|As mentioned in the last article Electricity 101, many of the problems that you will encounter on your vessel are electrical problems. It is because of this that the multimeter is an invaluable tool. With it you can do a lot of troubleshooting and track down potential problems without calling an electrician.
I prefer a digital multimeter that gives a more accurate digital readout over the less expensive analog models that have a needle that moves over a set of printed numbers on the dial. You can get a good, dependable multimeter at most marine supply stores or Radio Shack for well under $100.00.
Understanding AC verses DC
Multimeters can be used to measure voltage (pressure), amperage (flow rate), and Ohms (resistance). These measurements can be made on AC or DC systems. Before using a multimeter, make sure you understand the basics of electricity as outlined in the aforementioned article.
AC, or alternating current, is the same current that you find in your home that is used to turn on your lamps, television, radio, etc. This is usually 120 volts and can be dangerous. This same AC current comes to your boat via your shore power cord, an extension cord, etc. You might be simply operating a sander with an extension cord from the dock or, on larger vessels, operating all the appliances and comforts of home via your shore power cord when docked and your generator when underway.
DC, or direct current, is that current supplied by your batteries. This is what starts your engine(s), runs your electronics, your VHF radio, your DC lighting system, running lights, etc. This is generally a 12-volt system, except on larger boats where you may find a 32-volt system.
An important factor when using a multimeter is to make sure that you are using the correct scale depending on whether you are measuring AC or DC. (This should be clearly marked on the face of your multimeter.) If you measure AC current while on a DC scale you could destroy your meter and, worse yet, get a severe shock. If you don’t have at least a working knowledge of the basics don’t attempt to measure or test AC systems.
AC and DC systems on boats often share the same electrical panel. Make sure you know which is which and never work on the panel with the AC power still active from the dock.
Using the same water analogy described in Electricity 101, lets review what we can test for with our multimeter.
Measuring pressure (voltage). Water pressure is measured by inserting a gauge in the line and comparing the difference to the pressure in the line to the surrounding atmospheric pressure. Because atmospheric pressure is all around us, by leaving one side of the gauge open, the difference can be measured. Voltage is measured in much the same way except the reference point is not atmospheric pressure but ground. The meter cannot measure the difference without being connected to the ground reference point. Thus, we must attach our meter to both the positive and negative (ground) sides of the circuit.
Measuring flow (amperage). To measure water flow, the rate of flow could be measured by inserting a paddle wheel into a pipe and seeing how fast it spins as the water flows through. Your multimeter, using the amperage scale, is the electrical equivalent of the paddle wheel and measures the flow through the circuit.
Measuring resistance (ohms). As water flows (amperage) through a pipe at a regulated pressure (voltage) and this rate of flow is measured, the resistance can be deduced. The same theory is used to measure electrical resistance. The multimeter, when used to measure ohms, uses an internal battery to supply current (amperage) at a carefully regulated pressure (voltage) to the circuit being tested. Using ohm’s law the resistance can be calculated.