safety shutter sockets,
                             auto-hold

                             CE, IEC 1010-1, CAT III 600 V, duty cycle,
                             dB, peak hold, relative, percentage,
                             safety shutter sockets, auto-hold,
                             memory, RS-232, backlit, auto-off






Company                      Price

A. W. Sperry Instruments     $34.95
631-231-7050
www.awsperry.com

ATP-Amprobe                   $299
800-327-5060, ext. 7598
www.amprobe.com

B+K Precision                 $199
800-462-9832
www.bkprecision.com

                              $187



Extech Instruments            $299
781-890-7440
www.extech.com


Fluke                         $379
800-443-5853                   and
www.fluke.com                 $399


                              $214



GMC Instruments               $595
800-462-4040
www.gmcinc.com


Hioki USA                    $219.95
609-409-9109
www.hiokiusa.com

Omega Engineering             $345
203-359-1660
www.omega.com


                              $249





Protek Test & Measurement     $249
201-760-9898
www.protektest.com

Test Products International  $369.95
800-368-5719
www.tpi-thevalueleader.com

Wavetek Meterman             $49.95
877-596-2680
www.metermantesttools.com

                             $199.95



Yokogawa Corp. of America    $195
800-888-6400
www.yca.com


                              $379
DMMs

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Author:Lecklider, Tom
Publication:EE-Evaluation Engineering
Date:Feb 1, 2002
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