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Old 09-01-2009, 10:38   #1
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Is there a market for untrasound metal thickness testing?

I live on a steel yacht and found a nasty little bit of rust hidden away yesterday so I've bitten the bullet and ordered an ultrasonic metal thickness tester. Not cheap but I really want to know exactly what's what with my hull. Now to cheer myself up a bit it would be nice to think that it would be able to recoup the cost after a number of years. Assuming it does what it says in a few days I should be able to measure thickness of steel or aluminium to +-0.05mm through up to 20mm of coatings without scratching the hull and produce a cad drawing with the figures. Do you think there is a market there for this type of service? Not a full survey and no guarentees could be provided but I imagine I'll be picking up rusty pieces of metal for months now and chopping them up after measuring to check the accuracy. Currently in Canaries but should be Caribbean before long. So if you wanted new davits but didn't know the deck thickness or were worried about that bit of rust you just found would you give me 50 bucks to know for sure? This isn't an advert, just curious as to the interest there might be out there.
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Old 09-01-2009, 13:02   #2
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Sounds good to me.Especially if,over time,you became skilled at finding problem areas for owners.
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Old 09-01-2009, 14:50   #3
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It must be a very expensive unit if it isn't influenced by the thickness of the paint.

All thickness gauges are "time of flight" meaning that they measure the time it takes from sending a sound pulse to recieving it again as an echo. If you know the material through which the sound propagates, and the temperature, the unit can calculate the actual sonic velocity, and thereby the thickness.

So if you have different types of paint/coatings with an undefined surface roughness, and undefined sonic velocity in the paint, how does it calculate the correct metal thickness?

Unless you can calibrate the unit for each specific measurement with paint, I would be very wary of promising the kind of accuracy you are hoping for.

Hope this clears things up a bit..

Alan
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Old 09-01-2009, 16:32   #4
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A thickness test does not help to find pits.

Among other pass times (sailing is better), I am and API licensed petroleum tank inspector - I test tanks from 2,000 to 12,000,000 gallons.

Although thickness testers are very good for testing the general thickness of metal for structural evaluations, they will seldom find small pits - the kind that cause small leaks. One reason is that the probe must pass right over the defect - tedious. The other is that the transducer is typically bigger than the pit and will often span the hole.

You should certainly have a calibration sample - a piece of steel of similar known thickness with similar paint. That will give you the accuracy... except for one thing....

The unit must pick the "best" echo and give a thickness. If the surface is rough it gets many to choose from. If you are having trouble getting a "lock" in a given area, that can be a sign of corrosion.

You should be able to find lots of information on-line. Good luck. They are great tools, once you understand what they are.
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Old 09-01-2009, 16:46   #5
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it take an awfull long time to do the hull on a steel yacht.The probes are only about 3/8 diameter ,unless you are prepaired to do an Ultrasonics course you would have the rely on the basic equipment.

Ultrasonics are an art form that takes a long time to become profficient at.
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Old 09-01-2009, 17:12   #6
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We use one in my shop to measure the thickness of the chrome on the ball for large ball valves and for the thickness of castings. But like thinwater states, it will not pick up anything smaller then the transducer.

On a hull it might be good after a sandblasting in a bad area to check the average thickness before a probable repair.
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Old 09-01-2009, 18:23   #7
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I used to have to have the hull sonoguaged once every two years for the Coast Guard and for the insurance company. The work had to be done by a certified surveyor though. What I am getting at is there is demand for this type of work for the above scenarios but it must be done by the right person.
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Old 09-01-2009, 20:02   #8
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Quote:
Is there a market for untrasound metal thickness testing?
Fr commercial boats it's required in the US. One of the great cast iron hulls on the Chesapeake failed a test and they spent nearly 1 million dollars replating the hull. The "Miss Ann" formerly of Tides Inn fame (Irvington, VA) was replated based on a required USCG inspection because it was a commerical boat with an iron hull. So for commerical boats it is a requirement. For recreational boats it may be a good idea but it's not required.
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Old 09-01-2009, 20:10   #9
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Sorry, I should have said it was for a Coast Guard inspected commercial passenger vessel.
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Old 10-01-2009, 01:30   #10
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Quote:
Originally Posted by Nordic cat View Post
It must be a very expensive unit if it isn't influenced by the thickness of the paint.

All thickness gauges are "time of flight" meaning that they measure the time it takes from sending a sound pulse to recieving it again as an echo. If you know the material through which the sound propagates, and the temperature, the unit can calculate the actual sonic velocity, and thereby the thickness.

So if you have different types of paint/coatings with an undefined surface roughness, and undefined sonic velocity in the paint, how does it calculate the correct metal thickness?

Unless you can calibrate the unit for each specific measurement with paint, I would be very wary of promising the kind of accuracy you are hoping for.

Hope this clears things up a bit..

Alan
See here for how the latest technologies work.

Tritex NDT - The technology behind multiple echo ultrasonic metal thickness gauges
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Old 10-01-2009, 09:10   #11
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Certification is generally required for comercial work.

For tanks testing the technician must be ASNT Level II (Certification at the American Society for Nondestructive Testing), which requires classroom time and a test. I do not know wha the marine market is like.

In order to make sense of the results in a structural sense is surely rather involved - I only know petroleum tanks and pressure vessels.
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