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ELECTRICAL INSPECTION, DIAGNOSIS, REPAIR
ACCURACY vs PRECISION of MEASUREMENTS
AFCIs ARC FAULT CIRCUIT INTERRUPTERS
ALUMINUM SECs & WIRING
ALUMINUM WIRING HAZARDS & REPAIRS
AMPS & VOLTS DETERMINATION
AMPACITY - the LIMITING FACTOR
APPLIANCE EFFICIENCY RATINGS
BACKUP ELECTRICAL GENERATORS
BACK-WIRED ELECTRICAL DEVICES
BOOKSTORE - ELECTRICAL
BUILDING SAFETY HAZARDS GUIDE
Cadet & Encore Heater Recall
CIRCUIT BREAKER FAILURE
CIRCUIT BREAKER SIZE for A/C or HEAT PUMP
Classified CIRCUIT BREAKER WARNING
CORROSION in ELECTRICAL PANELS
CORROSION & MOISTURE SOURCES in PANELS
CUTLER HAMMER PANEL FIRE
DEFINITIONS of ELECTRICAL TERMS
DIRECTORY OF ELECTRICIANS
DMM Digital Multimeter HOW TO USE
ELECTRIC METERS & METER BASES
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRIC PANEL AMPACITY
ELECTRIC PANEL INSPECTION
ELECTRIC PANEL MOISTURE
Electric Power Frequency Table
ELECTRICAL DISTRIBUTION PANELS
ELECTRICAL GROUND SYSTEM INSPECTION
ELECTRICAL SERVICE DROP
ELECTRICAL SERVICE ENTRY WIRING
EMF RF FIELD & FREQUENCY DEFINITIONS
FEDERAL PACIFIC FPE HAZARDS
FIRE SAFETY Checklist, CPSC
GFCI PROTECTION,Testing GFCIs AFCIs
HEATING COST FUEL & BTU Cost Table
HEAT TAPE USAGE GUIDE
Hertz - Definitions of KHz MHz GHz THz
KNOB & TUBE WIRING
LIGHTING, EXTERIOR GUIDE
LIGHTING, INTERIOR GUIDE
LIGHTNING PROTECTION SYSTEMS
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MAIN ELECTRICAL DISCONNECT
MAIN DISCONNECT AMPACITY
MOISTURE SOURCES in PANELS
MURRAY SIEMENS Recall
PHOTOVOLTAIC POWER SYSTEMS
PUSHMATIC - BULLDOG PANELS
REMOTE ELECTRIC POWER, PHOTOVOLTAIC
RUST in ELECTRICAL PANELS
SAFETY for ELECTRICAL INSPECTORS
SE CABLE SIZES vs AMPS
SIEMENS MURRAY Recall
UNDERGROUND SERVICE LATERALS
VOLTS / AMPS MEASUREMENT EQUIP
VOLTAGE MEASUREMENT METHODS
WIND ENERGY SYSTEMS
WIND TURBINES & LIGHTNING
ZINSCO SYLVANIA ELECTRICAL PANELS
How to use a digital multimeter or DMM. This article explains what a DMM or digital multimeter is, how to choose one, and the accuracy of DMMs. We describe the typical DMM probe connections & control settings to make voltage measurements, resistance measurements, and current measurements. We also discuss the types of DMM protective circuits and safety features.
This article includes a table of typical DMM Functions, Ranges, & Accuracy Limits and a second table comparing the accuracy limits of DMM/VOM functions across most DMM/VOM manufacturer brands, using representative product models.
For additional details & examples of measuring amps or current see AMPS MEASUREMENT METHODS; for measuring volts or voltage levels see VOLTS / AMPS MEASUREMENT EQUIP. Page top photo illustrates the Fluke_15B-17B digital multimeter, photo courtesy Fluke India. Also see DMMs VOMs SAFE USE OF as well as SAFETY for ELECTRICAL INSPECTORS. At ELECTRIC MOTOR DIAGNOSTIC GUIDE we describe both offline and online electric motor circuit analysis (MCA) test procedures performed using a DMM or ammeter.
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How to Use a Digital Multimeter or DMM: choosing a DMM, control settings on the DMM, and DMM features
Safety Warning: Home inspection standards for electrical inspections do not require the inspector to insert any instrument into the service panel. Therefore electrical tests that require using a VOM or DMM are optional during a property inspection even if they are in fact necessary for other more in-depth troubleshooting.
Opening an electrical panel and approaching live electrical wiring, devices, & equipment is a dangerous procedure that can damage electrical equipment or worse, cause electrical shock, or even death. 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. See SAFETY for ELECTRICAL INSPECTORS.
Our photo (above left) 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 at VOLTS / AMPS MEASUREMENT EQUIP
A DMM is simply an electronic device for making electrical measurements of electrical properties such as resistance, voltage level, or current flow. A DMM may have any number of special features (such as the ability to measure and report decibels), but mainly a DMM measures AC or DC volts, ohms in one or more ranges, and amperes or electrical current.
Note: 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.
At left our photo illustrates the Sperry DSA-500 clamp-on multimeter/ammeter reading 0.28A (Amps) of current flow on an electrical circuit. Accuracy of this measurement is as well as exactly how this measurement is made are discussed below.
