Multimeters

If you are working on bench electronics, consider a bench multimeter. typically with better accuracies and resolution. Bench multimeters can be used in other applications. For example, if you are a service company such as an electrical or HVAC contractor or a facility with many technicians each with their own meter, have one higher accuracy/resolution bench DMM kept in a stable room temperature and annually calibrated can be used to check the portable meters for drift. Meters do drift and it is important to check and calibrate them regularly.

4-Wire Kelvin Resistance Measurement on bench multimeters is a superior way to measure resistance not possible with handheld DMM's that use a 2-wire method. Having four input jacks and test leads permits the bench meter compensating for the resistance of the test leads possible, making the resistance measurement significantly more accurate.

Basic measurements: AC/DC Voltage, AC/DC Current, Resistance and the ranges you will be operating. Before looking at other features make sure the meter covers your ranges from min to max.

Are there any additional measurements that are absolute musts?

Here are useful articles from FLUKE on TRMS and the errors of not having a true RMS meter:
Facts About True RMS Measurement          Why True RMS?

 

Learn more in the detailed discussion and video blog further below.

Then consider the extras...

Clear big digits for visibility and backlight. You will appreciate it next time you are in low-visibility conditions.

Min/Max/Peak Hold. Free you hands and need to watch closely the display by capturing min/max/hold at the touch of a button.

Continuity. Usually depicted as a button such as this .  Very useful to determine if a wire or circuit has a break. Complete circuits, or simply touching leads together will give a loud beep. Some meters do a better job at this feature than others with faster response and higher volume. See David Jones video blog below for a demonstration.

Relative Mode/Measurement feature permits making measurements relative to a stored reference value. The displayed value is the difference between the reference value and measured value. It can be particularly useful when measuring voltage or resistance. A press of the relative button can zero out the resistance of voltage probes.

Temperature Measurement. This can be a very convenient feature to have and a must for HVAC technicians. Includes K type T/C bead wire, IR thermometer or both.

Low Pass AC Filter. Low Pass AC Filter allows troubleshooters to take accurate voltage, current and frequency measurements on the output side of a drive at either the variable speed drive itself or the motor terminals.

Crest Factor is a measure of signal distortion and is calculated as a signal’s peak value over its rms value. This is useful when looking at power quality issues.

Low Input Impedance (LoZ). Better DMM's will have a rotary switch position labeled LoZ or similar which will have a low input impedance for AC voltage measurements. See the ghost voltage discussion in analog meters above.

Non-Contact Voltage Detector. Typically purchased separately as an inexpensive test pen but some multimeters have it as a built-in function. What is a Non-contact voltage detector? It senses the electric field around energized AC circuits and are useful troubleshooting  whether standalone or as part of a multimeters. A visible light and audible beep would indicate an energized circuit.

Datalogging and PC connectivity. Troubleshoot intermittent problems and datalog high energy circuits from a safe distance.

Detachable Display is available for some meters. Useful not only for convenience but for safety with high voltage/power circuits. Connect the meter on a deenergized circuit then monitor at a safe distance when energized.

Fluke goes a step past detachable displays with the wireless enabled, Fluke Connect® tools that empower you to save measurements to the cloud and work outside the arc flash zone. Fluke Connect enabled tools are available in variety of tool categories so that you can pair up your multimeter measurements with various tool readings such as vibration, power quality, and thermal imaging to better diagnose and document an issue. Make sure you take a look.

Fieldpiece offers a modular approach with their "Stick Meters" designed for HVAC/R professionals.

Thermal Displays have also been introduced to digital mulimters. Be sure to see the 279 FC which has a thermal display to reveal hot sops on high-voltage equipment and transformers, detect heating of fuses, wires, insulators, connectors, splices, and switches. Designed to unover electrical issues quickly, the 279 FC is two tools in one.

Build Quality. Meters above $100-200 will have better durability, fit and finish. The rotary dial will feel better and not permit landing in between settings. Probes will be insulated with silicone instead of PVC, some have CAT ratings printed right on the leads. Probe handles are bigger and tips sharper. Watch the comparison videos below.

Consider Safety when choosing a Digital Multimeter

Proper and Quality Fuses. Inexpensive meters may not fuse the amp side or if they do, use a low quality fuse. Perhaps acceptable for troubleshooting low voltage/power electronics such as AA battery operated device but not recommended for any higher voltage/power work.

Terminal Shutters or Test Lead Alert when test leads are in the A or mA terminals and the selected rotary switch position is different. Better meters will also have dedicated terminals for mA and Amps to  help avoid this dangerous situation versus inexpensive meters that combine mA and Amps with Volts and Ohms. Safety shutters prevent erroneous insertion of test leads into current measurement terminals and Fluke provides audible and visible alarm to prevent you from proceeding. A serious safety hazard occurs, for example, when connecting meter leads across a voltage source with the rotary switch in amps. It shorts the terminals and meter short will at minimum blow the fuse and possibly explode.

