Electrical continuity of steel structure

[DrWhoF]

continuity or steel

How to measure electrical continuity of steel structure submerged in sea water???

 

[JoeB01]

You might try building a very low voltage high current continuity tester. 100 millivolts at 10 amps might work well. Use large clips (similar to battery jumper clips) to connect to the parts of the structure to be measured and use a 4 wire Kelvin type measurement (separate sense wires) to measure the induced voltage on the structure. If the structure is very large you may need a lower voltage higher current tester. The sea water will be a shunt resistance so it is important to be able to estimate the effect since a totally open structure will still conduct through the water and you need to allow for this.

 

[fala]

If you mean electrical current through sea water (which is an electrolyte) then you need to setup an electrochemical. while it is too complicated to be answered in a post. In simplest form you need an anode and a cathode connected to a voltage source through an galvanometer. sediments and other effects can block cover the surface of your structure and change the electrical properties. All of these can be studied by such an experiment.

 

[DrWhoF]

Thank you both for your valuable contribution.
I don’t want to send electrical current through sea water, all what I need to measure is the quality of connection between elementsts of a steel structure.
Do you think I will be able to conduct 10A through steel at 100mV in see water?
D you have any idea what may be the ratio between currents flowing through this structure and surrounding water?
And how to build an instrument which can generate currents of 10A at 100mV?

Thanks
DrWho

 

[fala]

I think it all depends on scale of the structure. If you want to test DC resistance according to JoeB01, then all you need is a low voltage/High Current supply , power NPN transistor with heatsink and a galvanometer. Connect them in series. control the emitter voltage of NPN by applying appropriate voltage to base.

 

[DrWhoF]

Do you think ohms or mili-ohms wil be appropriate units to describe connection of metal parts in sea water?

 

[fala]

Again I really have no idea what structure you have in mind and you want to measure electrical continuity to determine what, All I can say is depending on what you want to determine the technique to measure it may vary widely. But resistance of a metal structure under water per se seems meaningless at least to me.

 

[johnyaya]

I don't think that the issue is measuring the resistance of the structure, that part is fairly straight forward. The harder part is trying to figure out what effect the salt water is going to have.
For example, if you had a copper (Cu) rod laminated with tin (Sn) and you measure the resistance of the rod, then what is the resistance of the copper part of the rod? The resistivity of copper is 1.68 ohm.m (10e-8 ohm / cubic meter @ 20C), and the resistivity of tin is 10.9 ohm.m.
Resistance = resistivity × length / area
So you can see that the cross sectional area of each metal in this example contributes to the over-all resistance.
Now, back to your steel structure in sea water. What is the resistivity of the sea water? The sea water's cross-section is huge, so how much effect will it have on the resistance measurement?

 

[DrWhoF]

Can you suggest any other method that may be independent of sea water?

 

[fala]

As I said it all depends on scale and kind of your structure and kind of information you need, Do you want to check if they are electrically connected or you want to make sure they are physically connected? do you want to check if they are connected at all or you want to check connections are firm enough or not? can it brought out of water or not? It is very hard to imagine your problem with such little information. , acoustic, X-Ray, impedance,....

 

[kender]

...acoustic...

Sound propagation and reflection through the struciture can give you some information. Of course, it depends in the shape of the structure, the mechanical nature ofconenctions etc.

- Nick

P.S. I'd like to invite your question to a yahoo group dedicated to sensors: **broken link removed** . I know there are a few marine sensor experts in this group.

 

[DrWhoF]

Thanks.
I don want to go into X-rays, soud propagation and so on.
All I need to check is how good some brackets are attached to the man body of a structure as far as conducting current through this stracture is concerned.

 

[fala]

Well, while I think current conducting method is not a very good option to check how good some brackets are attached but if you insist I think at least you can begin with some experiments.
I think if you want to measure resistance between top and bottom of your structure, it will not give you any meaningful result but you may take one bracket that has been very well connected to the steel frame as standard and compare it with some less than perfect attachments. If you could detect a meaningful increase in resistance because of less connection area. then you may check each bracket one by one to make sure they are right.
Some notes,
1-points that you select for measuring resistance should be exactly the same for all experiments.
2-you absolutely need 4-wire kelvin type measurement.
3-measurement points should be absolutely clean, free from any grease, chemicals, rust,...
you may even need to clean them with silicon bead papers.

 

[johnyaya]

If you are only going to measure resistance:
To differentiate between the conduction of the sea water and the metal structure I would measure both the sea water resistance alone and the structure (which includes the contribution of the sea water). Hopefully, the resistance of the steel structure (with sea water) will be lower than the sea water alone. You may be limited in the length of the structure you are measuring and may need to measure each joint of the structure.

I would consider time domain reflectometry (https://en.wikipedia.org/wiki/Time-domain_reflectometry), although I'm not certain how this would work in sea water.

 

[DrWhoF]

I have tried to measure very low resistance between two steel parts in sea water and the problem is that the reading is not repeatable. I am not sure but it looks like results depend on the temperature of sea water.
Is it possible?

 

[kender]

I have tried to measure very low resistance between two steel parts in sea water and the problem is that the reading is not repeatable. I am not sure but it looks like results depend on the temperature of sea water. Is it possible?

That's not impossible. The resistance of the water is dependent on the salinity, and the salinity of the seawater at your site could have some correlation with temperature.

 

[shaun_c_m]

ok, im just going to throw this idea out into the open. i dont know if it is possible though...

pass an ac current through the structure (200 Hz or something). the current needs to be large ie several amps.

current in a conductor generates a magnetic field which can be measured with sensor such as a (hall effect)? for example.

you can look for a 200 hz trace from the hall effect thus eliminating other stray magnetism. (you might need to faraday cage the area under test)

amplitude of signal from sensor is proportional to current through conductor at that point. if you know current through the conductor, and the continuity of seawater you should be able to map the current flow.

if you know what current is flowing where you know the relative resistances in proportion to that of the seawater...



alternatively throw a few kilo amps through the area you are testing and see what gets hot with infra red....there is your resistance

 

[fala]

DrWhoF, can you post some of your data so we can see if anything can be done. As I said in my earlier post the points of measurement should be absolutely the same and contact points should be absolutely clean. you are trying to measure milliohm or sub milliohm(if you had posted some data I had better idea) resistance very thin oxide layer on surface of your probe(what is your probe material it should have been gold plated) can give you hundreds of kilo ohm resistance. As I said you have to clean points of measurement with alumina paper to remove any rust or invisible oxide layer on your steel structure.