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Old 11-10-2015, 07:34   #1
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Corrosion Reference Electrode -discrepancy

When checking for sufficient cathodic (zinc) protection from electrolysis is it normal that there is a significantly different reading for the rigging including shroud and stay turnbuckles and mast step in comparison to the battery negative post and the engine block?


I suspended a silver/silver-chloride Corrosion Reference Electrode in the seawater and connected it's cable to the negative terminal of my digital multimeter.


Then I attached the positive terminal of the digital multimeter separately to:


  • The rigging including shroud and stay turnbuckles and mast step and I observed readings around -750mV.


  • The battery negative post and the engine and I observed readings around -910mV.


Both of these readings fall within the recommended range of -750 to -1000mV indicating that I have adequate cathodic protection – but why is there a difference?




DOES THE DIFFERENCE OF APPROXIMATELY 160mV INDICATE THAT THERE IS A PROBLEM WITH THE CONNECTIVITY OF THE RIG'S GROUND?


Note that in order to minimize any extraneous factors the battery selector switch was in the”Off “ position and there also no connection to shore power.


Also note that the Pacific Seacraft owner's manual states that the lightning ground system consists of a 4 awg. Conductor leading from the stays, shrouds and mast step to a keel bolt. Yet the lead keel is coated with Interprotect barrier coat, so does that not insulate the electric current connecting to seawater?


I would appreciate receiving any comments that would help clarify whether there is a problem.
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Old 11-10-2015, 07:41   #2
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Re: Corrosion Reference Electrode -discrepancy

Did you allow time for the reference electrode to stabilize and then repeat the readings? This can take 5 minutes.
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Old 11-10-2015, 08:12   #3
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Re: Corrosion Reference Electrode -discrepancy

Yes I did allow time and the electrode was probably immersed around 20 minutes during a period of testing various locations.
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Old 11-10-2015, 11:13   #4
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Re: Corrosion Reference Electrode -discrepancy

If they are bonded they will be the exact same vaule +\- 0.010 or so.

You either have bad bonding or none at all between those 2 things

It's possible all mast peices connect to keel for lighting. But are not bonded to ships common ground. Though I would guess the keel would be.

You'd only get the same reading if phiscally joined by a wire.

i would check for the connection between the mast system and keel. And ground system and keel. Test the keel bolt and see what vaule you get. You'll know what side it's connected to by voltage (it can't be both voltages)
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Old 11-10-2015, 11:30   #5
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Re: Corrosion Reference Electrode -discrepancy

From what you have suggested it sounds like the rig is not bonded to common ground since the only mention of bonding of the rig in the owner's manual is bonding to the keel for lightning protection.

Is this satisfactory? What is customary?

I would appreciate hearing from all boat owners who have knowledge of this with their own boats.
Bob
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Old 11-10-2015, 12:17   #6
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Re: Corrosion Reference Electrode -discrepancy

FWIW, my mast is bonded to a Dynaplate while all through hulls are bonded and zinced.
The two are not connected to each other.
I insulated the VHF antenna bracket as well as the WIFI Bullet from the mast because of this.
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Old 12-10-2015, 09:17   #7
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Re: Corrosion Reference Electrode -discrepancy

I worked for McMillan Marine in the '60s, installing C.A.P.A.C. systems in yachts and large ships. You might find someone with a hull potential meter to take accurate, meaningful readings. We used silver silver chloride on our reference electrodes and platinum on our anodes. I realize this doesn't answer your questions, hence, the advice to find a hull potential meter. No offense intended.
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Old 12-10-2015, 11:48   #8
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Re: Corrosion Reference Electrode -discrepancy

A volt meter with a silver cell is the hull meter of choice.

An on board meter is just a volt gauge but less useful with colours and not numbers.
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Old 12-10-2015, 12:10   #9
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Re: Corrosion Reference Electrode -discrepancy

"FWIW, my mast is bonded to a Dynaplate "
And here I thought the Dyna folks themselves said not to do that. Their product is an RF ground, and the sintered metal contains enough voids, filled with seawater, so that it will superheat and "explode" from the steam produced if it does actually carry a lightning strike to the sea. (Not at all the same as using a solid metal grounding plate.)
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Old 12-10-2015, 23:50   #10
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Re: Corrosion Reference Electrode -discrepancy

Quote:
Originally Posted by hellosailor View Post
"FWIW, my mast is bonded to a Dynaplate "
And here I thought the Dyna folks themselves said not to do that. Their product is an RF ground, and the sintered metal contains enough voids, filled with seawater, so that it will superheat and "explode" from the steam produced if it does actually carry a lightning strike to the sea. (Not at all the same as using a solid metal grounding plate.)
You are correct, and I haven't gotten around to redoing that.
It's just not been high on my list as we don't get much lightning around here.
Especially at my home's dock as we live in a rain shadow of the Olympic Mountains.
Seattle gets a bunch, but we might get lightning about 4 or 5 miles away once in 5 years. Never seen or heard of any closer than that.
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Old 13-10-2015, 11:35   #11
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Re: Corrosion Reference Electrode -discrepancy

"but we might get lightning about 4 or 5 miles away once in 5 years. Never seen or heard of any closer than that. "


Obviously you live in a better neighborhood, where the zoning laws don't allow lightning.(G)


In the past year or two I've heard official warnings go from "lightning can strike 20 miles out from a storm" to "thirty miles out" and more recently "If you can HEAR IT, GET INDOORS."


I have no idea what the line-of-sight to a storm can be, because those same sources try to tell me that "heat lightning" (the silent flashes across the sky) is simply lightning that is too far away to be heard. I still prefer to get into something built with, say, robust nine foot thick granite block walls. Lacking that...hey, is there room under that bed?(G)
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Old 13-10-2015, 13:13   #12
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Re: Corrosion Reference Electrode -discrepancy

Getting back to the issues brought up by smac999 yesterday, is it o.k. if the rig and engine are not connected to a common ground?

Also I have not mentioned (since I did not want to over-complicate the original question) that there are several local spots on sides of the lead keel where the Interprotect barrier coat has chipped off. The same spots continue to chip off again after being re-coated with Interprotect.
That gets back to my 2nd initial question - does not the Interprotect barrier coat insulate the electrical current in the keel from contact with the seawater consequently defeating the keel as acting as ground?
Bob
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Old 14-10-2015, 06:21   #13
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Re: Corrosion Reference Electrode -discrepancy

Quote:
Originally Posted by hellosailor View Post
...In the past year or two I've heard official warnings go from "lightning can strike 20 miles out from a storm" to "thirty miles out" and more recently "If you can HEAR IT, GET INDOORS."

I have no idea what the line-of-sight to a storm can be, because those same sources try to tell me that "heat lightning" (the silent flashes across the sky) is simply lightning that is too far away to be heard. I still prefer to get into something built with, say, robust nine foot thick granite block walls. Lacking that...hey, is there room under that bed?(G)
Indeed.

Lightning can strike as far as 10 to 15 miles from the area where it is raining. That's about the distance you can hear thunder. If you can hear
thunder, you are within striking distance.

http://www.nws.noaa.gov/os/lightning...ing-safety.pdf

When you see lightning without hearing thunder (so-called "heat lightning") it is really lightning that is over 15 miles away, and too far away for you to hear the thunder.
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