When choosing a multimeter, first determine CAT levels you will be operating. Then choose a meter with a voltage rating higher than the maximum voltage you will be testing at each CAT level. For example, an electrician working in CAT III and CAT IV areas can choose a meter rated CAT III 1000 V / CAT IV 600 V. Within a category, a higher voltage rating denotes a higher transient withstand rating, e.g., a CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1000 V rated meter thinking that it is superior to a CAT III-600 V meter. Select a meter with the voltage rating suitable for the location (i.e. CAT I, II, III, IV) where you will be operating.

What are Digits on a multimeter?

Digits refers to the resolution of the DMM, not the accuracy. It is common to see meter specifications with 3-½, 3-¾, 4-½, etc. digit display. The half means the most significant digit can go up to 1. It originated because of  7-segment LCD/LED  displays. All 7 segments are needed to display every digit. When for the most significant digit on the far left only 1 is to be displayed, only the two rightmost segments of the 7 are needed, hence the term ½ digit originated (i.e. a fraction of the 7 segments are used, very loosely approximated as a half). That was when most DMMs had a maximum reading of 1999. Recently more accurate DMMs became available, having readings up to 2999, or 3999, even 4999, so it became more complicated. Manufacturers started using the ¾ approximation. But still not very clear as you can see here

Use of Counts eliminates the fractional confusion of the Digits terminology
 

So what happens as the measurement goes up in value?
For example, a 3-½  digit, 2000 count meter is measuring 1.999 volts. Increasing the voltage to 2.532 volts results in a loss of the least significant digit. The display will read 2.53 Volts. One digit of resolution is lost.

What about Multimeter Accuracy?
Accuracy is not the same as resolution. Resolution as explained above has to do with the number of visible digits (i.e. to the right of the decimal). Accuracy has to do with how correct or true the value is compared to a standard value from NIST or other international body. Look back at the snapshot from the video blog of David Jones above. He gives a simple formula using counts to give a rough minimum accuracy. David warns against meters that have a published accuracy higher than the minimums on his chart for the count level. See the video in full below and click this link for a useful application note from Fluke regarding precision: Understanding Specifications for Precision Multimeters

Quick Question: I often see an accuracy specification like this 1% of reading + 3 counts (or digits). How can I tell what my margin of error would be?

A: It refers to the least significant digit on the meter on that range. As an example, lets work the math for measuring 120 volts ac on a 6000 count meter (remember, it therefore goes from 0 to 5999). To measure 120 volts ac, the meter will have to be on the 600.0 ac voltage range. The resolution (least significant digit) then is 0.1 volt. (120 V X 1% = 1.2 V + 3 digits) = (1.2 V + 0.3 v). Therefore the margin of error would be +/- 1.5 V.

Video blogger David Jones (EEVBlog.com ) “Digital Multimeter Buying Guide for Beginners” has 100,000+ views. He covers safety, accuracy, ruggedness, important to obscure features, takes apart popular models, and more in his colorful and funny style.

This 30 min Fluke Electrical Measurement Safety video gives life saving  advice when using meters. In the Meter testing tab, poorly designed meters are tested and shown sparking even catching fire compared to properly designed and fused meters .  It really illustrates the importance of not only having the proper CAT rating, but checking that the meter has independent safety approvals such as UL. If you do not see the approval in the data sheet, check the instruction manual, or ask our sales team.

Digital Multimeter Accessories to consider

Test Leads. The most popular accessory for multimeters are test leads. Test leads do wear out. They should periodically be inspected. Set your meter to ohms and continuity. Then press the tips of the test leads together between your thumb and index finger. resistance should be zero, but wiggle the test leads and watch the display for an O.L. or high resistance. It might indicate that the lead wires inside the sheath are breaking. Leads are not repairable. replace them.

Alternate test leads styles are available from banana, alligator, hook, and more. They make making measurements easier and more accurate.

 

Alternate test leads styles are available from banana, alligator, hook, and more. They make making measurements easier and more accurate.

Current Clamp Adapters. While having a separate current clamp meter may be more convenient, current clamp adapters are available for DMM's.

Temperature Probes. Meters with temperature option will usually come with a bead wire K type thermocouple, but as accessories you can purchase others pipe clamp mounting for measuring pipe surface temperatures, penetrating probes for meats and other foods, and many other styles.

Multimeter Temperature Probe Adapter. If you did not purchase a multimeter with temperature capability, all is not lost. You can purchase temperature modules as accessories that will accept a temperature probe.

Cases and Holsters. Protect your investment.

Stray Voltage Adapter. If your DMM does not have a LoZ function, Fluke offers a stray voltage adapter. Read more above in the analog meter section.