Five Ways To Use A Digital Multimeter
Story and Photos By Tim Murphy
A DVOM uses two leads: a black lead and a red lead. Plug your black lead into the device's "COM" port, or common terminal. Similar to a negative terminal, this is the reference lead. Plug the red lead into the port that corresponds with the setting you've chosen on the DVOM selector switch. On some DVOMs, these are combined in a single port; other devices provide separate ports for amps, volts, and ohms.
To understand what's happening inside your boat's electrical systems, the single best tool is the digital multimeter, sometimes called a DVOM (digital volt-ohm meter). Available for as little as $20, a DVOM reveals three main things about an electrical circuit: the electrical potential (voltage), the electrical current (amperage), and the electrical resistance (ohms). Voltage is like water pressure in a hose. Current is the rate of flow — like, say, water passing through a nozzle. And resistance is like a crimp in the hose, or even the hose itself: the force that restricts flow. What's a circuit? It's the chain of six components that powers a device: a power source (battery or distribution panel), a conductor (wire), a switch, circuit protection (a fuse or circuit breaker), the device, and the return path to ground (another wire). If the VHF works only intermittently or not at all, chances are one of those six components is letting you down. The DVOM will let you sort out what's at fault. Here's how to go about it.
Open-Circuit Voltage Test
The first step is to determine if the boat's power source is good, and exactly how much voltage it can supply, using the open-circuit voltage test to determine the battery's state of charge. A reading of 12.6 volts or more indicates a full charge; 12.3 volts, a 75-percent charge; 12.2 volts, a 50-percent charge; 12.0 volts, a 25-percent charge. Readings below 11.7 volts indicate a discharged battery. Follow the steps below.
- Turn the DVOM selector switch to DC volts (scale for less than 20 volts).
- Turn all circuits on the boat OFF (battery selector switch to OFF).
- Engine and charging system OFF.
- Touch and hold the DVOM black lead (COM port on the DVOM) to the battery's negative terminal, and the red lead (DC volts port on DVOM) to the positive terminal.
You should get a voltage reading that indicates your battery's state of charge. Write it down. If it's above 12 volts, continue to the next test. If not, charge the battery.
The Voltage-Drop Test
Voltage drop is the inevitable loss of electrical potential through a circuit. The American Boat and Yacht Council (ABYC) allows for a three-percent voltage drop in critical circuits (nav lights, VHF radios), and a 10-percent voltage drop in all noncritical circuits. A higher voltage drop indicates a problem — faulty connections, corroded terminals, undersized wiring — that must be corrected. You should've noted the battery voltage in the first test. Now you need to check the voltage at the VHF.
- Battery and circuit you're testing ON (flip the appropriate breaker on the DC panel).
- All other circuits OFF.
- Turn the DVOM selector to DC volts.
- At the VHF, touch your red lead to the positive terminal, black lead to the negative.
Compare the result to your voltage at the power source. A drop greater than 0.4 V (the ABYC deems a VHF radio a critical circuit) or 1.2 V in any circuit should send you scurrying to clean terminals and check for ample wire gauges. If the voltage reads zero, you've got continuity issues.
A typical electrical fault is "an unwanted open" — an undesigned break in the circuit — that interrupts the flow of electricity. That can come from something as simple as a blown fuse or something more complicated like a broken or extremely corroded conductor, or a loose or separated connection. Continuity describes a circuit that is closed, as it's designed to be. To find out if you have a break in the circuit:
- Turn the DVOM selector switch to ohms (Ω).
- Check the DVOM battery. With the leads separated, you should see "OL" for "overloaded" or no continuity. Now touch the leads together, and you should hear a beep for continuity or a value near "0" on the meter. If you don't get these, replace the DVOM battery.
- Turn off the circuit to be tested.
- Disconnect the two ends of the circuit component you suspect is bad (say, the wire leading to VHF from the distribution panel). If your DVOM leads can't reach, add a length of properly sized wire to complete the circuit.
- Connect your probes to each end of the conductor.
A beep or a meter reading near zero indicates continuity. "OL" indicates overload: no continuity. No continuity means there's a break in the wire. You can also check for blown fuses and tripped circuit breakers with this test by touching each end with your probes (breaker must be on and disconnected from power). It's also possible the device itself has burnt up, breaking the circuit. If the continuity is high, but not OL, go back and look for a loose or corroded connection, or jiggle the breaker or switch if they're part of the portion of the circuit you're testing. When these go bad, they sometimes work when pressure is applied, but not when it's released.
Measuring Current Using Leads
If the problem you're having isn't getting devices to turn on but rather keeping them on, you might be drawing too much current (amps) through a given circuit. Fitting older boats with new, power-hungry devices can exceed the capacity of the original circuits. If you don't have good documentation on how many amps a given device draws, here's how to find out. Use a pair of meter leads that terminate in insulated alligator clips. You'll need to disconnect the power lead somewhere in the circuit; the meter itself will complete the circuit. A good place to insert the meter leads is at the circuit protection (fuse block). If there's a fuse, remove it.
- Switch OFF the circuit you're testing.
- Set your DVOM's selector switch to DC amps, at the proper scale.
- Disconnect a wire where you'll measure; alternatively, remove a fuse from its holder.
- Clip the black lead to the terminal closest to the battery, the red lead to the other.
- Switch on the circuit, being careful not to let your body parts complete the circuit.
The meter will give you a reading in amps to record. If the total amp draw on a circuit exceeds its protection rating, the breaker will pop. Measure all the devices on a given circuit to find out if it's overloaded.
Current Test Using An Amp Clamp
Some newer multimeters come with a feature called an "amp clamp" that lets you measure current without disconnecting the circuit. But there are still a couple of tricks. First, when measuring DC current, make sure you understand your DVOM's directional component. Some come with a plus sign and an arrow, showing which side of the meter should face the battery's positive terminal. Consult your manual. Second, you'll need to measure current through just one conductor — particularly, the positive conductor (usually red). On marine DC systems, you'll sometimes find duplex wire, which contains both the positive and negative (usually black or yellow) conductors inside a single sheath; you'll need to separate them to do this test.
- Identify the positive conductor of the circuit you're testing.
- Set your multimeter to DC amps, at the proper scale.
- Open the jaws of the amp clamp, then encircle the positive conductor. You should get a reading to record.
For more detail on these and other tests, see Ed Sherman's Powerboater's Guide to Electrical Systems or find a "Basic Marine Electricity and Marine Corrosion Prevention," course near you at www.abycinc.org.
Tim Murphy is the co-author of Fundamentals of Marine Service Technology (ABYC, 2012).
— Published: December 2013
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