Now that you've purchased your digital multimeter, you're ready for the next step: choosing the right accessories. A complement of accessories can enhance the versatility of the digital multimeter, instantly changing it from a voltage meter to a current meter to a thermometer - and much more.
The key to gaining the most from your instrument is selecting the accessories that best fit your workload. Just as you took pains to choose a high-quality digital multimeter, you'll want to select high-quality accessories to maximize your instrument investment. Many categories of accessories exist, from different types of test probes and current clamps to a variety of protective cases and carrying devices. The specific accessories you need both depend on 1) what you're planning to test and 2) the environment in which you will use your instrument. This article provides an overview of the many types of accessories available and explains which ones are best suited for different types of workloads.
In choosing multimeter accessories, the first thing you need to know is what parameters you need to test. For each parameter (voltage, temperature, etc.), a wide range of test probes is available, each designed for a specific type of test environment. The following is an overview of the types of probes available for each type of multimeter functionality. Voltage test probes for measuring voltage are available in a full spectrum of forms. The best choices depend on the nature of your particular workload.
Voltage level. With what kinds of voltage levels are you working? For example, if your work environment includes high-energy systems in an industrial plant, you'll require a robust probe capable of measuring high voltages (600V or higher) while withstanding the high energy present in these environments. On the other hand, if you work with low-energy electronic circuits, a less-robust probe may be better suited to your needs.
Frequency. Another factor to consider is whether you need to measure frequency as well as voltage. When working with low-voltage electronics operating at high frequencies, for example, you may need a probe capable of accurately measuring both voltage and frequency.
Accuracy. Then, there's the nagging question: How accurate does a probe need to be? If you work with low-voltage applications, such as motor control or process control circuits, you may require a specialized probe that provides accurate measurements at the millivolt (mV) level. On the other hand, if your job simply involves making sure that line voltage is present - for example, at motor terminals - a standard-duty probe may be all you need.
Other considerations. Voltage probes come in many different styles. Keeping this in mind, choose a probe that is most appropriate for the type of measurement being conducted. If you plan to take readings over time, or will wait to take a measurement after a particular event occurs, a clip to attach to the test point is very useful. For small-gauge wires or test points, you'll need a small alligator clip; for larger-gauge wires or terminals, a large alligator clip is handy. If you need to reach deep into a recessed panel to contact a terminal strip, you'll need a long probe; if the terminals are closely spaced, a narrow probe is appropriate. And if you need to pierce through dirt or corrosion to take your measurement, you'll require a sharp probe. These are just a few of the form factors to consider when choosing the proper voltage probe for your specific task.
Temperature. Temperature measurements are another common use of digital multimeters. But in order to turn your multimeter into a thermometer, you will want the right temperature probe.
Contact vs. noncontact. The first question is whether you'll be able to make physical contact with the item being measured or will need to take measurements without touching the item. In most electrical workloads involving temperature measurements, the item being measured is also electrically "hot" and therefore dangerous to touch. In such a case, choosing a noncontact probe that uses an infrared sensor is wise. For example, a noncontact probe is appropriate for measuring the temperature of such electrically-hot devices as transformers, circuit breakers or electrical connections. Conversely, if you're able to touch the probe to the item being measured, you may prefer a contact probe. Immersion probes for measuring liquids or pipe clamps for measuring the temperature of pipes are two examples of contact probes.
Gas/liquid/solid. When measuring non-energized items, your choice of probe depends on whether you are measuring a gas, a liquid or a surface. For example, if you're measuring the temperature of exhaust gases, you'll want an air probe; for measuring the temperature of liquids in a process plant, you'll need an immersion probe; and for measuring the temperature of equipment surfaces, such as the door of an electrical panel, you'll use a surface probe.
Range, accuracy and response time. A final set of questions deals with what temperature range you are likely to measure, how accurately you need to measure it and how rapidly you need a response. Probes differ in each of these factors, so it's important to pick the probe that's right for your workload. If you aren't measuring extremes of temperature and standard accuracy will do, you'll probably do fine with a general-purpose temperature probe that can measure a broad range of temperatures. Such a probe would be appropriate, for example, when taking general atmospheric temperatures (for instance, to make sure a heating system is working properly). On the other hand, if your work requires more accurate readings within a narrower range, you might need a more specialized probe. If you deal with refrigerants, for example, you will need a temperature probe capable of reading below 0??? F; and if you measure exhaust gases in a furnace, you will require a probe capable of withstanding high temperatures as well as measuring them.
Current. Measuring current can be more difficult than measuring voltage because the measuring instrument typically needs to be placed in series with the current being measured - a step that may require the electrical circuit to be opened. Yet in many cases, shutting down a plant or a process to measure current may not be an option. That's where current clamps become a handy alternative. By using a current clamp, electrical current can be measured without interrupting the circuit. As with other types of measurements, your individual workload determines the necessary type of current clamp.
AC or DC. Are you measuring AC or DC current - or both? When measuring AC line voltage with a DC offset, you would need a current clamp that measures both AC and DC current to attain an accurate reading. On the other hand, if you are only measuring AC current, you can save money by purchasing an AC-only current clamp.
