Multimeters, What a concept!

Multimeters, What a concept! Proper selection, use, and care of electrical testers is important By Jim Sparks November 2000 Like most involved in the aircraft maintenance field, I started out as an inquisitive little tyke with a fond...


Certification of accuracy is also a factor in the aviation industry. Any electrical tester that provides information used in the determination of airworthiness must be calibrated at least yearly, plus the certificate of calibration should always be in the possession of the technician.
Calibrated OhmMeters have a value in being able to test circuit resistances. This can be very effective for testing temperature-sensitive resistors or possibly resistive type heating elements. Many aircraft use electrically-heated windshields. Often, the resistance values are published in maintenance or wiring documents. If actual values differ from the published ones, current flow through the element will be affected. The result may be excess amperage resulting in windshield overheat or possibly low current flow resulting in reduced heating.
Many technicians will also use an OhmMeter to test mechanical type variable resistors such as potentiometers and rheostats. Testing these devices will often give better results by having the system under normal operating conditions. This will, of course, require use of the "Voltage" scales instead of "Ohms," then apply normal system power and test the output of the variable resistor under electrical load. Some prefer using the voltage scale on an analog meter for this type of test; however, using a digital meter and watching for a significant voltage fluctuation can sometimes be easy to spot.

Troubleshooting with the V.O.M.
High resistance connections or grounds can cause some of the most unusual electrical discrepancies. Troubleshooting this situation is again ideally suited to a voltmeter. By connecting the red lead of the V.O.M. to the component end of the ground wire, and the black wire to a known, good ground on the aircraft, then energizing the circuit; any voltage indication on the meter is proportional to the resistance in the wire or attachment to ground. Thus, the more voltage observed, the more resistance in the circuit.
Multimeters are often capable of reading circuit amperage, some even include an "Amp Clamp." An Amp Clamp is a device that uses a jaw-like clamp to encompass the wire where current flow is to be tested. By measuring the inductance of a conductor when current is flowing, the Amp Clamp can provide a very accurate indication of current flow. Amp Clamps are designed specifically for AC or DC circuits; however, there are units designed to work with both AC and DC. The alternative on many Multimeters is to connect the test leads in series with the equipment to be tested. Most meters are limited to around 10 amps unless an external shunt is used. Unfortunately, if the current limits of the tester are exceeded significant damage can result. Extreme care must be taken when utilizing a multimeter. Some aircraft electrical systems use high voltages and improper techniques could prove lethal.
Multimeters on today's commercial market offer many different functions and adapters can be attached to provide some basic meters with extended capability. An example of this would be a temperature probe. By plugging an external module in the standard test lead connections of a multimeter, a number of different type temperature sensors can be connected and observed. In addition, Amp Clamp modules can be purchased and can be used with many V.O.M.'s. The information obtainable from a properly used multimeter can be applied to solve almost any electrical problem. Often, equipment manufacturers or suppliers conduct seminars to highlight the correct techniques to get the maximum from your investment. This is something to consider when selecting a source from which to purchase an electrical tester or accessories. Selection of test leads is every bit as important as choosing the correct meter. After all, a meter without test leads is like a screwdriver without a tip.
Selection of an electrical tester should be given careful consideration as one size and type does not work in all situations. In the aviation industry of today, a V.O.M. is as important as any other tool in the tool chest.

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