Ohm not Om...........Ed Sherman

We're not talking about a consciousness altering chant here, but one of the three constituants in our triad of electrical players. In my last several installments we looked at measuring volts and amps. Today we need to have a look at the Ohm guy.

The Blue Sea 8110 meter we have been discussing in these last few installments has the ability to measure from  0.1- 40 megohms (40 million ohms), a quite useful range around boats, but I say that with a caveat.

Ohms are the unit of measurement for electrical resistance, named after the early scientist attributed with their discovery, George Simon Ohm. The symbol used to identify ohms is the Greek symbol omega, shown below.

Electrical resistance can be used to make some things electrical function, but for the most part the boater is generally fighting with excessive electrical resistance. In some cases its desireable to have high resistance, a wire's insulation is a prime example of wanted resistance, which keeps the wire from short circuiting with other wires in close proximity. But generally, resistance, which I have already discussed as being caused by loose connections at terminals, corrosion at connections, or perhaps wire too small for the task at hand is an enemy to the boater. So, with that in mind you might think that being able to measure how much resistance you have in a circuit would be a great advantage. The problem is that if you don't have a resistance specification to work with, you really are guessing when acquiring ohmmeter readings. I don't mean to totally discount the value of using an ohmmeter, but I've just seen too many technicians draw entirely incorrect conclusions when using one. So let's walk through the basics here so you don't make some of the same mistakes.

Set your meter to the ohm scale as shown in the photo below:

The yellow icon you see below the omega symbol indicates a continuity beeper. If the wire or component you are testing has a complete circuit through it, connecting the meter's red lead to one end of the circuit and the black lead to the other end will give you a beep sound indicating a complete circuit. Incidentally, make sure you de-power any circuit you are checking as power in the circuit could damage your ohmeter, or blow a fuse internal to the meter.

When you are checking a wire or circuit if the continuity is good, your meter will give a beep and the scale on the meter will give you a reading in ohms, or fractions of ohms. Don't place too much stock in this reading, that's where mistakes get made, people tend to read too much into this reading. For example, wire typically used in boats does have some inherent resistance per foot, but engineering tables often only list ohms per 100 ft. of wire for various gauges of cable. You'd never be able to remember these numbers, nor should you. Suffice to say they are really small numbers. Conversly, remember that we are using multi-strand wires in marine installations (or you should be). One strand connecting two points in a circuit will give you a continuity reading, but the more relevant question should be, can the wire carry the amount of amperage it is required to? One or two connected strands will not, for sure.

 In the diagrams below, from my Powerboater's Guide to Electrical Systems show an ohmmeter is being used to check some typical ignition components. This can be useful for checking such things as ignition or charging system components because often the workshop manual for your engine will give expected resistance values for various components within the system, so you can match your readings to the specifications. If they don't match, the component is bad and needs replacement.

Testing a spark plug wire for continuity and specified resistance..

 Testing ignition coils for proper resistance and short circuit to ground (top diagram). When testing for a short circuit to ground, you don't want continuity. So, your meter should typically read "OL" or overload, meaning simply that the resistance value is so high the meter can't read it. This is exactly what you want when testing insulation integrity.

So, in summation, use an ohmmeter for sure, but understand what it can and cannot tell you. Don't confuse Ohm with Om.

Measuring Up..........Ed Sherman

All right, now that you know some of the basics of using your multi-meter (and I hope by now you've acquired an ampclamp type) let's look at some of the things you can measure when checking both volts and amps.

