How to Use a Multimeter
Not so long ago, the simple 12-volt test light was the tool to use when tracking down electrical problems. Increasingly, though, the digital multimeter is used by electricians and do-it-yourselfers to diagnose a wide range of problems in both DC and 110-volt AC marine electrical systems.
Even if you aren’t sure how to use one, having a multimeter aboard makes good sense for two reasons: If you run into trouble, chances are that someone else aboard or nearby will probably know what to do with the tool. And if you don’t have one on board, it’s guaranteed that you’ll never learn how to use it. Multimeters are relatively simple to operate, and the wealth of information they provide is priceless.
While meters come in a wide variety of shapes and sizes with different capabilities, features, and ranges, most function in the same way, and even an inexpensive one will do nearly everything the average do-it-yourself cruiser could ever hope for when troubleshooting common onboard electrical faults.
To get started, let’s get to know the control panel, where you set the meter to perform a specific task. Once you understand the basic controls, we’ll dig deeper into their various functions and explore the tasks they perform.
On my multimeter, the instrument’s functions are arranged in a clockwise direction from the Off position.
A: When this function is selected, the multimeter measures AC-power voltage for doing shore-power, generator, and inverter work.
B: This function measures DC voltage for a battery, an alternator, and the other primary onboard electrical components. With an optional attachment, the meter pictured also measures rpm for gasoline engines.
C: The millivolts scale, represented by the symbol mV, measures thousandths of volts DC and is useful, among other things, for troubleshooting corrosion issues.
D: At this setting, the multimeter measures resistance in ohms, represented by the Ω symbol. Use your multimeter set to this position to identify poor connections and to evaluate the bonding system.
E: This setting, represented by the symbol of an arrow striking a wall, is the multimeter’s diode-test feature. Use it to evaluate a diode or your galvanic isolator-equipped shore-power system.
F: At this setting, the multimeter measures DC current in milliamps, useful in determining how much current is being drawn by, say, a light, fan, or motor.
G: At this setting, the multimeter measures AC amperage of power from shore, a generator, or an inverter. Use this to determine how much current an AC appliance is drawing and thereby gauge if it’s working correctly.
In upcoming columns, we’ll delve into specific troubleshooting tasks for this all-too-useful tool as well as explore techniques for measuring amperage, voltage, and resistance. Stay tuned.
Steve D’Antonio offers services for boat owners and buyers through Steve D’Antonio Marine Consulting.