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How to use a digital multimeter or DMM to measure voltage.
Here we demonstrate how to use both an older analog VOM (Volt-Ohm Meter) and how to use use a DMM (Digital Multi-Meter) to measure voltage levels in building electrical wiring and electrical service. We using several different types of DMMs and a VOM we show what settings to select on the DMM or VOM, how to connect the sensing probes, and where to touch the probes to check voltage.
We cite safety warnings, then we describe DMM or VOM function and range settings and probe connections to make simple voltage level measurements. Page top photo: our Sperry DSA-500 DMM measuring voltage at an electrical circuit.
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Safety Warning: Opening an electrical panel and approaching any live electrical wiring, devices, & equipment is a dangerous procedure that can damage electrical equipment or worse, cause electrical shock, or even death.
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Such procedures should not be undertaken unless the person conducting the examination is trained and competent to avoid electric shock. If the inspector is not trained for this procedure s/he should never insert any instrument or tool into electrical equipment.
Our photo illustrates the Fluke 28 II DMM. Detailed safety advice specially applicable to using VOMs, DMMs and ammeters, including both personal safety and advice to avoid damaging the equipment is found
If you are not sure of the definitions of volts or voltage and amps or amperage,
A DMM or VOM is simply an electronic device for making electrical measurements of electrical properties such as resistance, voltage level, or current flow.
DMMs provide a digital readout while VOMs like the Jensen 310J shown here read voltage on an analog scale by noting the position of a movable pointer. Both types of instruments are fully capable of measuring voltage as well as other electrical functions.
While the DMM settings described in this article were written to describe using a Fluke DMM, most digital multimeters will have very similar controls and will use similar measurement procedures.
What is the Proper Function Selection When Using a VOM or DMM to measure Voltage?
Watch out: be sure to select the proper function for the type of measurement being made.
In particular, on some instruments, leaving the function selector set at Ohms ( Ω ) and then touching the probes to live AC or DC current may damage the instrument and may be unsafe.
At above-left we illustrate the proper function setting when using the Extech 430 DMM to measure voltage. The Extech DMM dial is set to point to the V on the green scale. For this instrument a second step is necessary: we press the "Select" button until we see "AC" indicated in the display. [Click to see an enlarged, detailed version of this or any other image at InspectApedia.com]
What is the Proper Connection of the Test Probes When Using a VOM or DMM?
By convention, all electrical test meters color their test probes: one probe is black, the other red.
Our photos below illustrate the probe connection points on the Extech 430 True RMS Digital Multimeter. The black probe is plugged into the COM port and the red probe is plugged into the Temperature & Voltage port. In this range the meter can measure up to 600V.
What is the Proper Range Selection When Using a VOM or DMM to Measure Volts or Amps?
If you are uncertain of the circuit properties you are measuring, or for general safety, always start a measurement at the highest range offered on the instrument.
For DMMs or VOMs that do not provide an auto-ranging feature, set the voltage to a level higher than that which you expect possible, or start at the highest voltage setting.
In our photo at below left showing the yellow Sperry DSA-500 DigiSnap Meter, the function dial is set to measure AC voltage in the range of 0 to 600VAC.
At above right we illustrate the proper probe connections and range setting for a typical analog VOM, in this case the black Jensen 310 VOM. The VOM range setting is at 300 Volts on the ACV Scale.
Particularly on analog VOMs this step minimizes the risk of damage to the instrument or its meter movement assembly. From this position and after reading the actual measurement obtained, if you see that the measurement is a much smaller number than the maximum range of the instrument, change the Range Selection to the next lower position, thus increasing the instrument's reporting sensitivity and precision.
Our photo above shows the Extech True RMS Multimeter 430 in use measuring line voltage at an electrical receptacle.
Where do I Touch the Probes to make voltage measurements using a Digital Multimeter (DMM)
Fatal Shock Hazard Warning: Inspecting electrical components and systems risks death by electrocution as well as serious burns or other injuries to the inspector or to others. Do not attempt these tasks unless you are properly trained and equipped.
You will see that the red and black probes were connected incorrectly at the receptacle slots in our photo at above left. When measuring alternating current, (such as 120VAC or 240VAC) which probe goes into which slot will not normally change the voltage reading. When measuring direct current circuits (DC) polarity matters. .
Other common voltage detection or test points are anywhere in an electrical circuit, at switches, light fixtures, at exposed wires in an open electrical box, and in the electrical panel.
Above we illustrate voltage readouts on the Extech and Sperry Instruments DMMs.
At left we illustrate reading voltage on the analog scale of the Jensen VOM. [Click the image to see an enlarged, detailed version of any of these].
On the VOM we are using the top most black AC-DC scale; since our meter selector was set to 300V we read the top arc of black numbers - the scale that ends in "300". You can see that the meter is indicating 125 Volts.
Note that each of these photos was taken at a different measurement time or location so the three meter displays would not be expected to be in agreement.
Watch out: Common situations that lead to DMM failure - do not do any of the following:
Article adapted from information provided provided courtesy of Fluke Corporation - India. Fluke offers a wide assortment of multimeters and has sales offices in most countries. Email: firstname.lastname@example.org
Accuracy for a DMM is usually expressed as a percent of reading. An accuracy of one percent of reading means that for a displayed reading of 100 volts, the actual value of the voltage could be anywhere between 99 volts and 101 volts.
Separately at DMM Digital Multimeter HOW TO USE we provide a table that describes the measuring ranges and accuracy of a DMM.
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Continue reading at VOLTS / AMPS MEASUREMENT EQUIP for a review of choices of a wide range of devices or equipment that can be used for detecting or for measuring volts or voltage levels or select a topic from the More Reading links shown below.
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