If you need to create a high vacuum, there are basically two options: turbomolecular pumps and diffusion pumps. Turbomolecular pumps require rotors spinning at many thousands of rotations per minute and must be carefully balanced to avoid a violent self-disassembly, but diffusion pumps aren’t without danger either, particularly if, like [Advanced Tinkering], you use mercury as your working fluid. Between the high vacuum, boiling mercury, and the previous two being contained in fragile glassware, this is a project that takes steady nerves to attempt – and could considerably unsteady those nerves if something were to go wrong.
A diffusion pump works by boiling a some working fluid – usually silicone oil – and creating a directed stream of vapor. The vapor molecules collide with air molecules and impart momentum to them, drawing them along with the vapor stream into a condenser. The condenser liquefies the working fluid, while a backing vacuum pump just past the condenser removes the entrained air molecules. The working fluid then flows back into the heating chamber to begin the cycle again. The earliest diffusion pumps did use mercury as a working fluid, a practice which has almost completely died out, but which did have one significant advantage: if, for some reason, air did flood back into the vacuum chamber, there was no risk of setting hot oil vapor on fire.
[Advanced Tinkering]’s diffusion pump is made of glass, which gives a good view of the internal process; It’s in equal parts fascinating and disquieting to see droplets of metal condensing on the glass parts. A Dewar flask of liquid nitrogen holds two cold traps to condense any mercury vapors leaving the pump: one on the line between the diffusion pump and the backing pump, and one between the diffusion pump and a vacuum gauge to make sure that mercury’s vapor pressure isn’t throwing off measurements. Another vacuum gauge is connected to the backing pump’s inlet, which lets the diffusion pump’s performance be measured. After a few hours of running, the pressure at the diffusion pump’s inlet was two orders of magnitude lower than at its outlet, and more vacuum-tight connections could probably have brought it even lower.
This isn’t [Advanced Tinkering]’s first time working with dangerous liquid metals, nor his first time building equipment for high vacuum. If you’re still looking for a safer vacuum, check out this budget diffusion pump.
Liquid mercury has a notoriously high vapor pressure — that is the source of most of the element’s real-world toxicity — and it would seem that a pump in which mercury was the working fluid could, by definition, not pull a vacuum harder than that vapor pressure. What am I missing?
He addresses this 3 minutes, 25 seconds into the video.
Yes LIQUID Mercury.
The vapor pressure of mercury at 25°C is 0.00258 atm
Mercury’s boiling point is 356.7°C
The vapor pressure of mercury at its
normal boiling point, 356.7 °C, is only 1 atm
Liquid mercury is not the working fluid in this pump. Mercury vapor is.
Diffusion pumps basically create a mixing zone between the evacuation chambers ever decreasing gas molecules, and its working vapor. The gas molecules become entrained in the vapors flow, dragging them out. When the working vapor is condensed, the evacuated gas molecules are trapped. The working fluid is then reheated and reintroduced to the system as vapor ready to drag more gas out of the system.
That will pull a pretty hard vacuum, if you want better, add getter.
Wrong! Use a titanium getter. Cheaper than both of those options.
A titanium getter is used for Ultra High Vacuum.
You dont use them alone. You must first pre-pump the system to at least 10⁻⁴ torr. Some high-performance, two-stage rotary vane pumps can achieve an ultimate pressure of 10⁻⁴ Torr. However, this is typically the very limit of their capabilities. For reliable, sustained performance in this range, Youre looking at a turbomolecular pump, or a diffusion pump.
No, you don’t actually need to do that. Titanium getters work at any pressure, and if sized correctly for the vessel they’re evacuating, will drop the pressure by a factor of roughly 100, since they react with all components of the atmosphere other than noble gases. Hell, if you burn them hot enough, they’ll start evaporating and trap the noble gases too! I’ve used them starting at around 10 millitorr and it was fine. Read fewer books and do more experiments.
10 millitorr = 10⁻³ torr
so youve used them starting outside of ideal parameters at the sacrifice of longevity and efficiency. Congrats.
You still used a prevacuum.
The titanium film has a finite capacity for trapping gas molecules. Once the surface is saturated, it loses its ability to pump effectively. The rate of saturation depends on the gas load. At higher pressures (> 10⁻⁴ torr), saturation is rapid and requires frequent sublimation. This is why no one with an inkling of knowledge or competence uses them at higher pressures. Then there’s you.
