What would be the best PM tester for a collector in 2024

I think of the Sigmas as electromagnetic testers. (Really, all of the things you listed are precious metal verifiers, aren't they? )

Moderately thick gold plating over anything will fool an XRF. Gold plating over tungsten will fool specific gravity, weight, and magnet. But I don't know of any approach that will fool an electromagnetic test and one of the others simultaneously -- gold and tungsten have different conductivity and magnetic susceptibility, and the frequencies used in Sigma-type testers will reach more than deep enough to scan an entire coin or bar.

I suspect an acoustic test would be difficult to fake, too, but I don't like people thwacking my coins, and I wouldn't trust their ears anyhow, and I don't know of any commercial testers that use acoustics.

 

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In that case I'm set. I thought I had read that some of the Sigma testers were better than others (i.e., the newer more expensive models) at depth.

I agree with categorizing the Sigma as an electromagnetic tester. I latched onto their "PMV" product nomenclature and didn't give it due consideration.

If an XRF can be fooled by plating (and for some reason I had thought it was the ultimate gold tester ... maybe I was misled by its ultimate price) then what are its advantages over a Sigma?

 

XRF machines work by sending high-energy (short-wavelength) X-rays into a sample, then measuring lower-energy (longer-wavelength) X-rays that are re-emitted from the various kinds of atoms in the sample.

X-rays can't penetrate dense metals very well. So the high-energy X-rays get absorbed before they can penetrate very deeply, and the lower-energy fluorescence X-rays get absorbed even more easily on their way back out of the sample.

I found one discussion that says 50keV x-rays (the kind that might be generated by an XRF gun) can penetrate gold to a characteristic depth of about 70 microns, but the generated X-rays (that the gun detects to do its analysis) can only penetrate 4-6 microns.

So if you've got a gold-plated object, where the gold is typically a few microns or less thick, the XRF analyzer will detect a signal from the underlying base metal. But if you've got a fake bar that's a millimeter or two of gold wrapped around a tungsten core, XRF can't see through the outer layer. An EM checker might be able to.

(When I posted earlier, I was confident that an EM checker would be able to see through a thick layer of gold. But I've done some fact-checking on myself, and it depends on the specific frequencies the tester uses. Sigma claims 1.5mm, which would be deep enough to detect tungsten-cored coins, but possibly not 10-oz or larger bars.)

 

Thanks for sharing that info.

No 10-oz gold bars in my immediate future.

 

Yeah, I'm still taking other paths to preserve wealth chunks that size. And if I wanted to spend a chunk that size, I'd be tempted go for the XRF instead!

 

Several years ago there was an article about the U.S. Mint using an ultrasound speed measuring device that would use a velocity about 3240m/s * rate through pure gold* as any other metal or lack of it in the object would change the velocity. They are used in several situations. Jim

Note: They are available rather cheaply in Amazon catalog,,,guess I will have to get one now

 

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