Ideally there should be no current flowing as the pipeline should be
insulated. This is intended to reduce corrosion of the pipe, it is called
cathodic protection. In inspecting pipelines the resistance between ground
and the pipeline is measured and if it is too low anodes are buried deep (>5
ft) in the ground and connected to the pipe to make up for the resistance to
prevent corrosion.

It's basically the same principle as putting anodes on a boat.

There are articles out there where they talk about the Alaskan pipeline
having a large potential difference between locations due to the aurora. 
Current in this case is not a problem as the pipeline is above ground.

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
rad0
Sent: Friday, October 22, 2004 12:29 PM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC]: Sensing a current

I should have also said,
this is the type of gas line from a gas well to
where ever that goes.


I guess I'll just get the guy out here
and walk around with him as he does it.
And have him explain it.

----- Original Message ----- 
From: "Peter L. Peres" <plp@actcom.co.il>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Friday, October 22, 2004 11:58 AM
Subject: Re: [PIC]: Sensing a current


>
> On Fri, 22 Oct 2004, Olin Lathrop wrote:
>
>> rad0 wrote:
>>> I was chatting with a gas pipeline man,
>>> and he told me that gas lines have a
>>> slight current running through them,
>>> and they use this to locate the position
>>> and depth of the pipe.
>>
>> Hmm, that's interesting.  I wonder where the return current is flowing.
>>
>>> Can someone go over how you might
>>> build a sensor to do this?
>>
>> Assuming the return currents are no nearby, the current thru the pipe 
>> will
>> create a circular magnetic field around the pipe.  With the right
>> instruments, it should be possible to subtract off the earth's magnetic
>> field, leaving only the one caused by the pipe.  You are directly over 
>> the
>> pipe when the magnetic field lines are horizontal.  How much they tilt as
>> you move known distances sideways will tell you how deep the pipe is.
>
> It takes tens of amps to do that for any sort of pipe depth. I don't think

> that the gas company is that eager to get rid of its pipes. Also the aerth

> field is very irregular around buildings and man-made areas. You can't 
> 'cancel' it.
>
> Peter
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