DMMs Proliferate and Prosper

Today’s DMMs are differentiated by their feature sets and prices. Basic low-resolution meters that only measure voltage, current, and resistance are available for less than $100. High-accuracy measurement platforms with a wide range of options and accessories may cost more than $2,000.

Manufacturers have adopted various strategies to position products between these extremes. A stand-alone DMM with excellent performance costs $500 to $1,000 or more but may not support multichannel data acquisition. Or, you could find a meter with an application-specific set of capabilities that targets a particular industry.

Generally, resolution, accuracy, and the length of the feature list increase with price. Nevertheless, nonelectrical parameters such as throughput can be very important in production applications, and a meter must be intrinsically safe for use in an oil refinery or mine.

The extent to which additional features command a premium price can be seen by comparing the Signametrics Models 2060 and 2064. Both are 7½-digit DMMs compatible with PCI, PXI, or USB. But the 2060 lists for $1,295 and the 2064 for $1,895 (USB), $2,180 (PCI), or $2,195 (PXI). The 2064 has a few more measurement ranges than the 2060 and slightly better accuracy although a 90-day period is specified for the 2064 vs. one year for the 2060. The bigger difference is in the extra features.

The 2064 measures capacitance and inductance, supports two additional DC voltage inputs, and contains a programmable 10-V DC source. This means that you can stimulate and measure load cells and strain-gage bridges with this one instrument. Perhaps the event counter or programmable AC voltage source or guarded six-wire ohms capability better suits your application. The $600 to $900 difference between the 2060 and 2064 is a small price to pay for versatility that may avoid the cost of additional test-system modules.

What Has Changed?

Truly New Features

Not too long ago, being able to magnetically attach your hand-held DMM to a steel equipment rack was a big deal. Today, magnetic hanging straps are included or optional with several meter brands. Perhaps the wireless display Fluke has developed for the Model 233 DMM also will be widely copied someday.

However, for now, it’s a new and innovative feature that really does address an actual need. Instead of requiring a second technician to watch a meter while you make a remote adjustment to a machine or circuit, now you can simply hook up the meter and take the display with you to wherever the adjustment is to be made. The Model 233 is only a 4,000-count meter, but with 0.25% DCV accuracy and 1% for several other functions, it’s well suited for this kind of industrial application.

Extech’s Model EX540 DMM features wireless datalogging to a PC. This may not be as self-contained as the Fluke approach but has the advantage of remote monitoring as well as data storage, trending, and analysis without physically connecting the DMM and PC.

Another innovation, the company’s point-and-shoot IR temperature measurement, isn’t a new concept, but it’s unique in a DMM. As described by André Rebelo of Extech, “We have included this patented, built-in infrared thermometer in many models. In addition to getting contact-based temperature readings using a type K bead probe, users now can point and shoot at target surfaces with the aid of a laser pointer. This allows them to make temperature measurements at a safe distance from hazardous electrical or mechanical components.”

Application-Specific Feature Sets
In preparing the comparison charts, it’s apparent from reading through dozens of DMM datasheets that many of these products have been carefully developed. Take the example of a hand-held, plastic-case DMM that’s being used for troubleshooting in an industrial environment with lethal voltages. Input overload protection is more than a convenience in this kind of application. Some meters are fused, some buzz when an incorrect connection is made, and many have at least 1,000-V isolation between the inputs and ground.

Assuming that you have correctly connected the meter to the source, how are you going to record the measurements to your PC when the DMM may be floating at a 500-V common-mode level? A meter with optically isolated I/O is the answer to this problem. And, some meters have dual displays or graphical panels that allow several related measurements to be conveniently and simultaneously displayed.

This level of attention to details really does make a difference and complements the trend to differentiate products by application. Just grouping useful measurements isn’t enough. To do the complete job, you need application-specific functions as well as things like input protection and isolated I/O. The best meters offer all the capabilities required for a particular kind of job.