One of the things we are concerned with on boats is excessive voltage drop. This is especially true with your DC system, typically 12 volts. With only 12 volts to start out with, a loss in voltage can have a really profound effect on how your DC appliances operate. This is most important for things like your electronic equipment, navigation lights and bilge blower circuits on gasoline fueled boats. In fact, the ABYC dictates that a maximum drop of only 3% of nominal voltage is allowed. Excessive voltage drop is caused by things like loose connections, corrosion, or sometimes by wire used in a circuit that is too small a wire gauge to handle the amount of amperage the appliance in the circuit needs to function properly. Symptomatically, excessive voltage drop will show up as an engine starter motor that is not cranking your engine fast enough, or lights that are glowing too dimly or a fish finder that is blinking diagonally lines and other forms of mishmash on the display screen. Remember, if voltage drop is excessive, inadequate amperage is flowing through the circuit in question. Both volts and amps are closely related. The third key player in this mix is resistance, measured in ohms. We'll talk more about ohms in a later installment, but understand that all three of these components must play together well, and the truth is, on a boat, they often don't. I've used the diagram below, which was first used in one of my very old Cruising World articles on this topic, to explain the interaction between volts ohms, and amps.

So, looking at the diagram above. Think of volts a pressure, amps as rate of flow, and ohms, well you can see what he's always trying to do, especially on your boat.

With more pressure, you can actually distribute high amperage through a smaller pipe (wire), but if voltage is a constant, and too low, amperage is just not going to get through the ohm guy.

OK, so now lets see how to use your voltmeter to measure somthing besides how much voltage your boat's battery has available.

In the diagrams below, I'm showing you how to measure voltage drop at different points in a starter motor circuit. In this case the symptom is slow cranking. You've already tested the battery and you know it is in good shape, and up to charge.

In the above example, the meter is set to the DC volts scale. The black lead from the meter is connected to your battery's negative terminal and you are tracking through the circuit with your red lead. You are looking for a voltage reading of something in the order of 0.1 or 0.2 volts at any given point in the circuit. The measurement is the amount of voltage drop in that particular leg of the circuit. It is cumulative, so if you want to truly isolate a given leg, put your negative lead on one side of the starter button and the positive lead on the output side of the starter button and depres the button. A voltage reading of greater that 0.2 volts tells you that the starter button switch has too much resistance through it, causing excessive voltage drop. Your slow cranking problem could be caused by the switch itself. A well connected starter circuit with everything in good order should not have a cumulative voltage drop of more than about 1 volt with the engine cranking. This methodology will work on any electrical circuit.

Measuring Amps

With amperage, its nice to know ahead of time what the normal current draw is for a given circuit. Some electrical appliances will give you this information, it'll be on a label on the appliance somewhere. (Actually required on AC appliances). The appliance might only tell you the operating voltage and the wattage of the appliance. That's OK, because with those two bits of information, you can easily calculate the third bit you need. A water heater for example might be rated at 120 volts, 1500 watts. By dividing the watts by the volts you can derive the amps needed to run the appliance at its maximum potential. 1500 divided by 120 equals 12.5 amps. OK, but what if you turn on the heater and clamp the AC hot wire (black wire) and measure less than 12.5 amps? Whats implied?

Here's where your thorough understanding of all of this becomes important. There are several possibilities here and you will need to decide which it is. The water heater above may only be rated to 500 watts, or the cable supplying the unit is too small to carry the amperage required. If its neither of those two things, it is possible the heater element is faulty, in which case the element will need to be removed for further inspection. The tables below show some typical requirements for popular on board appliances, both AC and DC. Use these as a rough guide. These tables come from two of my books, The Powerboater's Guide to Electrical Systems and Advanced marine Electrics and Electronics Troubleshooting. Don't forget, If you have questions regarding this material, or anything else I present in this blog, you can always just register and contact me. I answer all of my emails daily.

Let's Get Amped Up....Ed Sherman

I'm not talking about a caffeine overdose here, but having a feel for how much amperage (electrical current)  is running through the wiring on your boat being pretty useful. You see when all is said and done amperage is the stuff that can cause really bad problems on board. Uncontrolled amperage is the stuff that causes electrical fires and shocks. So, knowing how to measure amps and what the readings you get can mean is good. In this installment I'll walk you through some of the basics related to amperage and how to measure it with your multi-meter.