Pardon?
Dude it’s a titanium wire. Suck out the air with a rotary vane pump, close the valve to your chamber and burn the wire. Put a new wire in when you’re done. You don’t need to buy fancy-pants $$$ stainless steel nonsense, just two feedthroughs with some terminals to hold a wire. Please, I beg you, put down the books and try it out. It’s easy. You can achieve high vacuum for under $200. Stop being a snob.
https://www.edwardsvacuum.com/en-us/vacuum-pumps/knowledge/applications/working-with-ion-getter-pumps
“Unfortunately, they can be poor at pumping noble gasses, require high voltage and magnetic field, and need a turbomolecular or other secondary pump to create the starting pressure.”
https://www.rbdinstruments.com/blog/titanium-sublimation-pump-operation/
“You can use them starting in the mid 10-4 Torr range. In fact, they are very helpful at this vacuum level in helping start the ion pumps (which need to be in the low 10-5 or better vacuum to start). Typically the TSPs are operated after loading gassy samples to help the vacuum recover more quickly from the 10-8 Torr into the 10-9 Torr range.”
https://www.ibericavacuum.com/pages/tsp
“TSPs can operate from 10⁻⁵ to 10⁻¹² mbar”
Maybe you should stop pretending to have experience in everything and start reading a bit. Some factual knowledge will enhance the believability of your BS.
Huh. I guess I just didn’t know that what I was doing was impossible. I’ll remember that, next time I do it. Have a good one!
So basicaly any turbocharger from a diesel car.
You could go to a scrapyard and buy a turbo from something like VW Golf for maybe $30. Then you only need to attach an oil pump so it doesn’t seize while running. But no, let’s use the most toxicest element ever created because that’ll generate more YouTube views from SHOCKING EXTREME RADIOACTIVE TOXIC OMG YOU WILL NOT BELIEVE thumbnail crap.
It’s stupid -.-
A car turbocharger cannot be used as a substitute for a turbomolecular pump.
A Turbocharger uses one rotor to scavenge energy and one rotor to generate pressure. Each of these rotors has at most a dozen vanes.
A Turbomolecular pump typically has a series of 9 or more rotors, with a dozen or more vanes each.
Your proposal is akin to someone proposing an F16 could use a hair dryer instead of a jet engine.
My weekender has a 2 stroke that can do 15,000 rpm what’s the top speed of a Dremel 35k THOUSANDS of rpm is the joke here
When your weekender or dremels bearings fail your out a few bucks in parts. Their balance is relatively inconsequential. You wear some low dollar parts a bit more.
When a TMP fails catastrophically you can lose $5-10K in precision machined rotors.
Woosh
A turbomolecular pump is typically rotating at 70-90k rpm, with clearances between the rotor and the housing a few 100s of microns, its a touch more sensitive than your 2 stroke or dremel, and will fail catastrophically if simply exposed to atmospheric pressure when at full power.
Next time at least check Wikipedia before posting. Pretty clear your idea isn’t viable
Huh, sounds cool. Where’s your project writeup? You have actually done this, right?
There is also the Sprengel pump which uses Hg but forgoes the boiling. Can achieve one microPa or ~10^-11 atm.
https://en.wikipedia.org/wiki/Sprengel_pump
Sorbtion pumps are another option. A sealed container of molecular sieve (porous ceramic beads) is cooled with liquid nitrogen. Immersing the pump in a styrofoam bucket full of liquid nitrogen is adequate. It is basically a cold trap as gas molecules hit cold surfaces of the sieve and can lose enough energy to stick to the surfaces (adsorption). They are removed from the volume of the chamber which is what vacuum pumping is – moving gas molecules to where you want them. Eventually the sorption pump becomes saturated and won’t pump anymore. It must be heated to drive off the gas molecules. Up side: clean, no moving parts, fairly fast pumping speed, relatively low initial cost. Oh and no mercury vapors. Sorry mad hatters. Down side: it requires some (not crazy) amount of liquid nitrogen and time for regeneration.
One thing not mentioned in the video is the skill required to shape the glass. Beautiful.