eddy current testing

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raghead

Member
Joined
Sep 25, 2014
Messages
10
" Therefore using John’s theory, ETher NDE has added the capability to their EC conductivity meter, the SigmaCheck software (should it be necessary in the future) to penetrate thicker plating."
This is the catch to using this equipment. I think that ultrasonic testing, which uses the difference in velocity of a signal through metals and alloys, would be more effective, since it penetrates through the item in question, not just the 'skin'. It's a tested and proven method.

You can find videos on the subject on Youtube.
 
People from all over the world are on this site, no need to offend someone with a poorly chosen board name.
 
raghead,

> "i wanted to ask you if some one used or uses the eddy current method ?"
Some years ago I was also puzzled by this method. After talking with several specialists, the conclusion was the following:

Eddy current testing can be used to detect whether the content of a surface metal is mainly the element gold. With standard devices (<US$500) it is possible to achieve high accuracy up to 1/2" thickness (don't remember the exact number, only that all the interior of coins could be tested while bars don't). Almost all kind of discontinuities can be detected: cracks, voids, bubbles and insertions. The test is completely non destructive, very fast, accurate and don't require any kind of gel or consumable (as with ultrasonics and other electronic testers).
The only 'inconvenience' is that the reading panel is not very friendly for newbie users. The user must have some knowledge of circuit impedance, vector diagrams and phase angles in order to evaluate whether the sample is fake or not. Nothing extraordinary difficult, but requires some theoretical background.

> "[...] we all know what theory most times a little bit off reality"
This is an important issue.
If eddy current test is so good, why it is not massively used by goldsmiths?

> "if i find reference material for calibration it should give theoratically good results ."
Let us know if you have done such tests.

Regards
 
Just a small correction in the last post: "it is possible to achieve high accuracy up to 1/2 mm thickness"
 
Irons,

> "[ultrasonic testing] would be more effective, since it penetrates through the item in question [...]"
If you have a gold bar of >1/2", ultrasonic is definitely the right choice (even central bankers use it).
The only cumbersome is applying the gel before taking the measurements.

But if you have a gold coin of ≈1mm with several irregularities over the surface, there is no was to use ultrasonic. There's not enough precision bellow a certain thickness (roughly <5mm).

> "It's a tested and proven method."
This is a good point. Ultrasonic has a proven record, while eddy current don't.
If eddy current test works, why it is not being massively used by goldsmiths?

My guess is the following: In order to do the eddy current test, the surface of the sample must be reasonably flat. So, several jewelries (ex: necklaces, pendant, bracelet, ...) cannot be tested via eddy current.
Besides, there are several other tests which are less expensive that can do it. For example: acid test (although 'semi' destructive) and electronic testers (based on resistance). And, paying a little bit more than an eddy current test, you can do a XRF test which is more complete (provides the composition for any kind of surface type, although not as deep).

Regards
 
Mark Messa said:
Irons,

> "[ultrasonic testing] would be more effective, since it penetrates through the item in question [...]"
If you have a gold bar of >1/2", ultrasonic is definitely the right choice (even central bankers use it).
The only cumbersome is applying the gel before taking the measurements.

But if you have a gold coin of ≈1mm with several irregularities over the surface, there is no was to use ultrasonic. There's not enough precision bellow a certain thickness (roughly <5mm).

> "It's a tested and proven method."
This is a good point. Ultrasonic has a proven record, while eddy current don't.
If eddy current test works, why it is not being massively used by goldsmiths?

My guess is the following: In order to do the eddy current test, the surface of the sample must be reasonably flat. So, several jewelries (ex: necklaces, pendant, bracelet, ...) cannot be tested via eddy current.
Besides, there are several other tests which are less expensive that can do it. For example: acid test (although 'semi' destructive) and electronic testers (based on resistance). And, paying a little bit more than an eddy current test, you can do a XRF test which is more complete (provides the composition for any kind of surface type, although not as deep).

Regards

Please learn how to quote correctly in your posts they are hard to read.
 
Mark Messa said:
Irons,

> "[ultrasonic testing] would be more effective, since it penetrates through the item in question [...]"
If you have a gold bar of >1/2", ultrasonic is definitely the right choice (even central bankers use it).
The only cumbersome is applying the gel before taking the measurements.

But if you have a gold coin of ≈1mm with several irregularities over the surface, there is no was to use ultrasonic. There's not enough precision bellow a certain thickness (roughly <5mm).

Hi Mark,

I have done a lot of UT in the past and have developed some systems as well for this. You are completely correct when you say that surface irregularities can be challenging, but it is not impossible. Most systems will analyse the signal for you, but what you really want to do in this case is have a direct look at the signal. This is often referred to as the A-Scan. So you would spend a bit more time and need to know what you are doing, but it can be done. As far as small dimensions go, you would simply need to use a higher frequency. I just did a quick check since it's been a few years for me as well, but I saw that Olympus is selling 25MHz immersion transducers that would well do the job. At these frequencies you could go down to 1mm fairly easily and maybe 0.5mm if you have a good sample, experience etc...

As far as Olympus goes, if I remember correctly they bought a US manufacturer for transducers some 5-10 years ago and rebranded it. Assuming they use the same facilities I would say they are fairly good transducers. But there is several manufacturers that make custom transducers of extremely high quality. Comes at a price though. I remember having paid 5.000EUR per transducer for a special from France.

Bottom line is; it can be done, but it is expensive and you need a bit more background than you would need to operate a normal thickness tester of a few 100 bucks.

Joey
 
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