Hello Bill M, this topic can be diced up in a number of different ways. I went through an experience a few years back when the facility that I teach and test at was required to go through an audit for testing certification. In a nutshell, this is what I came away with when all the dust had cleared. A load-bank test will give you the most accurate idea of machine calibration as the load will be reasonably constant while the test is being conducted. Meter calibrations will likely be a bit easier and more accurate by using a load-bank.
     Using a clamp-on meter(even if it is calibrated) while having a welder run a weld with the machine will exhibit fluctuations due to arc length, cable size and length, and other human and machine induced elements to the point where it will be very difficult to make an accurate comparison to the meters on the machine and those of the test meter.
     Digital meters, in most cases, will be very accurate on MOST manufacturer's machines and not require calibration, they may not even have a provision for adjustment. Analog meters on the other hand are fairly easy to change and adjust to match calibrated test equipment.
     In my case load-bank tests and calibrations were prohibitive and cost-ineffective so we went with the calibrated clamp meter instead, even though this was not the most accurate method.
     Miller has some very good information available on their website concerning calibration, I can't remember the specifics, but I was able to contact a couple of different individuals at Miller who provided me with a substantial amount of information on the subject. Good luck on your quest, regards, aevald

Hello again Bill M, I'm going to try to include some information that I received from Dan Woods of Miller. Hopefully I can get it to attach to this post. For others who have an interest in this calibration thread, I did get a file to attach, but it doesn't show up on new posts, it only shows up on the original thread. Hope this helps some. Regards, aevald