2 Wire Ohms

[CalLabSolutions]

Just taking a WAG to answer the original question, I'd say the reason that 2-wire tests are still valid is that more than half the users have their meters configured and use them that way.

I agree we need to calibrate 2-wire ohms, just not 2-wire ohms 0 Ohms when the specifications state you have to zero out the lead resistance.

100 Ohms test where you short the leads press null. Then apply 100 Ohms... That makes sense.  But doing it with a 4-Wire short does not make any sense unless meter has a separate 2-wire zero ohms spec that does not require a lead zero requirement in the spec.

Mike

[measure]

34410-90010, 34410/11 service manual jan 2006 2d edition
34410-90010, 34410/11 service manual apr 2006 3d edition
34410-90010, 34410/11 service manual feb 2007 4th edition (this is the service manual on line at present) call for 5720A.
Please link the manual you are referring to.

Thanks Bryan. The current manual does, in fact, reflect the use of the 5720A.

As to the original question, I believe the intent of a 2 Wire Ohms test is to verify the integrity (e.g., thermal emf and contact resistance) of the input path. When you install a short and subsequently set an offset (null) on the meter (typically on the lowest range), you are 'mathematically' removing the effects of lead and contact resistance, with respect to the installed short, at a specific point in time. If, however, the input path has excessive thermal emfs or contact resistance (physical effects), for example, the display, though initially nulled out, will continue to drift and/or show excessive, out of tolerance readings. If out of tolerance conditions are detected when subsequently measuring 2 Wire resistance on higher ranges, it would also expose the possibility of range relay problems existing, perhaps where a 4 Wire Ohms test might not.

[Hawaii596]

Not familiar with it (or what for).  I hope you & yours are safe from the fires.   I don't think that I've ever seen it this bad in my time in TX (since 85).

BRYAN - Thanks for the thoughts.  It is really bad.  I live NW of Austin, and have had two wildfires very close to where we live.  One a few weeks ago destroyed 15 homes, and as we drove home, various streets were closed off, all along our street, everyone was using their lawn sprinklers as a precaution, we saw ashes floating down as we drove home, and there were forest fire planes and helicopters dumping retardant and water within a half mile of us.  Then in the most recent ones, there was another fire maybe a quarter mile further away.  But no one has had it as bad as the folks in Bastrop.  I heard that over 800 homes are destroyed there, and over 1000 homes destroyed in total (all the fires).  They have it pretty much controlled now, but it is not done.