Using a multimeter

A multimeter is used to test components and measure circuits in an electrical system. It may be digital or analogue; the former gives the read out in figures and the latter has an analogue scale and a moving pointer. Digital ones are easy to read and often they can be bought cheaply, but bear in mind if you pay a bit more you should get a much better bit of kit that offers a full range of scales. As well as giving additional measurements, more expensive meters will typically have longer leads and a more robust build quality. I have bought cheap multimeters in the past and over time have not found them to be durable or reliable. Choose an auto-ranging meter, if possible, because with manual multimeters you need to estimate the value before you test it.

Initially, to become familiar with your multimeter, it can be used for simple jobs such as checking the batteries or a fuse. It’s particularly handy for jobs such as the light installation described here and this is the ideal sort of practical wiring job that will also help you get used to it. For use on a boat, most of the time you will want to use a multimeter to measure the voltage in volts, the current in amps and/or the resistance in ohms. In practice that might mean you want to see you have a healthy 12V supply at a power output or connector block. When measuring the current in amps you will be able to determine how much power a piece of equipment consumes; this can be particularly important with heavy load items such as fridges or heaters, for example. When it comes to fault checking, testing resistance will be more relevant than voltage or current because it will tell you if there’s a break in the circuit. For example, you can tell using the resistance function if it’s a navigation bulb that’s at fault or if it’s the contacts holding it that have corroded.

Voltage, resistance and current are inseparable pals and team up to make what is known as Ohm’s law. The formulas derived from this are V = I x R, I = V/R and R = V/I. V is for voltage, I is for current and R is for resistance. This means you can calculate the current through a wire by dividing the volts by the resistance. A dim light might mean voltage drop caused by long wires. If you increase the voltage in a circuit, the current increases, but if you increase the resistance, the current decreases.

For our purposes the voltage will be DC (Direct Current) on a 12V or 24V system for a boat. In your house you will have a far more powerful AC (Alternating Current) supply of 240V. Mains voltages can be lethal, so don’t expose live AC circuits unless you really know what you’re doing. Even the most basic meter should come with a manual – keep them together at all times. When you come to use the meter, you don’t want to forget which test probe goes in which socket and what the correct setting on the dial should be. Also, remember to switch the meter off each time after you have used it so that you do not run the internal battery flat.

We look at some of the typical practical scenarios for using a multimeter on board on the next page.