How to measure tens of micro-ohm resistance?

The usual way is a 4 terminal measurement. You force a current through the busbar measure the voltage drop. Use two connections for the current and two more connections to measure the voltage drop. This is so that the resitance of the connections does not spoil your reading.

It's common to use low frequency AC for this kind of measurement to avoid thermo electric emfs.

If you use a test current of 1A then 20uR will result in 20uV which is easy enought to measure.

There were some home brew designs blogged about on E14 a while ago but I can't remember who did them.

You can buy a ready made instrument to do this (new or Ebay ).

Or you can bodge it together.  ideally you don't use 50Hz if you uses AC because mains interference can cause problems. 30Hz AC is OK and it's not that hard to make a narrow band pass filter to separate out the wanted signal and an audio amplifier to make the excitation current.

This will do it:

https://www.seaward.com/gb/products/resistance/ohmmeters/930139-cropico-do4002/

but I couldn't find anyone selling it.

If you want to lash up something simple then let me know what gear you have and I'll see what I can suggest.

MK

The usual way is a 4 terminal measurement. You force a current through the busbar measure the voltage drop. Use two connections for the current and two more connections to measure the voltage drop. This is so that the resitance of the connections does not spoil your reading.

It's common to use low frequency AC for this kind of measurement to avoid thermo electric emfs.

If you use a test current of 1A then 20uR will result in 20uV which is easy enought to measure.

There were some home brew designs blogged about on E14 a while ago but I can't remember who did them.

You can buy a ready made instrument to do this (new or Ebay ).

Or you can bodge it together.  ideally you don't use 50Hz if you uses AC because mains interference can cause problems. 30Hz AC is OK and it's not that hard to make a narrow band pass filter to separate out the wanted signal and an audio amplifier to make the excitation current.

This will do it:

https://www.seaward.com/gb/products/resistance/ohmmeters/930139-cropico-do4002/

but I couldn't find anyone selling it.

If you want to lash up something simple then let me know what gear you have and I'll see what I can suggest.

MK

Thanks! I don't know why I did forget about this technique.

I'm using a power supply that can deliver 50A, the DMM says 0.000774V so it's 15.468µΩ, that is close to my calculation (21.4µΩ).

I'm going to ask my manager to buy one of those micro-ohmeters to have a better measurement, but I doubt it, it is an expensive equipment.