[EugeneOregon]

I have learned that the use of polymers in my vacuum system needs to be kept to an absolute minimum. Lolz, like everything else I had already ordered $17 per foot premium Nalgene vacuum line before I learned this.

To be fair I am very glad I have the Nalgene. It is a DREAM to work with and it is worth every penny, but now I only use it to couple metal to glass in inch size pieces.

Any polymer I have learned is simply too permeable to operate in molecular distillation deep vacuum applications without leakage. My understanding is that even the o rings in the seals leak. The explanation is sort of the same deal as a helium filled balloon from the fair. The balloon will lose enough gas in one day through permeability to lose bouyancy because the pores in the rubber compound are big enough to let small helium atoms through. The same apparently with Nalgene in that it has enough permeability to allow enough small molecules through to impact deep vacuum attempts.

I have seen the vacuum gauge jump 100 microns when I forget to turn on the stir bar and then turn it on after things are 60C as measured. It pulls down again but it is obvious that at under one micron it takes VERY little extra gas present in the system to impact deep vacuum significantly. Being old enough to make most mistakes twice and three times at least, I have seen glass fail from my own stupidity. I respect the enormous pressures involved with deep vacuum. I employ a full spherical mantle for the efficiency while at the same time I enjoy the safety of having all the really hot stuff and glass (most likely implosion point) encapsulated in thick mat. A full spherical mantle in this regard is a great safety device. Inspecting each bulb takes only a moment and any cracks at all are grounds to toss the unit into the trash.

From experience let me pass on little tips. I always leave at least one joint unclipped but secured otherwise. In the case of an unexpected presurization (like a small pocket of water going superheated and making a small explosion in the flask) then the unclipped joint allows a point of pressure relief in a predictable location which is made safer. On my bulb set up I leave the fritted air inlet adapter on the end unclipped for this relief now. It will just pop off but is only connected by a few inches of Nalgene so nothng is broken. Once implosion or overpressurization breaks the vacuum the boiling ceases instantly. The next tip is this - NEVER peer at the process from directly over any glass joint. I made the mistake of restarting hot extract while still purging residual solvents after a short shut down. During he shut down the stir bar was stopped. Upon restarting the vacuum the gas that evolved instantly overpressurized the boiling vessel which then took on what looked like a whale clearing his breathing hole lolz! I learned that my Amish built dining table has an EXCELLENT finish against hot extract that sponges up easily lolz. There was even extract in my coffee mug. It got on everything on the table. If it had got into my eyes.... just rub your eyes after touching standard extract...ouch! I also "aim" every single opening that might fail this way in a direction away from my chair.

https://www.youtube.com/watch?v=l5qz65O2o7c

[SkyHighLer]

2.4.2 Outgassing
The broad relevance of the above to vacuum practice and outgassing is as follows. It should be stressed that in typical systems the condition of interior surfaces is not well known because of the variety and complexity of the processes which occur in the course of operating the system. In addition typical technical surfaces will be microscopically rough and oxidised, with many surface flaws.

When vacuum systems are vented back up to atmospheric pressure, direct adsorption from the gas phase leads to complete coverage of up to several layers of adsorbed gas. Large amounts of water vapour are adsorbed, as well as oxygen, nitrogen and other atmospheric gases. Some of this adsorbed gas will migrate into the interior near-surface region to become absorbed. It is prudent to try to minimise water vapour adsorption by venting to dry nitrogen, for example, because nitrogen is less strongly bound to structural materials such as stainless steel than is water vapour, and desorbs more readily in subsequent pumping down.

In pumping down from atmospheric pressure, most of the gas in the volume is soon removed, and, in typical systems, sub-millibar pressures of 10(-2) mbar or better are achieved in times of the order of minutes. Desorbing gas starts to contribute to the gas load below about 10(-1) mbar, and as the pressure continues to fall into the region below 10(-4) mbar the gas load (assuming no leaks) becomes increasingly due to outgassing. Of the molecules which desorb, a number will fmd the entrance to the pump and be removed immediately. But others, a majority in vacuum chambers of typical proportions, will traverse the chamber to another part of the vacuum wall, and further interactions with the surface, before being pumped away. It is this traffic of molecules in transit to and fro across the chamber, fed by desorption and diminished by pumping, which constitutes the number density n of molecules in the vacuum and determines its quality.

Gas that is loosely bound on internal surfaces is pumped away quickly. Gas that is tightly bound desorbs at a very slow rate and does not contribute a significant load. But water vapour, which has an appreciable probability of desorption, has been stored in large amounts at the surface by adsorption during exposure to the atmosphere. The result is that there is protracted gassing of water vapour from structural materials such as stainless steel and glass. The outgassing rate does decrease, but only slowly and desorbing water vapour accounts for the dominant gas load for times of the order of tens of hours, unless bakeout procedures, to be discussed shortly, are used. In circumstances in which pump performance did not limit the vacuum achieved outgassing would be observed to diminish as time progressed, with the contribution to it of the small amount of diffision of gas from the interior, particularly hydrogen, becoming more important and eventually, after very long times, becoming the dominant contribution.


