Digital Multimeters: One size does not fit all

One size does not fit all. Selection of the proper multimeter involves significant consideration. Cost is one, another is the specific purpose of the device. How it will be used and the qualifications of those using it are also important. The author...

One factor to always consider when dealing with electrical circuits is a term referred to "Flash Hazard Analysis." To determine the possibility and more important the potential hazard related to an electrical spark several factors are considered and include the operating voltage and the time required for circuit protection to react to a short. Most test equipment is rated for various applications where an electrical arc could occur and testing will verify the maximum safe voltage for use. When troubleshooting or testing electrical circuits, the test equipment is not the only thing requiring protection. Proper clothing, protective equipment and knowledge of the circuit being tested are all critical to ensure a safe outcome.

It takes a very low value of current, flowing through the human body, to cause death or serious physical harm. There have been many studies performed in this area with different values of current that cause each effect.

Most electrical test equipment is manufactured with some type of internal protection and it is always a good practice to create some type of standard procedures when using electrical measuring equipment. In some cases digital meters may be a bit more tolerant of installation errors than their analog counterparts. Some of the most common practices include:

  • Select a range with a maximum value greater than you expect to be reading.

  • Connect the meter, making sure the leads are positioned correctly. Digital meters can often be connected in reverse and display a reverse polarity, but an analog meter may be damaged.

  • If the reading goes off the scale: immediately disconnect and select a higher range.

Multimeters are easily damaged by careless use and precautions are needed!

Analog as well as digital meters contain a battery and in the case of the analog it is used as a source of electrical current for the measurement of resistance. In the digital application the battery also is a supply for reading ohms but it is also used to power the display so virtually no power from the circuit under test is required to run the meter. This means that on their DC scale voltage they have a very high internal resistance (usually called input impedance) of 1 Meg ohms or more, usually 10 Meg ohms, and they are very unlikely to affect the circuit under test. In some cases analog meters are preferred when doing circuit resistance checks as a somewhat higher current flow is required to displace the indicator.

Digital meters often have a special diode test setting which enables a higher current flow to adequately bias or switch on the diode or semiconductor.

The fact there is an active battery or external power supply required for meter operation carries a certain stigma. I know for an absolute fact that the 9-volt battery installed in one of my testers is without any doubt capable of detonating certain aircraft engine fire extinguishers. (Don't ask how I know!)

On or Off?

Understanding the symbol key on the meter face is paramount to successful operation (See chart at left).

The function selection on the face of the tester should also be a consideration. In some cases an OFF position is included and No, OFF is not an abbreviation for On Full Force.

This is of course the safest position to place the selector anytime the tool is not actively being used. In some cases multiple switches have to be configured to first determine what the meter is measuring and second determine the anticipated range of the requested parameter.

Reading a meter will first of all require an understanding of the prefixes which are displayed in metric units. Some multimeters have a feature called auto range. This means that when selected to a circuit the display will determine how the information is presented. When troubleshooting a circuit it can sometimes be rather confusing when using this automatic feature while trying to interpret the readout. The important thing to note is the abbreviation identifier often accompanying the reading in the meter display. (See chart.)

Frequently testers with auto range features have a minimum/maximum hold capability.

This is a valuable asset when diagnosing faults with modern-day aircraft. All too frequently voltage surges can cause numerous avionics components to behave in unusual ways.

Additional Troubleshooting Capabilities

Adaptability and availability of accessories can turn a basic VOM into a diagnostic dynamo. Devices such as amp clamps will extend the amperage monitoring range of the device from perhaps 10 amps to several thousand without even interrupting the circuit.

Various kinds of temperature modules combined with thermocouple probes enable the technician to monitor what might be otherwise undetectable areas for overheat or cold soak problems.

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