Ohm, not Om
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.
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.
