DIY Resistivity Tester

[TomsFocus]

I know we have a few electronics members on here and this is beyond my level!

I'm trying to work out if there's a cheap way to create a resistivity tester using a Raspberry Pi or similar.  Or at the very least, does anyone know what the main components would be?

Basically, I'm after something to verify genuine coins from their metal composition.  The current standard tester for this is linked below and costs around £1000, which I can't justify when the coins are mostly worth sub £50 each!  Needs to be able to test coins by just laying them on the surface, not connecting clips which could cause damage.

Original | Sigma Metalytics

I've been waiting for someone to make a cheap copy lol...  But seriously, I can't see why this tester should cost so much, and why no-one seems to be making a similar one in the UK.  Any knowledge would be appreciated.

[unofix]

To measure the resistance of something like a coin you need a 4 wire micro ohm (μΩ) meter. To build an accurate usable unit would probably cost more than a commercially produced meter.

Good quality professional meters under the £350 only go down to 100μΩ which would still not be good enough. What you are actually trying to measure is the resistance of an almost perfect short-circuit.

Many years ago I used to repair micro ohm meters for the National Grid. The calibration of the repaired units was carried out by the UK's National Physics Laboratory and if memory is correct was £400 ten years ago, just for the certificate.

Aliexpress do a 4 wire milli-ohm meter for £59.80 but I would doubt its accuracy and of course it's still not a micro-ohm meter.

[tazzman600]

As the saying goes. You get what you pay for.

Measuring resistance has always done my head in. 

[TomsFocus]

Fair point.  It seems there's no cheap way to do it so I have given up on the idea.

Thing is, even the proper one can be fooled quite easily by fairly common coins, despite it's 4 figure price.  I'm hoping they'll become more accurate over time, as well as cheaper, but obviously that doesn't help me right now.

Still not quite sure what makes it so expensive.  Can't work out if there's a rare element needed to measure such small amounts or whether it's just the very tight tolerance tooling needed that makes it so expensive.

[mjt]

I don't know if this description of a Kelvin bridge is of any interest. At least it does explain the principle behind 4-terminal measurements.