Multimeters, What a concept!

Multimeters, What a concept!

Proper selection, use, and care of electrical testers is important

By Jim Sparks

November 2000

ImagesLike most involved in the aircraft maintenance field, I started out as an inquisitive little tyke with a fond habit of taking things apart. One day, after spending a significant amount of time trying to get a friend's transistor radio to operate, I climbed on my bicycle and pedaled up the road to the local electronics retailer. It was my suspicion that the battery was dead but with the price of a nine-volt battery being about half of my friend's weekly allowance, I did not want to take any unnecessary chances. The sales clerk, after hearing my dilemma, reached into the showcase and withdrew a fantastic gizmo. It had neatly folded test leads and a display that had lots of numbers and a mirrored background and even a selector knob that had about a million different positions. The man behind the counter had removed the suspect battery from the radio and connected and adjusted the mirrored gadget, then told me that the battery was indeed depleted and would need to be replaced. I, on the other hand, was no longer concerned with a mere battery. I wanted a closer look at the mirrored marvel. He called it a V.O.M. and said that complete with test leads and battery it could be mine for $5.65. My birthday had already passed and Christmas was a ways off. Fortunately, I had a father who also liked to tinker and was equally enthused about the fantastic little gadget. Under the premise that the directions would be read carefully and the device only used under supervised conditions, my father bought me my first V.O.M.

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Assorted multimeters

It was only after reading the directions that I learned V.O.M. actually had a meaning, Volt, Ohm, Meter and was also known as a Multimeter. It could be used to test things called voltage, amperage and something called Ohms. I observed that anytime I selected the scale that had to do with Ohms, I could touch the two test leads together and make the pointer deflect all the way to zero on the mirrored scale. About a year later, a friend of mine who was old enough to drive purchased an old, used car. Unfortunately, the headlights would not come on when the switch was activated. I selected the scale labeled as Ohms and connected the test leads to each headlamp and observed meter movement. That told me the lamps were okay. The next step was to connect my V.O.M. to the light switch. Unfortunately, I connected the Red test lead to the light switch while the Black lead was resting on the uncarpeted floor. When I did install the black wire, there was no meter movement. I then removed both leads and touched them together as I had done many times in the past. Again there was no movement. I went back to the store where I purchased my meter and then ask the guru behind the counter why my V.O.M. no longer worked. This time he pulled out a Phillips screwdriver and removed the one screw that held the back on. After careful scrutiny, he informed me that I had blown a fuse. He then asked if I had used the meter as illustrated by the directions. I informed him what I was doing and was promptly reprimanded for having the familiar Ohms scale selected while connecting the meter to a circuit that had power applied. It was then made clear to me that anytime I connect a meter, it is always best to select the highest range of the voltage scale to see if any potential is available. If it is confirmed that the circuit is dead, then go to the Ohm scale. The cost of this lesson involved spending 59 cents to replace my blown fuse, plus have one as a spare, and listening to a 10-minute dissertation on how to properly use my meter.

Images Volt check on Falcon-20
battery connector

Back to the task at hand, I connected the red wire to the positive side of the switch and the black wire to the wire going out to the lights. With the voltmeter selected on a 50-volt scale,we connected the car battery and I observed 12 volts. When the switch was selected "ON," the meter still indicated 12 volts. Now we disconnected the car battery and then connected my V.O.M. to the light switch. By actuation with no meter movement, we were able to confirm a faulty switch.
Electricity has always had the power to intimidate those who never really spent the time to understand. Many claim to know the properties of hydraulic systems and realize a combination of fluid pressure and flow will make some device move or react.
Most everyone realizes that if pressure is available to a device and it is still not responding, there is either a problem with the component in question or there is something obstructing the fluid flow. To diagnose hydraulic malfunctions, we use pressure gages and flow sensors. Hydraulic pumps produce flow. It is only after flow is restricted that hydraulic pressure begins to increase.
Strangely enough, the flow of electrons is very much the same as hydraulic fluid. A pressure gage is used to tap into supply lines and look at internal pressure compared to atmospheric pressure. Many external hydraulic power units come complete with built-in flow sensors. These devices can make troubleshooting hydraulic circuits a breeze. These same concepts can be applied to electrical systems. An aircraft generator is an electron pump. Just like its hydraulic counterpart, an electron pump produces flow. Electrical pressure known as voltage is produced when the electron flow encounters resistance. Electrical flow is known as amperage and electrical resistance is measured in Ohms.

Analog or digital?
A multimeter can be used in an electrical circuit in a manner similar to a pressure and flow gage in a hydraulic circuit. The first decision when selecting a V.O.M. is to decide whether to go Analog or Digital. Each of these has both advantages and disadvantages. The analog device will employ a mechanical movement and like most mechanical devices, it is more prone to damage from impacts. This type of tool also includes some rather sophisticated switching internally to convert the meter movement from the voltage to amperage scales. Also, frequent compensation is required as the internal batteries begin to deteriorate.
Digital meters on the other hand are more impact resistant and frequently require no external adjustments for accuracy. When installed in a circuit to observe voltage, the analog device will apply a slight electrical load to the system. This means the system being tested provides the electrical current flow needed to operate the meter movement. Sensitivity of the meter movement will determine the amount of current needed to operate the meter and will in turn determine how much of a load the device will impose. The higher the sensitivity the lower the current flow. Even though this current flow is quite small it is nevertheless, a draw. A digital counterpart applies almost no electrical demand on the circuit. Sometimes this is advantageous but in other cases the application of an electrical load may cause an electrical problem to be more easily detected. Frequently, digital meters have an automatic range feature that can cause technicians to misinterpret the reading. Close attention must be paid to operating range.

Testing continuity
One of the most common uses of a V.O.M. is as a continuity tester. This involves installing the meter test leads with one on each end of the wire or other conductor in question. When the "OHMS" scales are selected, the meter will connect the output of the internal battery to the circuit being checked. Should the circuit be low resistance, the meter deflection will indicate low Ohms. This is a very effective feature for technicians involved in the wire installation process to verify proper configurations. Many meters will also include an audible tone that can be selected "On" or "Off." This makes the continuity testing process move even faster. By hearing the tone, the technician knows that the circuit being checked is complete and does not require visual verification of the meter display. Resistance checks are possible using power from the battery within the meter. Great care should be taken when installing this power source in certain circuits. Unfortunately, a nine-volt battery found in some Multimeters could detonate certain types of fire bottle squib, as well as damage sensitive electronic circuits. Knowledge of a circuit's characteristics should always be determined prior to installing any test equipment. Continuity testing is not always a good test to verify the load carrying ability of the system. Aircraft electrical wires and cables can be compared to the hydraulic pipes in the aircraft. The multimeter introduces a very small electrical flow in the system and in the event the wire is damaged or where it is incapable of carrying the required electrical current to operate the circuit, the small current from the Ohm meter might give the technician the opinion that the circuit is in tact. Many have experienced the condition where a hydraulic supply line is crushed or restricted. It is often possible to disconnect the line on each end and install a hydraulic hand pump and observe the flow of fluid through the line. However, during normal system operation, the higher flow rates will result in a significant pressure drop. This should always be considered when testing a circuit for high resistance.

Calibration required
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Temp adapter

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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