ohm meter to test for real from fake siler

This thread brings back memories of setting in a circuits lab for 8 hours a week and getting one hour credit for it.

 

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For me, it brings back memories of dropping a jumper cable and nearly spot-welding it to a metal bumper.

 

I see my first answer wasn't good enough. For me it brings back memories of the past 8 hours. Work.

Wanna have fun with a multimeter? Go around your house and check all your grounds. That's fun.

 

Another way to test is to ask at the jewelry store.

Very helpful. Thank you. Just investing in silver. I see some good deals on ebay but am reluctant to buy afraid these might be fakes.

I have heard that fake bars could contain another bar inside made out of similar weight metal. How do you test for that without cutting or using x-ray?

So I rather pay little more and buy from reliable source.

 

Another way to test is to ask at the jewelry store.

Very helpful. Thank you. Just investing in silver. I see some good deals on ebay but am reluctant to buy afraid these might be fakes.

I have heard that fake bars could contain another bar inside made out of similar weight metal. How do you test for that without cutting or using x-ray?

So I rather pay little more and buy from reliable source.

 

Another way to test is to ask at the jewelry store.

Very helpful. Thank you. Just investing in silver. I see some good deals on ebay but am reluctant to buy afraid these might be fakes.

I have heard that fake bars could contain another bar inside made out of similar weight metal. How do you test for that without cutting or using x-ray?

So I rather pay little more and buy from reliable source.

 

So...welcome, welcome, welcome to CT...you are responding to a post that was made in 2017


Yup, bars of tungsten carbide plated with gold will have the same weight as an identical 100% gold bar.

 

Tungsten metal, right? Tungsten carbide's density is about 25% lower.

 

OK, I went back and reread the thread from the beginning. Good times.

When I was talking about resistance-measuring techniques, I completely overlooked the thermoelectric effect (voltage generated across a junction of dissimilar metals as a result of a temperature difference). I'm guessing that would swamp the tiny voltages we'd be trying to measure. That makes resistance measurement even more of a non-starter.

 

Before the epidemic hit, I was getting good results using an ultrasound velocity meter after a month of playing with it. The US stored Gold bars are said to use this type of checking for purity. If you enter Amazon and search Ultrasonic Thickness Measuring Instruments you will see a range of instruments, including one used in this video. Jim

 

I agree that using a DMM’s resistance function will be futile unless the meter has 1 micro ohm resolution.

If I were to try this I would go with a four wire test. Get a current source capable of maybe 1 amp and connect it to the coin, with a current meter inline to measure the actual current. Then get a second meter to measure voltage at the current source contacts spots, using a separate pair of wires. 1 amp through 1 micro ohm would be 1 microvolt. Divide the measured voltage by the measured current to get the resistance.

The advantage of this technique is that I can measure much smaller current and voltage with a meter than I can resistance, and the voltage drop on the current source leads would not affect the voltage measurement.

Still might not work for reasons already stated (dirt on coin, etc.) but I think it would have a better chance than trying to use the resistance function on a "standard" DMM.

 

Four-wire measurement would be better, of course, but I'm pretty sure it still wouldn't be good enough. I know I don't have the calculus chops to figure out what voltage drop I'd expect across a chord of a solid disc with irregular thickness (the features on each face of the coin). And you'd still need to stick the current-source contacts onto the coin hard enough to leave permanent marks. And deal with thermoelectric effects, and, and, and.

Better to induce a current and measure its behavior, the way Sigma testers do. I'm still not convinced I totally trust them, but I don't think any contact-based resistance tester can do better.

 

I was just going to solder the contacts on!

 

I bought a Sigma PMV (original, not the Pro) and measured some of my 1 ozt bars and a few 1/4 ozt AGEs. The former measured about 2.2 microohms and the latter about 8.0 microohms. This is with one of the newer original PMVs that have a 'measurement' mode, not just the pass/fail mode.

The bars were still in assay cases and the AGEs were in air-tites.

Anybody else with resistance measurements to report?

 

I have more to report. I measured a 10 ozt silver bar and an American Silver eagle, both bare metal, no cases. The former measured 1.60 microohms and the latter measured 1.66 microohms.

I expected the silver to have less resistance than the gold, but I thought there would be a bigger difference between the bar and the coin. Maybe I approached the useful lower limit of the PMV, or maybe it's because the ASE is a big coin with plenty of space for electron flow. I measured both of these items on the "main" sensor, not using a wand.

 

Do the instructions/manuals say what "microohms" really means here? Is it microohm-meters?

I mean, if you have a bar that's twice as long as it is wide, and you measure its resistance point-to-point from one side to the other, you'll get fewer microohms than if you measure from one end to the other. Resistance through a material is directly proportional to distance, and inversely proportional to cross-section. That's why the unit of resistivity is ohm-meters -- it's actually ohms, times area (square meters), divided by distance (meters).

 

Measure two different lengths of the same gauge copper wire.
That will give you a different reading.
Ohm meter isn't going to tell you much other than it's conductive.

 

I'm pretty sure the Sigma would be reporting resistivity rather than simple resistance. Or at least pretending to.

 

You are correct, the PMV measures resistivity and not resistance. The original discussion in this thread was the application of a DMM to measure resistance, and resistance is measured in ohms.

Then the discussion veered off into using devices such as the Sigma Precious Metal Verifier (PMV), and that device measures resistivity in ohm-meters (Ω-m). My PMV instructions don’t explain much about the science behind the measurements, it doesn’t seem to even use the word “resistivity” much less μΩ-m, but I found Sigma references to resistivity in some of their other online documentation:

https://cdn.commercev3.net/cdn.bgasc.com/downloads/Sigma/FAQ-Sigma-PMV-Original.pdf .

Weeks ago I found a reference online (I can’t find the source now) that stated the PMV measurements were in microohms, and I didn’t think about it hard enough to realize that it should have stated microohm-meters. So all of my previous measurements should have been in μΩ-m.

But regardless of the actual units, the PMV provides a number in its measurement mode, and my objective here was to provide my results of that number for the benefit of others also using a PMV in that mode. Others might find it beneficial to compare results. I would find somebody else’s results useful just as a sanity check that I was doing it right.

I’m assuming a consistency of results so that a comparison of coin or bar measurements gives consistent results across different samples of the same bar/coin and across different PMV sets. I don’t have experience with a different PMV, but I can say that my gold bar measurements had the following characteristics:

Sample size: 4
Sample average: 2.1875
Minimum value: 2.17
Maximum value: 2.21
Standard deviation: 0.01708

My AGE measurements had the following characteristics:

Sample size: 16
Sample average: 7.99375
Minimum value: 7.72
Maximum value: 8.48
Standard deviation: 0.24268

So that provides some level of confidence in the consistency across different samples of the same type, although not across different PMV units.

Tough crowd!

 

I suppose I could also make a dozen measurements of the same bar or coin and check for consistency. On the other hand, "don't ask the question if you can't stand the answer." I might prefer my existing state of bliss.