Accessories provide another dimension of functionality to your digital multimeter.
Have you ever tried to troubleshoot a problem, only to find that your meter couldn't perform the particular measurement? A meter accessory might have fixed that problem inexpensively. While situations do arise where you simply need another meter, you can often bridge the gap with accessories that allow you to add more than just measurement capability. To make the right accessory decisions, start with a needs analysis.
Doing the analysis.
What are your measurement tasks? If those tasks involve repetitive measurements, a dedicated device — rather than an accessory — usually makes more sense. To identify those tasks, walk through your procedures for preventive maintenance, testing, and troubleshooting, making sure you note the following:
Type of measurement, such as current and temperature.
Number of measurements.
Location considerations — you may not want to carry three meters up a ladder.
Equipment considerations — is the area too small?
Once you complete your list, review it against the test equipment you already have. As you go over the list of measurement tasks, note what you can't do with your present test equipment and consider the following:
So you want to know the load on a 400A breaker that's experiencing frequent nuisance trips? If you don't already own a current clamp (Photo 1 above), you'll need to buy one, but before doing so, make sure it and your meter carry the proper category rating. In this case, it would be at least Cat. 3 and 600V. If you don't own such a device, spend the money to buy a meter capable of that measurement, rather than combining a meter and an accessory.
Some people avoid accessories because they have seen the chaos created by combining too many components. However, that problem stems from poor planning, rather than the accessories themselves. Accessories like temperature probes or current clamps tend to be single-function items. How many do you need? Should you buy a multifunction meter to do those tests with a single instrument? Keep in mind how often you run each test, and which of them could be combined in one unit before making a decision.
One strategy is to provide a quality, general-purpose digital multimeter (DMM) to each team member, and specialized accessories to use as required for infrequent testing situations. Another is to build dedicated accessory kits. For example, you know maintenance work for Line 3 always includes measuring current on the drive motor, temperature on the motor bearings, and light on the fiber optic signal cabling. Rather than waste time finding accessories when it's time to work on that line, just grab the Line 3 test kit. The cost of providing such a kit is nothing compared with the labor savings and reduction in downtime.
What if the cost of your proposed test equipment exceeds your budget? Don't let this cause you to make bad decisions. One approach is to buy generic, but cutting corners can cost more down the line than buying quality equipment from the start. For example, a poorly constructed clamp-on device has usability and accuracy issues that may render it worthless. Or it may fail prematurely. Even worse, it may be unsafe if it has the wrong category rating for the application. Going cheap won't save you money or help you meet your testing needs — and it may cause serious injury and/or death. Buy from a reputable manufacturer, not an unknown clone, and remember those energy ratings.
Another approach is to buy piecemeal, per the existing budget, but that budget probably didn't result from a needs analysis. If your analysis shows your test equipment arsenal doesn't match your measurement needs and is costing the company money — in extra downtime, technician errors, or injuries — you need a different approach.
Submit a funding — capital or expense — request that includes the analysis you just made. If a test kit for Line 3 cuts downtime by 10 hours, that's 10 more hours of top line revenue. Apply this analysis to all production equipment, and you'll be able to show that the cost of buying the right test equipment is nothing compared with the increased cash flow you get from using it. If your funding request flops, ask for a temporary budget increase based on a prioritized short list. Once you know how much buying power you have, you can begin the selection process.
When you test the temperature of transformers, circuit breakers, or electrical connections, the item under test is usually energized. So for safety's sake you want a noncontact probe. What about the range, accuracy, and response time requirements? You want the probe to match the anticipated measurement as closely as possible. Opting for the widest possible range means you sacrifice the accuracy and precision you'd get from the right sized probe. If you don't have a range in mind, get a general-purpose probe — but again, not necessarily one with the widest possible range. Trying to save a few bucks by getting a cheap probe with a long response time automatically puts you in a contest between inaccurate measurements and higher labor costs — either way, you lose.
Something else to consider when choosing a contact-style temperature probe is meter compatibility. Some of these probes only work with certain meter models; these probes are typically thermocouples that plug into a thermocouple jack built into the meter for that family of probes. These two approaches, — contact vs. noncontact — balance tradeoffs in cost, performance, safety, and convenience, so refer to your needs analysis before buying.
A look at current clamps.
DC current measurements represent less than 10% of current measurements in a typical plant. The AC/DC style clamps are about twice the price of AC clamps, and the jaw openings for these combo-clamps often won't fit around multiple conductors. The solution is to have separate clamps for DC and AC measurements — but that may not mean just two clamps.
Don't try to use one clamp range for all loads. A clamp with excessively large jaws will give you problems in tight spaces, and you'll lose low-current functionality and resolution. How can you avoid this loss when you have large DC bundles and need the capacity a large clamp provides? Buy a flex clamp — it has a flexible loop conductor that wraps around the bundles.
Leads and cases. Since test leads (Photo 2 right) are inexpensive, cost isn't an issue. But using the wrong test leads can be a fatal error. Choose leads with voltage and current ratings that match the environment. It's smart to buy only leads that qualify for the highest energy rating you'll need; if you have a mix of Cat. 2 and Cat. 3 testing, just buy Cat. 3 test leads. Your time is too valuable to spend untangling leads, so don't use cheaply made, stiff leads.
A case prolongs the life of your meter and prevents lead damage during storage and transportation. Cases come in several styles and colors. Determine how you'll carry your meter and accessories around, then get the appropriate case.
You need the right tools for the job, and you need to make the best use of your budget. With a good needs analysis, your purchasing plan will use a combination of meters and accessories to satisfy both requirements.
Allen is a product manager for Meterman Test Tools, Everett, Wash.