Handheld Multimeter

Handheld Multimeter

Multimeters are the instrument of choice when you need to measure voltage, current, or resistance. Multimeters originated as analog Volt-Ohm-Millliammeter (VOM), some would say Volt-Ohm Meter, combining the ability to measure voltage, resistance, and current. Another related acronym is DVM standing for Digital Volt Meter, which are dedicated for voltage measurement. Some oscilloscopes have DVM capability.

Multimeter Types

Multimeters are available either as Analog or Digital. Today a digital multimeter (DMM) is more common than an analog multimeter.
 

Analog Multimeters

Analog multimeters still have a place because some have a preference for the analog display, peaking adjustments, fast moving trends, and other cases where the precise digital value of a DMM is not needed. Also, analog meters typically have low impedance versus high impedance DMM. High impedance of a DMM is a benefit when measuring electronic circuits, because it will not load the circuit. In other words, an analog meter across your circuit, is like sticking a low ohm resistor across it. In many circuits, this will have no practical effect, but be careful. Most DMMs have a high input impedance, so they are effectively unseeable, as far as many circuits are is concerned.

Stray or Ghost Voltages
One noteworthy application for low impedance analog multimeters is checking for stray or ghost voltages. Stray voltages are caused by the capacity between the de-energized wires and adjacent energized wires. It is important for safety to check a circuit is de-energized before starting work. Analog meters will load the circuit and the needle will fall to zero. Because of the very high impedance DMM input, this stray voltage has nowhere to go which will lead to an incorrect reading on DMM’s.

DMM ghost voltage solutions:
Better DMM's will have a rotary switch position labeled LoZ or similar which will have a low input impedance for AC voltage measurements.

Fluke has an accessory solution for DMM's, a stray voltage adapter that lowers input impedance.
 

Digital Multimeters

Digital Multimeters convert analog measurements and display digital values. DMM provides better accuracy and resolution than an analog meter. Some DMM's have an uncalibrated analog bargraph to mimic the sweeping needle capability of an analog meter.

How to buy/choose a digital multimeter (DMM)?

Digital multimeters have evolved to include more measurements and features. First, think about your application: home, hobbyist, professional electrician, electronics or HVAC technician, etc. Consider getting a second meter such as a clamp meter, so you can measure voltage and current at the same time, and a maybe even a pocket multimeter too to keep in convenient locations.

Think about Bench 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 from canceling the resistance of the test leads possible, making the resistance measurement significantly more accurate.

Next consider these features

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?

• Capacitance (F)
• Frequency (Hz)
• Diode Test

Safety, Safety, Safety! Make sure the meter has the proper safety ratings for your voltage and current ranges. Please see the detailed safety discussion below and videos.

Accuracy and Resolution

• Focus on the accuracy of the basic measurements Volts and Amps.
• True RMS (root mean square) refers to the conversion process of the sinusoidal AC current signal into the displayed digital value. True RMS meters, while more costly, provide much better measurement. TRMS calculate an accurate reading even when it encounters irregular waveforms such as square, sawtooth, or rectified waves. Learn more in the detailed discussion 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. 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 the 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 their CNX Wireless System launched in 2013. Wireless modules that connect to your CNX meter or PC. Make sure to take a look.

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

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.
 

This symbol indicates a meter is protected throughout   by double insulation or reinforced insulation, an  important safety feature. Look for it on the back of  the meter and instruction manual.

CAT Ratings for Multimeters and other Instruments
 

Photo from Fluke's "ABCs of Multimeter Safety" showing locations where CAT ratings I - IV apply

 

Overvoltage Category	In Brief	Examples
CAT IV	Three-phase at utility connection, any outdoor conductors	Refers to the "origin of installation," i.e., where low-voltage connection is made to utility power Electricity meters, primary overcurrent protection equipment Outside and service entrance, service drop from pole to building, run between meter and panel Overhead line to detached building, underground line to well pump
CAT III	Three-phase distribution, including single-phase commercial lighting	Equipment in fixed installations, such as switchgear and polyphase motors Bus and feeder in industrial plants Feeders and short branch circuits, distribution panel devices Lighting systems in larger buildings Appliance outlets with short connections to service entrance
CAT II	Single-phase receptacle connected loads	Appliance, portable tools, and other similar household loads Outlet and long branch circuits Outlets at more than 10 meters (30 feet) from CAT III source Outlets at more than 20 meters (60 feet) from CAT IV source
CAT I	Electronic	Protected electronic equipment Equipment connected to (source) circuits in which measures are taken to limit transient overvoltages to an approximately low level Any high-voltage, low-energy source derived from a high-winding resistance transformer, such as the high-voltage section of a copier

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.

Digital Multimeter Accuracy And Resolution Discussion
 

Snapshot from David Jones Video Blog.  About 3/4ths into the video is an explanation of  DMM counts, digits, accuracy and resolution.   Below is the synopsis and some background   definitions to understand counts, digits,   resolution, and accuracy.   And do watch his complete video below.

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
 

Digits	Display
3	+/- 0 through 999
3-½	+/- 0 through 1999
3-¾	+/- 0 through 3999 (typically), but can also mean 2999 or even 4999
4-½	+/- 0 through 19999

What are Counts on a multimeter?
Counts are a better way to represent display resolution. Let’s go back to the table.
 

Digits	Display	Counts
3	+/- 0 through 999	1000
3-½	+/- 0 through 1999	2000
3-¾	+/- 0 through 3999 (typically)	4000
4-½	+/- 0 through 19999	20000

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.

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.
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.