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Ultrasonic Testing of Gold Bars


Application:

Nondestructive verification of the physical integrity of gold bars.

Background:

The rise in fraudulently adulterated gold bullion bars by inserting slugs of inexpensive base metal of similar density causes much concern in the gold markets. These insertions are difficult or impossible to detect through weighing or radiography technologies. X-ray fluorescence is a very reliable way to confirm the purity, but XRF is a surface technique. Some precious metals processors have even resorted to drilling or cutting bars to verify integrity. However, a simple ultrasonic test can quickly and reliably locate inserts with no need to drill, cut, or otherwise alter the bar. Combined with XRF surface testing, the processor can be confident of the purity and integrity of their gold.

Equipment:

Any Olympus flaw detector or phased array instrument can be used for this test. These include the EPOCH 600, EPOCH 650EPOCH 1000, OmniScan SX, and OmniScan MX2. The recommended transducer frequency will typically be 2.25 MHz. 

Procedure:

Adulteration of a gold bar by means of inserts will cause predictable changes in the way ultrasonic waves pass through the metal. Inserts of material other than gold in a bar will change the pattern of wave reflections, a response that can also be caused by internal voids. Large inserts that fill most of the volume of the bar can also be detected through changes in sound velocity.

1. Pulse/echo reflection method

Ultrasonic waves traveling through any medium will continue propagating in the same direction until they strike a boundary with a different material, which will cause them to reflect back to their source. Ultrasonic flaw detectors and phased array instruments generate pulses of high frequency sound waves with small, hand-held transducers. The sound energy is coupled into the test piece and the instrument monitors and displays the pattern of reflected echoes. Reflections coming from inside a gold bar, rather than the opposite surface, change the pattern and indicate either an insert of another metal or a internal void.

Gold Bar test with UT

This test is set up by coupling the transducer to a known good bar and identifying the echo from the bottom surface. A flaw gate may be used to monitor the interval ahead of this back wall. Any echoes appearing within the region marked by the gate indicate that the sound beam is reflecting off a discontinuity, and the bar should be further inspected by other means. Typical screen displays are seen below.

Conventional flaw detector images from solid metal (left) and metal with discontinuity (right).
Note: Echo appearing in region marked by red gate.

Conventional flaw detector image from solid metal
Conventional flaw detector image metal with discontinuity

Phased array images from solid metal (left) and metal with discontinuity (right). The discontinuity appears as an indication in what should be a white region of the image.

Phased Array Screen from solid metal
Phased Array Screen from metal with discontinuity

2. Velocity method

The sound velocity in pure gold is 3,240 m/S or .1275 in/uS. The harder gold alloys used in jewelry will typically be somewhat faster, but will similarly have a specific velocity associated with a given alloy. If the sound velocity in a bar deviates from the expected value, then the metal content has been altered.

This test is set up by coupling the transducer to a known good bar and identifying the backwall echo. That backwall echo may be marked by a flaw gate. If the position of that echo changes with no accompanying change in bar thickness, then the sound velocity in the metal has changed and the bar should be further inspected by other means. The most common adulterants increase sound velocity and thus move the echo to the left, as seen above.

Note: With small modifications these test can be applied to other precious metals such as silver and platinum as well. Contact Olympus for further information.

Olympus IMS

Products used for this application


Serie EPOCH 1000

Das EPOCH 1000 ist ein hochenwickeltes Prüfgerät für konventionellen Ultraschall, das in einer zugelassenen Olympus Kundendienststelle mit der Phased-Array-Darstellung aufgebessert werden kann. Seine wichtigsten Merkmale sind Konformität mit EN12668-1, 37 einstellbare Digitalfilter am Empfänger und Impulsfolgefrequenz von 6 kHz für schnelle Prüfung.

EPOCH 650

Das EPOCH 650 ist ein außerordentlich leistungsstarkes Ultraschallprüfgerät, das für zahlreiche Anwendungen eingesetzt werden kann. Dieses intuitive und robuste Prüfgerät mit zusätzlichen Funktionen ist eine Fortführung des bekannten EPOCH 600.

GoldXpert Mobile XRF AnalyzerNEW

Das GoldXpert ist ein Blickfang in jedem Verkaufsraum. Es ist kompakt, wiegt nur 10 kg und kann in fast allen Umgebungen zum Prüfen eingesetzt werden. Das GoldXpert ermittelt genau die chemische Zusammensetzung und bestimmt die Karatzahl bzw. den Feingehalt in Sekundenschnelle.

OmniScan MX2

Das OmniScan MX2 verfügt ab jetzt über das neue Phased-Array-Modul PA2 mit einem UT-Kanal und das UT2-Zweikanal-Modul für konventionellen Ultraschall, das auch für TOFD (Laufzeitbeugung) eingesetzt werden kann, sowie über neue Softwareprogramme, die die Leistungsfähigkeit der erfolgreichen OmniScan MX2 Plattform steigern.

OmniScan SX

Das OmniScan SX für Einzelgruppen ist leicht, besitzt einen hellen 8,4 Zoll (21,3 cm) großen Touchscreen und stellt eine kostengünstige Lösung dar. Zwei Modelle des OmniScan SX stehen zur Verfügung: das SX PA und das SX UT. Das SX PA (16:64PR) sowie das SX UT verfügen über einen Kanal für konventionellen Ultraschall für IE-, SE- und TOFD-Prüfungen.
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