Guide to Multimeters
When choosing electrical test equipment it is important to know exactly what features a product has. Digital multimeters can have a variety of functions and range in their level of precision. Check out PE Facts to find out which digital multimeter is best suited for your task!
The digital multimeter, also known as a DMM, is one of the most practical and useful electrical test instruments on the market today. Digital multimeters are the best meters available that are capable of working with energized, as well as de-energized, electrical circuits. There are a variety of functions that can be helpful to both amateur and professional users such as measuring AC and DC voltages and currents, continuity, resistance, capacitance, diodes, Ohms law and power formula, frequency and duty cycle.
Applications include electrical engineering design, inspection, quality control, maintenance, and any application requiring electrical servicing and diagnostic testing. When using the DMM, as with any instrument involving electricity, safety is critical. The user needs to know what to look for and must follow safety precautions at all times. When selecting a digital multimeter, you should determine all of its functions and whether or not it has extra protection features, such as high energy fuses or overload protection. You should also be aware of any new international safety standards for low voltage (1000 volts or less) test equipment and how they affect digital multimeter use.
Selection
When selecting a digital multimeter, you should base your decision on the types of electrical measurements necessary to your work.
For example: A precision digital multimeter instrument is essential if you need exact small value measurements in resistance and current. It also allows you to check transistors and diodes.
Digital meters are usually accurate within 0.1% to 0.5% of the reading.
Auto ranging, or dialing, is also a valuable feature to look for in a digital multimeter instrument. It allows you to quickly move between the various measuring scales while maintaining accurate readings.
Whether or not it's a True RMS (root mean square) meter is another important factor to consider when choosing a digital multimeter instrument. The RMS value is also known as the effective or heating value of an AC signal. It is equal to the DC voltage that causes the same amount of heat to be generated by a resistor as AC voltage would when applied to the same resistor. A useful 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).
So, what does all this mean? Most digital multimeter instruments are not True RMS (root mean square) meters. Therefore, they will produce false voltage readings if they are used to measure anything other than a DC signal or sine wave.
A digital multimeter instrument that includes "True RMS Measurement" is for nonlinear voltage and current loads. This function is required when taking AC voltage and current measurements.
There are various ways to include the RMS feature into a digital multimeter instrument. The three basic types are:
- The rectifier type of meter produces RMS values for sine waves only. It measures the average voltage and multiplies it by 1.11 in order to do so. Using this instrument with any other wave form besides sine waves will produce misleading RMS values.
- The average-reading digital volt meter measures the average AC voltage.
- The True RMS meter uses an intricate RMS conversion system to generate RMS values for any type of AC waves.
Although RMS values can be very helpful in certain situations, True RMS meters can be rather costly. Therefore, it is important to identify the specific features that would most facilitate your needs in order to choose the best type of digital multimeter for you.
Summary
When using electrical test equipment, the most crucial thing to remember is to handle and maintain the instrument according to the manufacturer’s guidelines. Proper training is recommended for all individuals operating a digital multimeter, along with any other electrical test equipment.
Commonly Asked Questions
Q. When taking resistance measurements, can I set the test current myself?
A. No, the test current is pre-set depending on the range of the instrument. Typical ranges 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 DMM is approved or certified?
A. Although the IEC (International Electro-Technical Commission) sets standards, it does not test or inspect for compliance. Manufacturers often claim to “design to” a standard without having independent verification. In order for an instrument to be UL-Listed, CSA or TUV certified, a manufacturer must employ the listing agency to test the product’s compliance with the standard. If you want to be certain that your DMM is certified, look for the listing agency’s emblem on the meter.
Q. What safety issues should be considered when selecting a DMM?
A. For safety reasons, it is best to 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
- Independent certification
Q. Are there specific safety practices I should employ 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
Please Note: The information contained in this publication 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 questions should refer to the cited regulation or consult with an attorney.
