How do I use a multimeter to test an anode?

[Colvic Watson]

We now have an electro eleminator fitted to properly connect the hull anode to the shaft and thereby protect the new prop. How do I use a standard multimeter to test the connections? I'm afraid my multimeter skills extend to checking if there are more than 12v in the batteries!

 

[Gwylan]

We now have an electro eleminator fitted to properly connect the hull anode to the shaft and thereby protect the new prop. How do I use a standard multimeter to test the connections? I'm afraid my multimeter skills extend to checking if there are more than 12v in the batteries!

Can you measure resistance or check circuit continuity?
Using either you can measure between two remote points in the circuit either the resistance across two points or check for a continuous circuit. Effectively amounts to the same thing.
There are all sorts of other issues about the circuit. But either these two tests will tell you if you have a reasonable circuit.

 

[Colvic Watson]

I could if I only knew how - so imagine I'm thick (quite easy say many) - and tell me how to measure resistance

 

[johnalison]

You should have a setting with a little beep sign, a bit like a contactless card sign. At this setting, if it detects little or no resistance it will beep. Job done.

PS it is necessary to put the probes on the places to be tested first.

 

[maby]

We now have an electro eleminator fitted to properly connect the hull anode to the shaft and thereby protect the new prop. How do I use a standard multimeter to test the connections? I'm afraid my multimeter skills extend to checking if there are more than 12v in the batteries!

By "electro eleminator", do you mean a galvanic isolator (zinc protector)? One of them may make effective testing using a basic multimeter more difficult. Basically speaking, you set the meter to its resistance range, touch one probe to the earth connection in the boat and the other probe to the hull anode and check that the resistance is low. Most multimeters will have several resistance ranges - you need to select the lowest. Touch the two probes together and watch the reading on the meter - it should show zero, or very close to zero. That shows that the resistance between the probes is very low - good in this case. If your anode is properly installed, the resistance between it and the boat earth line should also be pretty close to zero.

Now, unless the boat is extremely small, you are going to have practical problems in that most meter test leads will simply be too short to reach from the anode outside the boat, round into the hull and make contact with the earth line. You could check the resistance between the end of the anode bolt inside the hull and the earth line, but this is not such a good test - poor installation of the anode or corrosion on the bolt could introduce additional resistance that you will not detect. Alternatively, bodge up an extension to the meter lead using a few yards of thin wire and a crocodile clip.

The other potential pitfall comes if you have a galvanic isolator fitted. They are designed to break the earth connection for low voltages - that is how they protect the anode. Ordinary multimeters use pretty low voltages to measure resistance - to avoid damaging sensitive components in electronic equipment under test - the meter may show the isolator as quite a high resistance if its test voltage is around the threshold of the isolator. If your initial test seems to indicate that the anode is not connected, then try measuring to both sides of the isolator to see if it is the problem. It might be wise to disconnect the shore power before doing this.

 

[dolabriform]

You should have a setting with a little beep sign, a bit like a contactless card sign. At this setting, if it detects little or no resistance it will beep. Job done.

PS it is necessary to put the probes on the places to be tested first.

Failing that, look for the Ohm Symbol Ω . This is used to measure resistance.

There will probably be various ranges, choose the lowest one.

Put the probes in the right places and look at the measurement. The lower the amount the better ( a short straight wire will give 0 ohms resistance on a normal meter ).

 

[alahol2]

If you've got a multimeter you should have a section of the selector dial that is marked with an omega symbol (pair of headphones) or the word Ohms. Select the 200 range (say). Put one probe on the anode and the other on the prop. If you get a reading then you are measuring the resistance (in ohms) of the circuit. Virtually any reading on the 200 range means you've got a circuit so the anode should be protecting the prop..

 

[sailorman]

Rainbowpaul is a whiz with wiggly amps

 

[Mistroma]

By "electro eleminator", do you mean a galvanic isolator (zinc protector)? One of them may make effective testing using a basic multimeter more difficult. Basically speaking, you set the meter to its resistance range, touch one probe to the earth connection in the boat and the other probe to the hull anode and check that the resistance is low. Most multimeters will have several resistance ranges - you need to select the lowest. Touch the two probes together and watch the reading on the meter - it should show zero, or very close to zero. That shows that the resistance between the probes is very low - good in this case. If your anode is properly installed, the resistance between it and the boat earth line should also be pretty close to zero.

Now, unless the boat is extremely small, you are going to have practical problems in that most meter test leads will simply be too short to reach from the anode outside the boat, round into the hull and make contact with the earth line. You could check the resistance between the end of the anode bolt inside the hull and the earth line, but this is not such a good test - poor installation of the anode or corrosion on the bolt could introduce additional resistance that you will not detect. Alternatively, bodge up an extension to the meter lead using a few yards of thin wire and a crocodile clip.

