“Faraday Wax” – recreating old school high-vacuum epoxy
eevblog.com
eevblog.com
Glass to metal is the only tough nut at the low end of high vacuum. Glue works but you will always have to worry about thermal stresses crating a leak someday.
There's a new fs pulse laser technique for directly bonding glass to stainless, but its still not quite there yet.
At my company we developed a new type of all-metal seal as drop in replacement for viton gaskets. These can get you way below where viton can, and work well for arbitrarily shaped ports. It's awesome. Still a few bugs to work out to make sure you get a tight seal before cracking the glass.
I'd think that there would be an issue with thermal coefficients going directly to stainless..
The base looks like an audio 12AX7 and fits the same size socket, the metal pins are supposed to be Kovar coming through the glass.
But this electron tube is not sealed at the top, it is wide open with a Kovar metal precision adapter ring fused to the glass instead, and a Viton o-ring is used to lock it into the vacuum measurement port.
As a vacuum tube, it "simply" behaves differently depending on residual trace gases present. It is connected to a circuit optimized to give calibrated vacuum readings.
No doubt I could connect it to an audio circuit instead, and get some excellent output. It's a well-made tube, only a single triode though.
I guess you could have an operation large enough to custom fabricate something, but that's not in my experience
The only way I can see getting to -10's with Viton seals is having a tiny small amount or a huge pump.
One of the dangers in early experimentation was the church, because of course.
To get down below a certain pressure, you have to heat everything and "bake out" residual volatile materials. This precludes a wax from even being tested at the lowest pressures.
Future cryogenicists: glycerine+soap is amazing because you can wash it away with hot water before you finally take the part to get re-welded. Most epoxies/sealants can't be cleaned so easily, fouling any future welds.
If needed I could silver braze the Ultra-Torr into a larger object. It's just a bit of a puzzle to figure out what sizes of glass tubing mate with the various metal fitting sizes, because glass tubing sizing can be variable.
There are a few people who make most of the copper to borosilicate glass joints in the world. It's really hard to do, and they're quite good at it. Don't try to make your own as a cost-saving measure!
A major challenge is matching thermal expansion coefficients. Copper expands quite a lot, and borosilicate barely moves at all, so they make a joint to a glass that moves less than copper, but more than boro, then they add a little bit of another glass, and so on until they've gradually transitioned from the copper to the borosilicate. Those well-made joints can be cooled to liquid helium temperature (4 K) without issue.
It was pretty expensive - you’re only supposed to use each O-ring once. We had a big box of them that we’d occasionally send off to recycle.
There is a big selection of high vacuum greases and waxes you can buy. As you mention they are basically highly pure, very heavy hydrocarbons that have almost no vapor pressure.