It’s easier than ever before to get a pretty good hand-held DMM at a low price. Whether that’s due to integration, high volumes, offshore manufacturing, or maybe all three factors, you can make meaningful measurements for less than $100.
Making accurate measurements under difficult conditions costs more. For example, low-cost DMMs often don’t include a true rms capability. This means that the AC voltage and current ranges only are calibrated when measuring sine waves.
For real-world power waveforms that may be distorted, you need a true rms meter. Of course, there are limitations even then, such as the maximum voltage-Hertz product the meter can handle.
The DMM’s maximum peak-to-rms ratio or crest factor is another relevant spec. However, these are second-order problems compared with not having the true rms feature at all.
Moving up the price scale, resolution and accuracy improve considerably. The ADCs used in higher-price DMMs have greater resolution. In DMM terms, the meters have more counts. A DMM with 4,000 counts full-scale might cost $89, but a $249 instrument would more likely come with 50,000 counts.
Measurement resolution typically follows directly from ADC resolution. For example, the last digit of the 50-V scale on a 50,000-count meter reads millivolts.
Accuracy depends on many parts of the DMM circuitry as well as the ADC. Most DMM manufacturers specify accuracy as a percentage of reading and an amount of uncertainty expressed in counts, such as ±0.1% ±2 counts. With this specification, a 10.000-V signal measured on the 50-V range of a 50,000-count meter could read anything between 9.988 V and 10.012 V.
Temperature drift and long-term aging also affect accuracy, but a meter’s basic analog circuitry and ADC are the major contributors. Depending on a DMM’s design, higher ADC resolution may not make a lot of difference.
With a 4,000-count meter, for example, even if the analog performance were perfect, the overall accuracy would be limited by the ADC resolution. But the design of a 50,000-count DMM may result in a large amount of uncertainty. The basic accuracy could be 0.025%, but if the uncertainty is ±10 counts, the last digit of resolution is worthless.
Often, higher-price DMMs will have lower amounts of uncertainty. This is one of the differences to look for when comparing DMMs with a similar number of counts but widely varying prices. Nevertheless, as is obvious from the comparison chart that accompanies this article, today you can buy a very good DMM with a number of worthwhile secondary features for less than $300.
More expensive meters must offer more than just high accuracy on basic measurements. Above about $250, manufacturers have provided separate DMM models with feature mixes targeting specific applications.
For example, Agilent Technologies offers the U1240 Series with 10,000 counts for general-purpose electrical installation and maintenance. Meters in this series cost about $220. For electronic test applications requiring higher accuracy and a greater selection of features, the U1250 Series has 50,000 counts, built-in datalogging, pulse width measurement, and a programmable pulse generator output. These DMMs are in the $400 range.
Fluke’s Model 287 at $499 is distinctive in having a ¼-VGA display rather than a backlit LCD. This means that several readings can be simultaneously displayed such as min, max, and average together with the times they occurred. The meter also includes TrendCapture, a strip chart-like display of data logged in the background. The 287 is intended for electronics applications. The Model 289 is similar but includes features such as a low-pass filter to reduce measurement noise in industrial environments.
The highest price DMM in the comparison chart is AEMC’s Model MX 57EX at $895. Its outward appearance is similar to many other DMMs, but the MX 57EX is intrinsically safe. This means that in dangerous applications, such as mining or oil refining that may involve explosive atmospheres, this meter is considered a passive device. Achieving safety-agency approval as an intrinsically safe instrument required special precautions throughout the design and development process.
Trends in DMMs
Although each manufacturer no doubt sees the future somewhat differently, the DMM selection guide on the Yokogawa website helps to put today’s products into perspective. On a single page, several of the company’s DMMs are listed along the vertical axis of a matrix. Horizontally, features are listed, divided into categories.The categorization is especially interesting.
The company has divided features into those related to the display, measurement items, and additional functions (Table 1). Looking at the large number of items contained in Table 1, you are immediately aware of just how comprehensive DMMs have become. It’s also easy to imagine how several families of instruments can span a wide price range by including or excluding features.
The items printed in red indicate DMM advances that have been or that could be made. There is no intention to imply that Yokogawa may or may not adopt these features. The company’s categorization of conventional DMM features simply was a convenient starting point for the table.
Display
A DMM that displays the maximum value as well as the present signal level is useful when an adjustment is being made. For example, if several parameters interacted in a product, it could be very difficult to know how each should be changed to arrive at an overall maximum output. Displaying the maximum value allows you to change as many parameters as you like, knowing that as a new maximum is reached, the display will be updated.
A dual-display capability is used to present two separate but related readings. For example, you can measure AC voltage and also read the AC signal frequency. Another combination that manufacturers support is input voltage together with the level expressed in dBm.
Bar graphs are featured in several models, being either a linear presentation along the lower part of the display or sometimes an arc across the top of the display. It’s interesting that manufacturers highlight a fast update rate for the bar graph although the basic meter readout may include more averaging and update at a slower rate. Apparently, it’s easier to appreciate signal-level variations graphically displayed than as a series of rapidly changing numbers.
Backlighting obviously is necessary unless you always use a DMM in a well-lit area. Nevertheless, it’s indicative of the degree of refinement in today’s DMMs that the Agilent U1240 Series offers a dual-intensity backlight. The more expensive U1250 Series has high-intensity backlighting. In general, DMMs costing above $100 almost certainly will have an integral backlight, but this is a feature sometimes left out of very low-price meters.