Choosing a DMM for the job requires not only looking at basic specifications, but also looking at features, functions, and the overall value represented by a meter’s design and the care taken in its production. Reliability, especially under tough conditions, is more important than ever today.
[- Why? - Ed.]
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.
At left our photo at left we illustrate measurement of Amps at 0.28A on the Sperry DSA-500 clamp-on multimeter/ammeter, the function dial has been set to the 0-40A range, the proper ampacity testing range in order to report with finer precision when measuring lower current levels than if we had used the 400A range setting. But what about accuracy differences in different DMM measuring ranges?
Sperry's documentation for this DMM indicates that the accuracy of the instrument when measuring AC current (Amps) is +/- 2.0% rdg+/-6dgt when making measurements at 23+/-5degC and 45-75% relative humidity. The company is in essence warning that at more extreme temperatures or humidity levels the accuracy of the instrument may vary from this level. Details of the procedure for measuring amps are provided below at How to make current measurements.
Below we provide a table that describes the measuring ranges and accuracy of a DMM using this instrument as an example, followed by a table comparing DMM/VOM Specifications & settings for Actron DMMs, Equus / Innova DMMs, Extech Electronics DMMs, Fluke DMMs, Mastech DMMs, Simpson DMMs & VOMs, and Sperry DMMs.
Watch out: don't confuse measurement precision with measurement accuracy. In the expression of measurements, precision refers to the number of decimal places or digits in a number obtained by the measurement, while accuracy describes the margin of error in the measurement.
People who do not understand this precision - accuracy distinction can be misled with regard to the reliability (accuracy) of numbers that are presented with much precision if the margin of error in the measurement was significant..
But if the possible range of error in our measurement is 10%, then our measurement of 129.4 OR our measurement of 120.2939480 both could be expressed as +/- 12 (since 12 is 10% of 120). This means that the range of accuracy of our measurement of 129.43939480 +/- 10% means that
Watch out: don't confuse the accuracy of the DMM instrument itself and its individual readings with true measurement accuracy.
The DMM instrument and an individual measurement may be quite precise and quite accurate for conditions at the moment that the measurement was made. But external variables such as time, temperature, humidity, weather, voltage supplied by the electricity provider, electrical loads on the system and other factors mean that from one measurement to the next the results could be quite variable.
When measuring the voltage level of an electrical circuit in our office in Mexico we find that from time to time the actual voltage level can vary by about 10%. So while we might report that at a given measurement we measured the voltage level at 108V, that measurement should be presented as 108VAC +/- 10%, telling our client that over time voltage in that location typically varies between 98VAC and 118VAC (excluding periods of power loss when voltage is zero).
For further explanation of these sources of inaccuracy in various types of measurements, see ACCURACY vs PRECISION of MEASUREMENTS.
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.
What is the Proper Function Selection When Using a VOM or DMM?
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.
In our photo the function dial is set to measure current (Amps) in the range of 0 to 40A. On this Sperry Instruments device there is an OFF position that should be used when the instrument is not in use or is to be stored.
Tip: for VOMs and DMMs whose function selector dial does not include an "OFF" position, when we are finished using the instrument we leave the selector set to AC-Voltage at the highest voltage range - a choice that minimizes risk of possible damage to the equipment should some fool touch the probes to live current without first checking the function dial position.
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.
Referring to our photo just above, for measuring AC voltage the highest voltage range would be the position indicated by the 3-green arrow. For measuring current or Amps the highest Amps range would be the position indicated by the 2-orange arrow.
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.
Select V~ (AC / alternating current) or V (DC / direct current), as appropriate for the electrical power source.
For details of using a DMM or VOM to measure voltage, see VOLTS MEASUREMENT METHODS
For a description of electrical equipment used to detect or measure volts or amps see VOLTS / AMPS MEASUREMENT EQUIP.
Watch out: Make sure the electrical power is off to the device or circuit being tested before making resistance measurements.
Tests of Electrical Motors by Measuring Resistance
DC electric motor resistance tests
Hobbyist LB Miller has described the design and function of a simple test fixture useful for determining the electrical resistance of DC motors by providing a 1A current to the motor and measuring the voltage drop across the motor, thus giving motor resistance in milli-ohms. Note that his approach is for DC motors. 
AC electric motor resistance tests
Testing an AC motor by measuring resistance across its windings or by making an apparently "simple" test of a motor (disconnected from all electrical power!) by measuring resistance across its power wires sounds appealing - as we might be able to deduce something about the condition of a motor that is itself not readily accessible, such as a submersible well pump located close to the bottom of a well.
We looked for some AC electric motor voltage, amps or current draw, and offline (power off/disconnected) electrical resistance measurement diagnostic rules of thumb. Unfortunately it's not quite so simple as motor types, designs, and specifications vary.
For details see ELECTRIC MOTOR DIAGNOSTIC GUIDE where we offline motor circuit analysis (MCA) test procedures that can through resistance (ohms) measurements identify shorted or open electric motor windings.
DMM Settings & Procedures for Making Current (Amps) Measurements
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