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...


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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