I'm taking the Level 2 class for Eddy Current nondestructive examination and we need to determine the percentage of conductivity using IACS (International Annealed Copper Standard).  I'm having no trouble getting the measurements and determining full screen height and width measurements.  My trouble is with the math and determining the percentage between known samples.  

I know this should be easy but I'm just having vapor lock today.  

Air (null) is at 10% FSW (full screen width) and 80% FSH (full screen height).  The 100.05% copper is at 10%FSH and 90% FSW.  The material that is a 57.57% IACS is approximately 28% FSH and 82% FSW.  There are 7 other known points plotted that I can't recall now so I'm focusing on the unknown material that comes in between 57.57% and 100.05%.  Assume this point is at 22% FSH and 87% FSW.  How do I go about getting the percentage IACS of this unknown material.  

The instructor told me this was more of a ballpark guess exercise today but I want to be able to get more accurate in my determination.  Anyone with experience in this?  I won't have the lab itself back until next Thursday and will post some pictures of it if that would help.  Surely, there is a formula for this but I don't know it.  

Oddly enough, I was able to bullshit my math for one that turned out accurate but I couldn't reproduce the work on the other unknown samples.  

If you're confused by this, don't worry.  I am, too.

Wish I could help but its been like 20 years since I have done ET.  Hell, its been 10 since I last did any NDT other than having the department report to me.
Should have kept my  Level III ASNT certs. Is that even a thing anymore???

Level 3?  Hell yeah. The certs don't travel with you from job to job.  If you've passed Level 3, you can easily get a Level 2 if your current employer offers it.  On second thought, I think there are a couple of orgs that offer level certs that will transfer but I think you have to travel to take the tests, and they aren't cheap.  I think one is in Canada and another in Europe.  I may be mistaken.   This is based on short trips down memory lane,so to speak, by our instructor.  Good dude.  Just tested (and passed) for MT Level 3 and UT Level 3.  He worked several years for Olympus and doing RT in the field.  Some of the stories are interesting.  One of them, horrifying.  Some RT guy wound up with a RT pill in his pocket when he went home.  Wound up with an amputation and eventually died.  I'm not sure if that's one that falls into the category of urban legend or not.  

So far, I've got the educational requirements done for MT Level II, PT Level II and Eddy Current Level I.  Working on ET Level II right now.  Doing VT Level I/II next semester while I knock out the math requirements to get into UT and RT.

The problem has always been that you need to know pretty
accurately what alloy you are dealing with.

The technique works when you are trying to determine uniformity of alloying elements in a piece.

It is not useful in determining WHAT alloy elements are present.
As in more than a single alloy element.

There are x-ray (and proton-neutron) testing methods that can accurately determine multiple alloy element concentrations.

They are expensive to do and only done if you really need to know what material you have.

Taking tiny test samples and running them through a mass spectrometer is a lower cost approach.