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 4¾- and a 5¾-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°F to +160°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 hand-held 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 so-called 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°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°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.
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February 2002