Equipment for Electrical Conductivity Measurements

If you are trying to measure the resistance of pure metal, you are down in the micro-ohms range, and you need a micro-ohmmeter. Try this device:

http://www.keithley.com/products/currentvoltage/?mn=6514

Its lowest range is 20 micro-ohms, full scale. If you want to buy something less expensive, you can find the predecessor device, the Keithley 580, used. It's not quite as sensitive; the most sensitive scale is 200 milliohms full scale, with 10 micro-ohm sensitivity.

How sensitive a meter you need depends on what you are trying to do with it. Most electrical bond specs are in the milliohm range, and the 580 and a host of competing devices will do quite nicely. I myself use an obsolete HP 4328A; the modern version is the Agilent 4338A. Both of these are excellent devices measuring down to at or below 1 milliohm.

One thing you want to keep in mind when purchasing a milli- or micro-ohmmeter is what is the purpose of the measurement.

If you are doing an incoming inspection on sheet metal, any meter with the proper sensitivity will do.

If you are measuring bonds between different pieces of metal, then the purpose of the bond enters into the picture.

Aircraft have stringent bonding requirements so that end-to-end they are 2.5 milliohms dc resistance, or less. That protects against lightning damage. Since lightning is not only a high current but also a high voltage phenomenon, you want a meter that puts out some Volts when it makes the measurement. With lightning, you don't want a low potential meter because if there were a little grease or corrosion between the pieces being bonded, the low potential output of the meter wouldn't arc across it - it would see it as an open-circuit or large resistance, and you would fail the measurement, whereas lightning would burn right through it. So you want your meter to "burn right through it," also. Biddle and Shallcross bond meters are good for that.

But if the bond protects against radio frequency interference (rfi) effects, then you want a low output potential meter, because the rfi is at milli-or micro-volts and it can't punch through anything. That's where your Keithley and HP meters shine; their output potential at the most sensitive ohmmeter range is 20 mV.

Finally, when you are making low potential, low resistance measurements, it is quite easy for a thermal junction (inadvertent thermocouple) to bias the measurement by adding a potential to the low potential that is being measured by the meter. In order to avoid that error, the HP/Agilent meters use a 1 kHz ac signal and average the readings they get for each direction the current flows. The Keithley does something similar. Both meters average out the thermocouple potential to zero.

If you're like me and try not to pay top dollar, you can usually find a wide variety of these meters on ebay. But you have to know what you want, and now you should have a better feel for it.