5.2 COMMONLY USED MATERIALS

5.2.1 Metals
Austenitic stainless steels.
Advantages: types generally preferred are EN58A, EN58B (US321), EN58E (304) and EN58F (347) with EN58B and EN58F chosen most frequently for satisfactory welding by an argon-arc. If low magnetic permeability required, EN58B is used. All have a low outgassing rate as shown in figure 5.1.
Disadvantages: EN58F will not accept a high polish and welding can cause considerable distortion in all grades.

Aluminium and aluminium alloys.
Advantages: aluminium and magnesium alloys are most used, but a high zinc content should not be used. Good corrosion resistance, easily machined and jointed.
Disadvantages: strength at high temperatures is poor. Alloys with copper content present welding problems. High distortion when welding which may lead to further machining.

Nickel alloys (Inconel, Kovar).
Advantages: high strength at high temperatures, excellent corrosion resistance. Disadvantages: not readily available, high cost and present machining problems.

Copper.
Advantages: oxygen fiee, high conductivity grade (OFHC) excellent for vacuum systems, easily machined, good corrosion resistance.
Disadvantages: brazing in a hydrogen atmosphere sometimes difficult.

Brass.
Advantages: suitable for some exceptional applications, care must be taken in grade selection, good corrosion resistance. The use of castings should be given careful consideration to avoid virtual leaks.
Disadvantages: brass contains zinc (15 - 20%). Zinc evaporates out at temperatures over 100 degrees C.

Mild steel.
Advantages: may be used generally down to 10” mbar, can be used at lower pressures if plated after welding.
Disadvantages: liable to rust.

Titanium. A good clean metal in vacuum, light in weight and ductile. Used mostly for its gettering properties e.g ion pump cathodes, and getter pump filaments.

5.2.2 Plastics
Plastics generally desorb large quantities of gas and have a high permeability rate compared with metals, so that they must be carefblly considered and generally kept to a minimum. Outgassing rates for some plastics are given in figure 5.2.

PTFE. Low outgassing rate, good electrical insulator, can be used at a higher temperature than most plastics, self-lubricating. Glass-filled PTFE - a form of PTFE which has less tendency to cold flow.

Polycarbonate. Moderate outgassing rate and water absorption, good electrical insulator.

Nylon and acrylic. High outgassing and water absorption rates, self-lubricating.

PVC High outgassing and water absorption rates, flexible PVC tubing useful for backing lines and temporary connections (e.g. leak detectors).

Polyethylene. Only suitable if well outgassed. PEEK is a high temperature polymer, useful as an insulator where low temperature bakeout is used.

Electrical wires. Electrical wires coated with Kevlar have good electrical insulation. Ceramic beads are also useful and prevent trapped volumes.

5.2.3 Synthetic resins
Not recommended.

5.3 SEALS

5.3.1 Elastomer seals
Nitrile rubber. The most commonly used sealing ring is nitrile rubber which can be easily jointed (in situ for large installations).

Viton. Is most suitable for seals at lower pressures, has low outgassing rates and is heat resistant. It has a tendency to remain deformed for some time after compression. New viton should be vacuum baked at 100 degrees C for 1 hour to remove moulding release agents.

PTFE. Has a very poor compression set and is not generally recommended.

5.3.2 Metal seals
Copper ring. Used with knife edges machined into opposing flanges (e.g. Conflat). Can be baked up to 450 degrees C. Care must be taken that the knife edges do not become damaged in store or during installation.

Aluminum diamond edged disc seal. Fits between flat machined flanges. Self-aligning, bakeout to 200 degrees C. Flange finish required - 0.8 pm.

'Wills' rings. Can be used at high temperature and pressure. Needs higher flange bolt loading than knife edge and requires a higher flange finish than the diamond edge seal.

Indium wire. Very soft, continues to flow after initial tightening of flanges. Can be easily re-extruded.

Aluminum wire. Easy to manufacture from annealed wire by electric butt-welding. Fits between flat machined flanges. Requires aluminium foil centring tabs. Flange finish required - 0.8 pm.

Gold wire. Inert to all gases, bakeout to 450 degrees C. Needs high flange bolt loading and well finished flange faces (0.4 pm). Expensive initially but good cost recovery for scrap.

5.3.3 Proprietary brand seals
Essential to follow manufacturers' tightening instructions. Bake-out up to 450 "C. (Vaculok, Swagelock, Betabite etc.)

