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Everything Eddy Current


SigmaCheck 2 Eddy Current Conductivity Meter & the Gold Bullion Test!

Is gold a good investment? Of course, generally the answer is “Yes”… but be cautious, otherwise you may need a SigmaCheck 2!

Alongside the attractive feature of Gold being a reliable investment as part of a diversified portfolio, runs the endemic problem of Gold (Au) bar and ingot forgery. The production of fake gold bars has become more common-place as criminals use this precious metal’s popularity to their advantage. A common forgery method used is to plate Tungsten (W) with a thin layer of gold (about 0.1mm). Tungsten is used as it has a very similar density to gold but worth 1% of the value. With this approach the result is that bars can be created weighing the same as a genuine gold bar. A standard gold bar held in reserves by central banks and traded among bullion dealers is 400-troy-ounce , equivalent to 12.4kg or 438.9 ounces. NDT methods are often used to test whether a Gold bar is legitimate or not, but NDT inspectors have discovered that these methods of forgery defeat the current favoured NDT inspection method of X-Ray Fluorescence (XRF), as this only tests the material surface. Drilling a hole to take a sample for an assay analysis is not permitted as this would remove valuable material. These challenges have led to the enquiry and exploration to find another NDT method which could be used, with reliable results, to test for this condition.

This is where ETher NDE come in, as we were approached to examine whether Eddy Current inspection could be used, as an alternative to the X-Ray methods, to test Gold (Au) bars, reliably establishing authenticity.

SigmaCheck 2 success…

In response John Hansen, ETher NDE’s General Manager, took up this challenge and established that the Gold bars could indeed be tested effectively using Eddy Current. John states

"I established in theory this was possible at 60kHz using our SigmaCheck 2 conductivity meter, as the conductivity of Gold is 45 Mega Siemens per m (MSm-1) (or if you prefer 77.6% IACS or resistivity 2.2 whereas Tungsten is 18 Mega Siemens per m (MSm-1) (or if you prefer 31.3% IACS or resistivity 5.5”

The Skin Effect

John continued…

“However there is of course the skin effect to overcome (this is a phenomena that limits the eddy current concentration to the material surface). The good news here is that at 60kHz the traditional frequency for conductivity measurement the skin depth (average depth of penetration) in gold is 0.3mm and the gold plate is no more than 0.2mm, so this works very well."

Following this cycle of in-house research and test, our enquirer came back to us reporting that they had conducted further tests tests on forged and real gold bars and that using the Eddy Current NDT with the SigmaCheck conductivity meter performed very well.

The SigmaCheck 2 now carries this capability as standard, with enhanced software enabling the penetration of thicker plating.

As well as forged or materials plating detection the SigmaCheck 2 Meters are also ideal for the following test scenarios:

  • Material Verification and Sorting
  • Heat Treatment Verification
  • Heat or Fire damage investigation
  • Lightning damage inspection
  • Material Purity/composition
  • Material ageing assessment
  • Measurement of phosphor content of CU Alloys
  • Condition assessment of highly stressed parts monitoring of deposition process for non ferrous materials on non-ferrous substrates