Joe Gwinn
Stainless
- Joined
- Nov 22, 2009
- Location
- Boston, MA area
I can't remember how many probes there were, this was back in the early 90's. I do remember the inspector being very careful to calibrate the meter with the calibration coupons before testing parts.
It might have had only 2 probes, as a lot of parts they had to check were contoured with no flat surface for 4 probes to contact simultaneously.
If only two probes total, contact resistance will likely dominate, rendering the measurement useless. This would be especially true of aluminum, which forms oxides readily.
If the article has a standard, known shape (machined part or standard coupon), one can use two groups of two probes. Nor is it absolutely necessary that the four probes be in a line, although that's simplest.
I would guess that Inspection measured the resistivity of a known-good sample, and tested for the same answer (within some small tolerance), thereafter.
I was a contractor at McD, and the company i worked for were making parts for the Wind Tunnel shop at McD. One day I get called down and shown a part they had made, the inspector said it was supposed to be made from 7075, but the electrical conductivity showed it to be 6061 (there was only 7075 and 6061 at the shop so it was an either/or situation).
The difference in bulk resistivity is fairly large for those two alloys:
6061-T6 4.066e-8 ohm-meter.
7075-T6 5.210e-8 ohm-meter.
7075-T73x 4.310e-8 ohm-meter.
The smallest ratio, 4.310/4.066, is 1.06, or 6% difference.
Which way exceeds the density difference, and is easier to measure in the shop.
Turned out a part got scrapped out on night shift, they'd substituted the material that should have been 7075 with a piece of 6061. and being night shift did tell anybody what they had done. After that debacle every time new parts came in I'd get called down to the inspection lab so they could do an incoming inspection with me there. They were always hoping to find another part that had the wrong material. Luckily it never happened again.
Yeah. The night-shift folk thought that nobody could tell, and when they found out different, stopped attempting that dodge.