Agilent Technologies has upgraded the U1250 and U1240 Series DMMs to CAT III 1,000 V and CAT IV 600 V, as explained by Ban-Kwan Wong, hand-held DMM product manager. He said that a 30-kA fuse is used for input protection and to avoid injury from an exploding fuse. In addition, hipot testing beyond the requirements of IEC 1010-1 is performed to ensure the meters withstand extreme voltage conditions.

Ross Ignall, systems application manager at Dranetz-BMI, said, “To distinguish our products from standard DMMs, we include special functions such as insulation resistance, temperature, low-resistance measurements, low-pass filter, and graphic display of memory. We work closely with customers to develop new functions, but it’s difficult to improve operator convenience because additional functions add complexity. The real benefit is the possibility of replacing several instruments with a single device. Our products are marketed as the DMM for the electrician or the automotive technician.”

Intrinsically safe, waterproof, and dustproof instruments are special cases within the application-specific classification. According to one definition, the theory behind intrinsic safety is to ensure that the available electrical and thermal energy in the system is always low enough that ignition of the hazardous atmosphere cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area and that all electric supply and signal wires are protected by zener safety barriers. Sometimes…a galvanic isolation barrier may be used.¹

The IP67 rating is commonly associated with meters claimed to be more rugged than usual. The rating can be broken into the IP prefix that means ingress protection and the two numbers that indicate the type and degree of protection. In this case, the 6 means totally protected against dust, and the 7 indicates indefinite submersion to between 15-cm and 1-m depth. So, an IP67-rated DMM is truly dustproof and waterproof to 1 m.

Fluke’s new 27 II and 28 II industrial meters have the IP67 rating. The specifications also include an extended temperature operating range from -15°C to +55°C, and the meters withstand a 3-m drop. Approval from the Mine Safety and Health Administration is pending so it’s clear that these instruments are intended to stand up to harsh environments.

AEMC’s datasheet for the Model MX 57EX TRMS DMM lists oil refineries, mining, and pharmaceutical plants as typical applications. A footnote claims “operating voltages are limited to 60-V peak values or currents to 500 mA for intrinsically safe operation.”

LXI
Another change that’s apparent is the increased number of LXI-compatible DMMs. Tom Sarfi, vice president of business development at VTI Instruments, said, “The EX1200 Series is a family of configurable and integrated switch/measure instruments certified to LXI Class A. A 6½-digit DMM is included in some models together with expansion slots for switching and control. An integral analog bus can be used to route multiple inputs internally to the DMM for simplified field wiring.”

Mr. Sarfi explained that the company’s VXI products are well suited to large-scale ATE. In comparison, the 1U rack size of the EX1200 and its capability to operate as a stand-alone instrument with no additional chassis/controller have allowed VTI to address smaller channel-count scanning temperature/voltage requirements. Typical EX1200 applications include temperature monitoring, automotive ECM testing, process monitoring, datalogging, cable/harness testing, battery testing, and functional test.

Bustec is another company offering more LXI products. The latest is the Class A ProDAQ Model 3475-AA 7½-digit DMM. Geoff Hoekstra, the company’s USA national sales manager, described the flexibility of the 3475 concept. “The key development that made this DMM possible was the recent release of the ProDAQ Model 6100 LXI Carrier that accepts four ProDAQ Function Cards,” he said. “This means that you can have up to four 7½-digit DMMs in one VXI slot or occupying only 1U of rack space with two side-by-side 6100 Carriers. Of course, if only one DMM is needed, you could fill the rest of the VXI slot with up to 144 measurement channels of various types or do the same in a 1U LXI space.”

Keithley Instruments has long supported LXI and most recently combined this capability with USB and GPIB in the same instrument. According to Jerry Janesch, the company’s market development manager, “The Model 3706 is an LXI Class B-compliant instrument. It incorporates a 10/100MBase-T Ethernet connection, graphical Web server, LAN-based instrument triggering, and IEEE 1588 precision time protocol synchronization.

“Greater test-system versatility is provided by the three built-in remote interfaces: LXI/Ethernet, GPIB, and USB,” he explained. “Additionally, system control can be greatly enhanced by using Keithley’s test script processor (TSP^®) technology, which provides smart instruments with the capability to perform distributed processing and control at the instrument level, often eliminating the need for a PC controller. A front-panel USB device port provides convenient and virtually unlimited memory storage.”

