You can buy high accuracy reference “air” resistors for calibrating and testing ohmmeters and related instruments. IET is one seller, and I think Fluke also offers them. By “air” they mean that you can use them in air, sitting out on the lab bench, rather than needing a temperature controlled oil bath, because their temperature coefficient of resistance is small enough relative to their stated accuracy.
What is inside one of these?
I use them, and I also use Vishay foil resistors, and I suspect the Vishays are what’s actually inside the “air” resistors. The specifications look consistent and it would make sense.
I have some I can open, to see (though the identity of the part inside might turn out to be obscured of course). Doing so ruins a calibration sticker, though, which I don’t want to do just now.
I might have many more such calibration standards if I find out they only cost what the little part does, and not the fancy box, which is as much as 10X pricier.
I have no idea. I always assumed they were simply made out of an alloy that had an extremely low TC.
FYI, at work we have dozens of L&N standard resistors that we keep in a Guildline oil bath. Many of them are Thomas-type. We use them for calibrating other resistors (using our Measurements International bridge) and temperature calibrations (Using an SPRT and ASL F-18 bridge).
Yes, I think they must be, but that’s only the beginning - like a car’s engine might be made out of aluminum.
The Vishay foil resistors are made out of a low TC alloy, in fact they fairly recently introduced what they call “Z-Foil”, which has a very low TC. In some places they say it’s zero, which of course can’t be a very complete description, but it certainly seems lower than other options. At least in the 5 ohm to maybe 50 kohm or 100 kohm range, these resistor components seem to be the nicest around. If I were selling air resistors, I don’t know what else I’d make them from, except making them from scratch - which considering the price difference between the finished product and the Vishay component, doesn’t seem necessary to turn a profit.
BTW I have a special fondness for Vishay because a good friend of mine worked there about 40 years ago, and also because they have their headquarters in Malvern, PA where I used to go visit my great uncle. It’s not like I know their product is way better than their competitor’s, though.
A conclusion! I took apart an iet resistor. “iet” looks like a typo but is the manufacturer’s name. The product was a SRL-100 Resistance Standard. I expected to see a resistor made by Vishay or one of their competitors.
Surprisingly, what I actually saw was a heavy black box, apparently a potted assembly, with iet’s name on the side, maybe 3/4" thick and 3" square, with four wires coming out of it and going to the front panel terminals. But there were also two huge connection points coming up out of its middle, little parallel walls of copper almost 1/8" thick, close to 1" square, with slots in them. There was also a smaller (but still pretty heavy) solder post sitting on a blue standoff. There were three resistors, the standard circuit board type, soldered between the huge connection points and the smaller solder post.
I guess they hand-trim the things by soldering these resistors in.
Opening this thing invalidates its long-out-of-date calibration. However, I have several nice ohmmeters and standard resistors that are all overdue for calibration but agree with each other to about 10 ppm, and one Vishay that was independently calibrated recently within 1 ppm, that the meters agree with.
On the basis of all of these, the iet resistor seems to be 200 ppm low. Since it is supposed to drift less than 6 ppm per year, and seemingly has drifted 200 ppm in 5 years, something seems all messed up here. I’m going to try sending it back for recalibration, but also flagging it to see if they conclude it met with some dark fate somewhere along the way.