Range. What current range will you be measuring? For example, if the measured circuit is carrying less than 200A, you can use a smaller current clamp. But if you're measuring 600A, a larger clamp capable of fitting around larger conductors is needed.
Voltage. You also need to know the voltage of the circuit you're measuring, since you need to be sure to choose a current clamp that is rated safe for the voltage involved.
Light. If fiber-optic cables are part of your work environment, you may need to measure the amount of light they transmit. A cable that has been nicked, bent too sharply, crimped or broken will not transmit light as it should, but with the proper light module, a digital multimeter can be used to isolate any transmission problems. When choosing a light module, a key consideration is the volume of light carried by the cable. Some light modules are designed for the lower end of the spectrum (around 850 nm); while others are designed to perform at the higher end (around 1,300 nm).
Test leads. Test leads vary according to several parameters: their insulation, flexibility, length and voltage and current rating. In each case, you'll want to choose the type of lead that best fits your work environment. Insulation test leads may be made of either PVC or silicone. PVC leads, which cost less, are appropriate for mild environments such as in a lab or on a workbench. Silicone leads, which retain their flexibility in cold environments and won't burn or melt in hot environments, would be a better choice if you work outside in cold weather or around hot industrial equipment.
Flexibility. Test lead flexibility is determined in part by how many conductor wires the lead has and how fine the wires are. A lead with fewer and larger conductor strands will be stiffer, but will cost less because it's less expensive to manufacture. A lead with many fine conductor strands will be more flexible and less prone to tangle, which could save you a considerable amount of time if you store your leads in a toolbox. Another factor affecting flexibility is the type of insulation. Typically, silicone leads are more flexible than PVC leads.
Length. Test lead length can range from 2 ft to 6 ft. If you don't need the extra length, you'll probably find it more convenient to choose a shorter lead. But if your leads are too short for the application, your job will be more difficult. In such a case, you could either buy a new set of longer leads or a set of test lead extensions to lengthen your existing leads. Voltage and current rating. Finally, it's necessary to choose test leads whose voltage and current ratings match those of the item you're measuring. These ratings are described as Cat. I through IV. For example, if you're measuring line voltages going to industrial motors, you may want a lead that can handle up to 1,000V and 10A continuously. Then again, if you're testing circuit-board components, you may need only a 300V probe rated for 2A.
Safety considerations. An important set of considerations when choosing your accessories is safety. The two key factors are 1) the rating of the accessory, and 2) tour work practice. The nature of your work environment will actually determine what rating and construction quality you need.
Category rating. With regard to the safety rating of your accessory, it should relate to the environment in which it is used. In addition to the proper voltage rating, the accessory should also have the proper overvoltage installation category rating (e.g. Cat. I, Cat. II, Cat. III, Cat. IV.)
The different rating categories relate directly to energy levels occurring in different electrical environments. Cat. I indicates a low-energy and Cat. IV represents a very high-energy environment.
For more information about accessory safety, call (800) 443-5853 to receive Fluke's ABCs of Multimeter Safety.
No tool by itself can guarantee your safety. It's the combinations of the right tools and safe work practices that give you maximum protection. Here are a few tips to help in your work:
- Work on de-energized circuits whenever possible.
- Use proper lock-out/tag-out procedures. If these procedures are not in place or not enforced, assume that the circuit is live.
- When working on live circuits, use protective gear.
- Use insulated tools.
- Wear safety glasses or a face shield.
- Wear insulated gloves; remove watches or other jewelry.
- Stand on an insulated mat.
- Wear flame-retardant clothing, not ordinary work clothes.
- When making measurements on live circuits: hook on the ground clip first, then make contact with the hot lead. Remove the hot lead first, the ground lead last.
- Hang or rest the meter if possible. Try to avoid holding it in your hands to minimize personal exposure to the effects of transients.
- Use the three-point test method, especially when checking to see if a circuit is dead.
- Use the three-point test method, especially when checking to see if a circuit is dead.
First, test a known live circuit. Second, test the target circuit. Third, test the live circuit, again. This procedure verifies that your meter worked properly before and after the measurement.
- Remember the old electrician's trick of keeping one hand in your pocket. This lessens the chance of engaging a closed circuit across your chest and through your heart.
PROTECTING YOUR INSTRUMENT A final set of considerations to keep in mind when choosing accessories is how you will protect your instrument. A variety of specially designed cases are available to protect your multimeter and keep it readily accessible. The type of case that's best for you depends on your work environment.
Hard cases. If you drive a service truck, you may want to keep your multimeter in an oversize case with other instruments and accessories. In that type of rough-and-tumble work environment, a hard protective case with separate compartments may be the best choice.
Soft cases. If you carry a tool bucket or wear a toolbelt, you may prefer a soft case (available with or without a belt loop). Oversized soft cases are also available.
Holsters. Another way to protect your instrument is with a rubber holster, or boot, which protects the body of the device while leaving the front exposed for use.