Using Your Multi-Meter.........Ed Sherman
In my last post I discussed my meter of choice for marine electrical trouble-shooting. Today, I'm going to explain how to measure voltage, both AC and DC. The Blue Sea 8110 is quite versatile in this area because it has the ability to measure very low voltage as well as sufficiently high voltage. Its range is 0.001- 600 volts on the AC scale and the same for DC. So with AC, you will typically be measuring 120 Volts here in North America, or in some cases with larger boats you may be looking for 220 volts. The basic tolerance for this is + or - 10% as an exceptable variation. Voltage that is too low can cause components to overheat, ultimately damaging them. Voltage too high is just not a reality unless you are measuring voltage output from an on board generator that may have a malfunctioning governor that controls the unit's RPM. But, with shore power delivered AC, excessively high voltage is never a problem. Low voltage on the other hand is a genuine possibility, especially if your boat is the last in a long string of boats using shorepower. Many docks use transformers to "bump up" voltage on the outer piers. The issue here is what we call "voltage drop". You see whether AC or DC some voltage is lost as it travels down a wire. This is caused by the inherent resistance in the wire or cable. Things like wire that is too small for the job or loose connections and corrosion all cause excessive voltage drop because they introduce excessive electrical resistance. If the voltage falls below the design parameters for a piece of equipment, then it will not function properly.
So, to check voltage you need to ensure that your meter is switched to the correct scale. On most meters this will be indicated by the letter "V" with either a flat line (DC) under it or a wavy line (AC) under it. Your red meter lead will be plugged into the socket on the meter that has the red coding around the socket and the black lead will fit into the terminal socket marked either "com" or with the black color code around the socket.
If your meter is what we call "self-scaling" or "auto-ranging" it won't matter what level of voltage you are attempting to measure, it will automatically give you a correct reading. If you have a less expensive meter that does not have auto-ranging capability, you will need to set the meter on a scale that is approximately two times the expected voltage reading, say 20 volts for a 12 volt reading or 200 volts for a 120 volt reading. The 8110 unit mentioned in my last post is auto-ranging, which is one of the reasons I like that meter. One less step to worry about. The diagrams below, which come from my book The Powerboater's Guide To Electricity illustrates how you would connect the meter to measure both AC and DC volts.
In the diagram above, I'm measuring the voltage across the battery in your boat. This is a DC reading. I have the red lead from the meter connected to the battery positive post and the black lead connected to the battery negative post. My reading is 12.6 VDC, which indicates a battery that is nearly fully charged.
In the diagram above I'm checking the volatge available at an outlet. Observe carefully which slots the meter leads are plugged in to. The red goes to the shorter of the vertical "slots". This is the AC "hot" terminal. The meter's black lead goes to the longer of the two slots, or the AC neutral. Also, before checking, make sure that the outlet is turned at your main AC panelboard. Also, note that your outlets may be oriented 180 degrees from the one shown above. Both ways of orientation are acceptable and the choice is often made based on regional preferences.
In my next installment, I'll talk about measuring amperage, so keep coming back as I add more information to this series. By the time I'm done, you should feel pretty confident using your multi-meter.