A fourth example of an LXI system DMM is the EADS North America Test and Services Model 4102, a 7½-digit DMM intended for use with the company’s Model 1830 Source/Measure Switch unit. Similar in concept to the VTI EX1200 approach, the 1830 is a rack-width chassis that accepts a number of proprietary plug-in modules. The 1830 is larger and accommodates up to nine switch plug-ins as well as a 4102 DMM. The lower-cost 4101 DMM has specs similar to the 4102 but with no inductance measurement and no built-in precision source.

Charles Greenberg, senior product marketing manager, explained, “The DMMs in the 1830 system are well integrated with switching assets by means of an internal signal raceway, a parallel architecture, and SCPI scripting engine…. Serial or parallel operation of the DMM is orchestrated by the scripting engine, a true programmable state machine capable of executing any combination of switching and DMM monitoring and data-collection applications.”

What Stayed the Same?

Datasheets
Unfortunately, the correlation between very low price and poorly presented specifications remains unchanged this year. There’s no doubt that it costs money to test products, and if a parameter value is to be guaranteed in a DMM’s specification, then it has to be tested. On the other hand, a specification such as the reading resolution available on a particular range doesn’t require testing. It’s inherent in the meter’s design but still may not be listed in the datasheet.

Even if the basic accuracy and resolution specs are listed, the datasheets for low-cost meters may not include further details such as reading uncertainty or time and temperature dependencies. These things are important because it’s easy to make one set of specifications appear better than another simply by referencing a 90-day calibration period rather than a one-year period. This level of detail generally is listed for 6½- and 7½-digit bench and system DMMs but not for hand-held meters.

The Amprobe and AEMC instruments are good examples of inexpensive DMMs that still manage to present relatively complete and easy-to-understand specifications. Amprobe does a better job because each model has a separate datasheet with detailed specs. This means that there’s room to list the specific ranges and show how the accuracy changes for them. Nevertheless, even though AEMC groups several similar meters in a single brochure, you still are given the best resolution for each function for each meter as well as the highest and lowest measurement ranges.

Another thing that hasn’t changed is the confusion some manufacturers cause by combining resolution and range under the range heading. Range refers to the full-scale value, such as the 10-V range. If the meter has 1-mV resolution, some manufacturers claim that you can measure from 1 mV to 10 V. Maybe you can if the noise is sufficiently low, but the range is not 1 mV to 10 V. The DMM range is 10 V, and the resolution on that range is 1 mV.

Ranges typically increment in decades, such as 1 V, 10 V, and 100 V. Depending on a meter’s design, the ADC resolution may be more or less closely coupled to the range values. For example, in a meter with 4,000 counts, it’s convenient to have ranges such as 400 mV, 4 V, 40 V, and 400 V. The ADC counts directly correspond to the displayed digits.

This is the case for the Signametrics 7½-digit DMMs with 24,000,000 counts and a 0.24-V range. However, Agilent Technologies, Keithley Instruments, and National Instruments all have a 0.1-V range on their 7½-digit meters.

VXI
VXI support continues, but extensive new DMM development does not. Only one VXI-compatible DMM is even indirectly listed in this year’s comparison chart. Bustec’s Model 3475 functionality is available in either an LXI or VXI configuration depending on the type of carrier used for the ProDAQ Function Module.

Further Opportunities

Among the many benefits of today’s DMMs, the flexibility afforded by a rich feature set stands out. But what if your application still resists solution? The NI 4071 PXI DMM might be the answer. Travis White, NI product manager—modular instrumentation, said, “The 4071 FlexDMM has 7½-digit DC performance but also operates as an isolated digitizer at up to 1.8 Msamples/s…. For applications more specific than capacitance, inductance, frequency, period, and temperature, you can customize measurements through analysis software such as NI LabVIEW.”

Finally, when you need a new DMM, be sure to look beyond the test instrument companies with familiar names. DMMs continue to be introduced by a wide range of companies.

Reference

1. Intrinsically safe: http://en.wikipedia.org/wiki/Intrinsic_safety