Today, the only intelligent way to measure amperage is with a clamp-type meter; another reason I love the Blue Sea 8110 I've been mentioning in my meter posts here. The 8110 has a clamp and the ability to measure both AC and DC amperage (they are very different in terms of measurement) the range of measurement for both is 0.01-400 amps. This is a very useful range for boat use. On the low end, you can scout out elusive, really low level leakage current (I'll discuss that more in later installments) and up to 400 amps covers most equipment used. Things like anchor windlasses and bow thrusters, even starter motor current in many cases can be checked with this meter.

Using the amp clamp is easy, just clamp it around the wire you want to measure the current flow in. Just remember to set the meter for either AC or DC amps. Just turn the dial on the face of the meter to either the "A" with the wavy line under it for AC or the "A" with the straight line under it for DC. If the circuit is powered by your battery it is DC current, if it's powered by shore power or your generator or inverter its AC current. The face of the meter showing the A settings is shown below:

The other nuances you need to be aware of are that especially when reading low amperage values, you may need to zero out the guage. Looking at the above meter face you can see under the DC A setting, "zero" in yellow. With the rotary dial on this setting, just hit the yellow "select" button to zero out the meter before you attempt to take a reading. The other feature on this meter that can be useful is the "hold" button shown above. Sometimes amperage readings will jump around a bit while you are measuring them. If you are attempting to get a "peak" reading, or the highest value measured, just hit the hold button to lock on that maximum reading when it appears on the meters screen.

The only other question that comes up relative to using an amp clamp is the orientation on the wire you are checking. The clamp on the 8110 is quite helpful in this regard. On the side of the clamp you will see an embossed arrow. Also, on the face of the clamp, you will see a + sign. These indicate both the source of current and the direction of current flow. Orient the clamp so the + sign is toward the source of the current you are measuring, let's say the battery or pwoer distribution panel. The arrow is point toward the appliance that the wire is supplying, let's say the cabin light circuitry. This will determine whether or not the meter gives you a + or - amperage reading. This is important in cases where you may be trying to determine which direction the current is flowing, as there are some wires on your boat where it is possible for current to flow in both directions depending on whats going on in the circuit at the time of measurement. (More on that in a later installment.) The arrow and + sign are shown in the photo below. Oh, and don't worry if you get it backwards, it really won't hurt anything, it will just make things confusing when you really are trying to determine current direction.

One last thing. Make sure you turn the circuit on!

In my next installment, I'll tell you what is implied by various current and voltage measurements. This will be the beginning of making you an effective electrical trouble-shooter. In the meantime, HAPPY NEW YEAR! It's New Years Eve here. I'm taking tomorrow off, but I'll get the next installment in over the weekend.

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. 

Become a Multi-Meter Master!

A lot of boater's keep a multi-meter on board. These units, which are also referred to as a DVOM (digital volt/ohm meter) are the key to troubleshooting electrical system gremlins, provided you know what to look for and how to read the meter. Most boaters fall short on that last requirement. So with that thought in mind, I'm going to devote a few blog entries into showing you how to use the multi-meter and understand what exactly it's telling you. The diagrams I'll use for this are excerpted from my book The Powerboater's Guide to Electrical Systems. The sailors in the crowd should not be intimidated by that title, because electricity really doesn't know whether its on a powerboat or a sailboat, all of the same techniques apply.

Selecting a useful meter

There are literally hundreds of meters available, but not all of them have the functionality you will need. The $9.95 analog meter available through Radio Shack will tell you a few things, but is far from the best choice if you really want to figure out what's going on with your electrical system. The best choice right now will include the ability to measure amperage, both AC and DC, volts AC and DC, ohms of resistance and electrical continuity. Go digital. Analog meters are going the way of pay telephones, they work, but are harder to use and not as convenient as they used to be.  My current favorite is distrubuted by Blue Sea Systems and is available online for about $175.00. Look for model 8110. West Marine also sells this unit. This unit offers the best ranges of measurement for both amps and volts for most all marine applications. The mini-clamp type meter is shown here:

You can check out the specifications for this meter at: http://bluesea.com This site is one of my absolute favorite references for all things marine electrically related.

Tomorrow, I'll begin showing you how to use this meter, or just about any meter you select.