A graphics capability must be the ultimate display improvement. It is a completely flexible solution that includes the existing functionality and supports virtually unlimited possibilities. Of course, it costs more than a traditional LCD. There’s nothing quite so all-encompassing that can be added to either of the other columns in Table 1.
Please click here to view/print Hand-Held DMM Comparison Chart
Measurement Items
The basic volts/amps/ohms capability has been improved in a number of directions. This point was made by Tee Sheffer, president of Signametrics: “Customers shopping for 5½-digit DMMs usually are looking for the lowest price, while those considering 6½ or 7½ digits are looking for performance. However, in either case, a versatile DMM can reduce the size and cost of a test system by doing the job of two or three other instruments.” 1
True rms and AC + DC capabilities continue to differentiate low-cost from more accurate meters. In some cases, you may only care that more than 100 V is present at a point, and a low-cost DMM has all the resolution and accuracy you need. To know more precisely what the signal level is, you need a better meter.
A feature that has only recently appeared in general-purpose DMMs is a square wave or pulse source. In the past, some DMMs specifically intended for use in process control work included a 4- to 20-mA output. This allowed you to simulate a control signal and measure resulting electrical signals caused by it. The programmable pulse generator fills a similar need but for digital circuitry.
Agilent’s Kamala Ravindaran K. Manickavasagam, marketing manager for the Basic Instruments Division, explained. “One of the new features of the U1252A DMM is the capability to generate pulse trains up to 2.8-V amplitude at a 4.8-kHz maximum frequency. This enables pulse width modulation (PWM) analysis in digital circuits. The signal’s duty cycle is modulated to encode a specific analog signal level using a high-resolution counter.”
Other manufacturers also have included a digital source in their meters. You need to understand the flexibility your application requires before you can determine if a particular DMM is suitable. Implementations of the PWM source are not all equivalent.
Additional Functions
How would you like your DMM to react to a signal peak? Do you need it to communicate with a PC? How much data analysis should it perform on basic measurements? These are the kinds of questions addressed by items in the Additional Functions column of Table 1. This is the longest of the three columns, and it’s a good bet that you can look for even more entries in the future.
As with all of the items in Table 1, a feature’s performance depends on its implementation. Different DMMs will not all behave identically with regard to data memory, for example. One meter will have more memory than another, or you may have more control over how the memory is used.
The Fluke 287’s TrendCapture feature is a good example of memory control. Chris Gloger, marketing manager for digital multimeters, said, “The TrendCapture feature quickly documents design performance and graphically shows you what happened. The Model 287 has two types of logging capability. Interval logging allows you to select how often a measurement is recorded. Event logging runs in the background and only records events that trip a preset threshold, so memory is used only for out-of-bounds readings.”
For the mathematical items such as relative value or logarithm computation (dB), you should get the same result on any meter, limited only by its accuracy and resolution. Similarly, maximum and minimum values should behave in the same way regardless of DMM model or manufacturer.
A less standardized item is communication. Connecting to a PC is very desirable in test setups and has become much easier to accomplish because of isolated I/O ports. Because a DMM is battery operated and most models have insulated plastic cases, engineers don’t hesitate to connect them to large off-ground voltages. For example, you might want to measure a bus within a switching power supply.
This setup is safe enough, especially if you simply read the display without touching the meter. But what about the I/O connection? Unless it’s isolated from the meter ground, you can’t simply wire the I/O port to the corresponding PC port. Many meters provide optically isolated USB or RS-232 ports for this reason. If you intend to use a DMM in this way, isolation is not just a nice feature–it’s absolutely necessary.
The red timestamping entry at the end of the column in Table 1 goes with a logging capability. Accurate timestamping means that you can reconstruct complex scenarios among several signals measured on separate meters. Each data point is recorded along with the time it occurred.
Summary
What features does your present DMM lack? Have you found a meter in the comparison chart that would do your job better? Chances are that unless your DMM is relatively new, you can buy more performance at a lower cost today. Nevertheless, perhaps it’s also never been more difficult to buy a DMM or at least to decide which one to buy.
As occurs in all test instruments, features migrate down from high-price meters to lower-cost DMMs each year. As the comparison chart shows, apparently similar feature sets can have a relatively wide range of prices. Sometimes you are paying a premium for a manufacturer’s reputation. You might find that it’s money well spent should you need a DMM repaired or replaced under warranty.
Other times, a lower price may indicate a less expensive implementation of a feature and perhaps a less useful one. Now that there are so many features packed into virtually all DMMs, you must consider many more details than just basic voltage or current accuracy. How does the accuracy of the AC voltage reading vary with AC frequency? Even if you only work with 50/60-Hz systems, how accurately will your DMM measure higher frequency harmonics?
Unfortunately, there is a direct correlation between the size of a DMM’s datasheet and its price. If they both are small, you may not be able to answer questions about more obscure specifications such as crest factor. It’s not listed. A more complete datasheet usually goes hand in hand with a higher-performance meter.
So, what does the future hold for DMMs? Certainly, there will be even more features available next year and for years to come. An idea of what they might be follows from looking at how other types of instruments have progressed. For example, several models of DSOs now can extract only unusual waveforms from the millions presented. How about a smart DMM with a similar exception recognition capability?
Reference
1. Lecklider, T., “Full-Function DMMs Streamline Test Systems,” EE-Evaluation Engineering, February 2006, pp. 16-22.
February 2008