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Multimeter Functions

Quick Tips #101
Introduction

Digital multimeters, also known as DMMs, are among the most versatile testing instruments. 

A digital multimeter functions well with energized and de-energized electrical circuits. They can be used for measuring AC and/or DC voltages, resistance and continuity, AC and/or DC current, Ohms law and power formula, frequency and duty cycle, diodes and capacitance, making them are a popular choice for professional and amateur users.

Applications include electrical engineering design, maintenance, quality control, and inspection, and any application requiring electrical servicing and diagnostic testing.

Safe practices should be followed while working with electricity. When selecting a digital multimeter, be aware that some have extra protection features such as overload protection, high energy fuses, etc. There are also new international safety standards for low voltage (1000 volt and less) test equipment and how they apply to DMMs.

Multimeter Selection and Functions

Choosing a DMM should be made based upon the electrical measurements needed. For example: A precision DMM is required if you need more exact small value measurements in resistance and current and the capability to check transistors and diodes.

Digital meters are generally accurate within 0.1% to 0.5% of the reading.

Autoranging, or dialing, is also a useful multimeter feature. It allows the user to quickly move from the instruments various measuring scales while maintaining accurate readings.

Another consideration to take into account when selecting a DMM is whether it is a True RMS (root mean square) meter. A RMS value is also called the effective or heating value of an AC signal. The RMS value is equivalent to DC voltage that provides the same amount of heat generated by a resistor as AC voltage would if applied to that same resistor. Since an AC signal's voltage rises and falls with time, it takes more AC voltage to produce a given RMS voltage. A handy formula to remember is: Peak Volts AC x .707 = VRMS (volts root mean square). A power grid that must produce 169 volts peak AC turns out to be 120 volts RMS (.707 X 169). What does all this mean?

The typical DMM is not a True RMS (root mean square) meter and as a result it will produce misleading voltage readings when used to measure anything other than a DC signal or sine wave.

DMMs featuring True RMS Measurement are for nonlinear voltage and current loads. This feature is necessary when taking AC voltage and current measurements.

There are several methods to incorporate the RMS feature into a DMM. Each handles AC differently. The following are three basic types:

  1. A rectifier type multimeter indicates RMS values for sinewaves only, (sine wave is also called cycle and is measured in Hertz-i.e. 60 Hertz or 60 cycle) It does this by measuring average voltage and multiplying by 1.11 to find RMS. Trying to use this type of meter with any wave form other than a sine wave will result in erroneous RMS.
  2. Average reading digital volt meters are just that, they measure average voltage for an AC.
  3. A True RMS meter uses a complex RMS converter to read RMS for any type of AC waveform.

Careful consideration should be given as to what type of DMM will best suit your specific needs.

Summary

The most important factor to keep in mind is that electrical test equipment should be used with respect and maintained in accordance with the manufacturer's guidelines. As with all electrical test equipment, individuals using a digital multimeter should receive training in its proper operation.

Commonly Asked Questions
Q.   Can I set the test current when making resistance measurements?
A.   No, the test current is set depending on the range of the instrument. Typically they are:
100.0000 ohm 1 mA
1.000000 kohm 1 mA
10.00000 kohm 100 uA
100.0000 kohm 10 uA
1.000000 Mohm 5 uA
10.00000 Mohm 500 nA
100.0000 Mohm 500nA||10Mohm
 
Q.   Are there any specific criteria to be used when working with digital multimeters?
A.   Yes, IEC 1010 standard (International Electro- Technical Commission) is the new safety standard for low voltage test and measurement equipment.
 
Q.   How can I be sure my new digital multimeter is approved or certified?
A.   The IEC (International Electro-Technical Commission) sets standards but does not test or inspect for compliance.

A manufacturer can claim to design to a standard with no independent verification.

To be UL-Listed, CSA or TUV certified, a manufacturer must employ the listing agency to test the product's compliance with the standard.

Look for the emblem of the listing agency on the meter.
 
Q.   What safety issues should be considered when selecting a digital multimeter?
A.  

Use a meter with:

  • Fused current inputs (high energy fuses)
  • Overload protection on the ohms function
  • Test leads that have shrouded connectors and finger guards
  • Recessed input jacks
  • Is independently certified
 
Q.   Practice safety when using a DMM.
A.  
  • Use meters within their rating
  • Use meters designed for power circuits and energy circuits
  • Use replacement fuses approved by the manufacturer
  • Use high quality safety related testing leads
  • Whenever possible work on de-energized circuits and follow proper lock-out tag-out procedures
Sources

FLUKE
International Electrotechnical Commission

(Rev. 11/2012)

 

Find even more information you can use to help make informed decisions about the regulatory issues you face in your workplace every day. View all Quick Tips Technical Resources at www.grainger.com/quicktips.

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Please Note:
The content in this newsletter is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific compliance questions should refer to the cited regulation or consult with an attorney.

©2012 W.W. Grainger, Inc.

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