Using a Multi meter on Your Boat – BoatSafe.com

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### Basic Electrical Theory for Boaters

Multimeter – The Invaluable Tool

Using The Meter

As with any piece of equipment, you should carefully read the manufacturer’s instructions prior to use. Various multimeters have basically the same features but the features may be selected in different ways. Obviously, the more you pay the more features you will get. The following methods will be based on a fairly inexpensive generic multimeter.

Although it does not matter which leads from your multimeter you use to test AC current, when testing DC, it is imperative that you use the positive (red) lead on the positive (+) side and the negative or ground lead (black) on the negative (-) or ground side of DC circuits. It is a good idea to get into the habit of using the positive and negative leads consistently even on AC current.

To measure voltage you simply need to touch the positive (red) lead to the positive side of a circuit and the negative (black) lead to ground or the negative side of the circuit. For instance you could put the positive lead on the positive (+) side of your battery and the negative lead on the negative (-) side of your battery to measure the voltage in the battery. A new fully charged battery should read approximately 12.5 volts.

Your meter will have several voltage choices to choose from. For instance mine has, on the ACV(volts) side: 750 and 200 and on the DCV side: 200m, 2000m, 20, 200, 1000. Unless your meter has a feature that automatically selects the correct voltage, always start at the highest voltage selection available first. Why you ask? Suppose you have a situation where you have both 120v AC and 240v AC circuits. If you select the 750v option first, you wonÂ’t blow out your meter if you mistakenly touch the 240v circuit. Once the voltage is confirmed in the read out you can select the lower range to get a more accurate reading.

What other situations might prompt you to measure voltage? LetÂ’s say you have an aerator pump in your live well that connects to, and is powered by, your battery. It worked fine last time you went fishing however today you put in the bait, turn on the switch and nothing happens. Do you have enough voltage in your battery to run the pump? You check this and find that you do. Next you use the meter to check the positive (red) and negative (black) wires going into the switch. HmmmÂ…no voltage. The solution must be that there is a loose or broken wire between the battery and the switch.

Amperage (current) can be measured (both AC and DC) by connecting the meter in series with the appliance you are measuring. On board you may have many appliances such as water pumps, fans, stereos, radios, electronics, etc. All these items draw current from your battery. These appliances may draw from .5 to 6 or 7 amps. Your battery, however, only has so much current that can be drawn before its voltage drops to a point that it will not run anything.

Most appliances will have ratings on them that tell the voltage required and the amps or current that they use. However, lets say you bought a 12-volt lamp at a marine yard sale that has no rating listed. You want to find out how many amps the lamp will draw. By connecting the meter in series with the lamps wiring and your battery you can measure the amperage that the lamp will use.

You can measure resistance in Ohms using your multimeter. Make sure that you only measure resistance on circuits that are free from a power source or you will blow your meter. The meter itself supplies the power source from its internal battery. This battery will discharge over time and may need to be replaced. Why would you want to measure resistance? Basically when you are measuring resistance you are measuring to see if you have a complete circuit. When you place the meter in a circuit that has been disconnected from its power source you can tell if the circuit is complete. You can start by checking the circuit in the meter itself. Turn the selector switch to the area labeled OHM. Depending on your meter you may get a display of the number 1 or the infinity symbol. This indicates that there is a break in the circuit. Now touch the ends of the positive and negative leads together. You reading should be zero or very close to zero. SoÂ…what you ask?

LetÂ’s take an example, say you have two light bulbs rolling around in a drawer. You remember that one was burned out and you just threw it in the drawer in order to make sure you got the correct replacement. You got the replacement and threw them both in the drawer thinking you would change them out later. Now you donÂ’t know which one is the bad one and which is good. They are both of the frosted variety so you canÂ’t physically see the element to see if it is broken in one bulb. You donÂ’t want to do the trial and error test because you just bought this cool multimeter. How do you identify the good bulb? You can use the multimeter to measure the resistance to find which one has an open circuit. This would be the bad one.