The other potential pitfall comes if you have a galvanic isolator fitted. They are designed to break the earth connection for low voltages - that is how they protect the anode. Ordinary multimeters use pretty low voltages to measure resistance - to avoid damaging sensitive components in electronic equipment under test - the meter may show the isolator as quite a high resistance if its test voltage is around the threshold of the isolator. If your initial test seems to indicate that the anode is not connected, then try measuring to both sides of the isolator to see if it is the problem. It might be wise to disconnect the shore power before doing this.

I doubt that OP is talking about a galvanic isolator. Post said "electro eleminator fitted to properly connect the hull anode to the shaft and thereby protect the new prop". This sounds more like an electro eliminator designed to stop RF interference created by shaft rotation. It will also provide a good electrical connection between engine and shaft. I don't know why OP had it fitted but he mentioned protecting the shaft rather than any interference problem. Perhaps he has a flexible coupling on the shaft or just a poor connection from the engine via the oil filled gearbox and bearing.

 

[davidej]

I doubt that OP is talking about a galvanic isolator. Post said "electro eleminator fitted to properly connect the hull anode to the shaft and thereby protect the new prop". This sounds more like an electro eliminator designed to stop RF interference created by shaft rotation. It will also provide a good electrical connection between engine and shaft. I don't know why OP had it fitted but he mentioned protecting the shaft rather than any interference problem. Perhaps he has a flexible coupling on the shaft or just a poor connection from the engine via the oil filled gearbox and bearing.

I think he is talking about one of these

http://mgduff.co.uk/yachts-and-powerboats/products/electro-eliminators/ee1

I fitted one to my boat but it wasn't a great success

 

[RichardS]

This sounds more like an electro eliminator designed to stop RF interference created by shaft rotation.

How does a shaft rotating cause RF interference?

Richard

 

[Mistroma]

How does a shaft rotating cause RF interference?

Richard

I imagine that there a few possibilities. What happens if there's a DC voltage on the engine and that connects to the sea via the shaft. Once the shaft rotates there will be rapid changes to resistance and potential (pardon the pun) for RF interference. That's one possible mechanism I can think of but perhaps we'll get an expert to provide a better explanation. I mentioned RF interference because I looked at electro eliminators once and remember bits of the advertising blurb. Of course it could be snake oil but I expect it is a well documented phenomenon.

 

[PaulRainbow]

Kinnel, soon gets complicated on 'ere.

This was solved in post #4. Just touch one probe on the shaft and the other on the engine/gearbox/anything that's connected to the anode. If it buzzes/beep/farts it's making a connection.

If you're still not sure, connect the live from the shore power to the shaft, nip outside and put your hand in the water while touching something that's earthed. If your hair stands on end, it's working.

 

[VicS]

How does a shaft rotating cause RF interference?

Richard

Kinnel, soon gets complicated on 'ere.

The blind leading the blind tonight ?

We now have an electro eleminator fitted to properly connect the hull anode to the shaft and thereby protect the new prop. How do I use a standard multimeter to test the connections? I'm afraid my multimeter skills extend to checking if there are more than 12v in the batteries!

Perhaps a few Photos will help

This shows a cheap multimeter measuring the resistance ( 9.4 ohms) of a length of resistance wire

You can see in this close up that it is set to the 200 ohms range ( its lowest), that the black lead is in the common negative socket and that the red lead is in the socket used for measuring volts, ohms and milliamps

Here is a picture of a slightly different meter doing the same thing

This close up shows that again the 200 ohms range is selected (This meter also has a bleeper on this range which sounds if the resistance is less than 60 ohms. This is useful for continuity testing)

This close up shows that with this meter the black lead is in the common negative socket while the red lead is in the socket used for milliamps, ohms and battery testing

 

[Colvic Watson]

Thanks all, I was on night shift so just catching up. Yes electro eliminator is there to connect the shaft to the hull anode to stop the prop fizzing away, we also fit a shaft collar anode but it falls off mid season - because they just do! Thanks for the explanations about how to use the continuity functions it makes complete sense now. Much appreciated.

 

[sailorman]

Thanks all, I was on night shift so just catching up. Yes electro eliminator is there to connect the shaft to the hull anode to stop the prop fizzing away, we also fit a shaft collar anode but it falls off mid season - because they just do! Thanks for the explanations about how to use the continuity functions it makes complete sense now. Much appreciated.

Not it you fit an MGD anode with the steel strap in it

[lmg]http://mgduff.co.uk/files/large/039adbbdd7f17dd89960cf373e29c111.jpg[/img]

 

[Colvic Watson]

Not it you fit an MGD anode with the steel strap in it

[lmg]http://mgduff.co.uk/files/large/039adbbdd7f17dd89960cf373e29c111.jpg[/img]

That's a shaft anode not a collar anode