5.4 CERAMIC SAND GLASSES

5.4.1 Ceramics
Fully vitrified electrical porcelain and vitrified alumina are most suitable for vacuum components and insulators and can be used up to a temperature of 1500 degrees C. Care must be taken in handling these materials as they are brittle and the surfaces mark easily, so they should be handled wearing lint fiee gloves. Selected ceramics can be brazed to suitable metals by the use of a molybdenum - manganese fired coating onto the ceramic. Machinable ceramics are available and are satisfactory for specific components. They may be machined, drilled etc following manufacturers' instructions.

5.4.2 Glass
Can be used for constructing vacuum systems or parts of systems. The most generally used is borosilicate glass (e.g. Pyrex) which can be obtained in matching components fiom stock. Glass is also used for viewing windows and selected optical glass can be used for the transmission of ultra-violet light. The permeability of glass varies with temperature and glass has a high corrosion resistance.

From Basic Vacuum Technology 2nd Edition - Chambers, Fitch, and Halliday

[mobin]

i like my super think gum rubber hoses. less out gassing way less permeable.

[flatslabs]

I use the big ass gum rubber hoses also

[BigJohnny]

so am I to understand that using sched 40 PVC piping would be problematic for use as a vacuum line?

[mobin]

Originally Posted by BigJohnny

so am I to understand that using sched 40 PVC piping would be problematic for use as a vacuum line?

id bet it out gasses pretty good.

[BigJohnny]

Originally Posted by mobin

id bet it out gasses pretty good.

So what does that mean exactly, if it's under vac, will I not be able to achieve full vacuum?

[mobin]

Originally Posted by BigJohnny

So what does that mean exactly, if it's under vac, will I not be able to achieve full vacuum?

pretty much.

you need clean hoses too. residues build up quickly and have the same effect.

clean glass. clean pump oil. empty and cold cold trap etc.

[BigJohnny]

Originally Posted by mobin

pretty much.

you need clean hoses too. residues build up quickly and have the same effect.

clean glass. clean pump oil. empty and cold cold trap etc.

Alright, so how does this affect the end product then?

If it outgasses then this means that when the lines are under vac, that the vac won't hold steady for 24 hours.
When I'm finished with my piping i'll do a test to see how long it holds vac in clean and new lines.


So if there's stuff in the lines that condensed out of the gasses flowing through the lines, then I can also see how this would affect vac as the volatiles in that will still boil off.


but I'm still missing the connection between how the porosity or outgassing will affect the end product.

[EugeneOregon]

Originally Posted by BigJohnny

so am I to understand that using sched 40 PVC piping would be problematic for use as a vacuum line?

Any polymer has a degree of permeability. I use stainless steel vacuum bellows now and stainless steel fittings except for the few inches that need coupling from glass to metal. Pubchem lists Dronabil (trade name for THC) as boiling at .02 mm Hg (Torr) and 200C. This .02 mm Hg is also said as 20 microns of vacuum.

My pump easily pulls down to three quarters of one micron (Edwards EM 28) and I distill at a much lower temp. However, having both data points and using a single point conversion chart allows a crude visualization of the temps needed to boil THC at pressures higher than 20 microns.

For instance, my Robinair dual vane pump is listed as having an unltimate vacuum of 75 microns. Using the single point conversion chart and my data collected, this means that at 75 microns the crudely computed boiling point will be high enough that the heat mantle max temp of 400C on my full spherical mantle will be exceeded because rhe mantle needs a higher setting than boiling point. Temp to boil climbs geometrically as pressure departs further from absolute vacuum.

So if adding three feet of polymer hose prevents the ultimate vacuum through permeability, and it does, then you are stuck chasing the temperature all around during distillation. My hunch is that the commercial operations that I have seen employ in their videos polymer vacuum hose is that they likely chase the heating mantle temperature around with frequent adjustment of at least 10% and more up and down and probably often in order to chase the ever changing boiling point of a system with intrinsic leakage designed into it.

An example of how little leakage can cause major grief is seen on my vacuum pump display graph function. When I pull down to under a micron and allow the flask to come up to 50C or so and then engage the stir bar magnet, just the act of beginning to stir can cause a jump of 50 microns easily, which of course pulls back down in a few minutes. So the boiling points return to the same value but everytime the pressure changes the boiling point changes which changes the pressure which changes the boiling point....etc. So if the vacuum is not rock steady stable the only way to overcome this is frequent temperature changes or running a temperature much higher to overcome the constant variations in pressure.

Permeability also impacts initial pull down times because there is no avoiding vacuum hose contamination of some sort. At least some will absorb into the hose which requires then purging (or disposal) of its own to completely outgas the stuff. With stainless steel bellows I can pop off the pipe in a few seconds from the kf25 fitting, Rinse the thing out with methanol and let dry in the sun. If I choose I can then bake the bellows in my oven at 300-400F to completely purge it. This has the advantage of sparing the pump oil from purging impurities. The downside to stainless steel bellow is cost - generally about $35 per foot or so.

https://www.youtube.com/watch?v=XMW1mYiSU04