Achieving 10e-4 Torr - Some questions

[Hadez411]

Hello all,

I have a few questions for anyone with the physics knowledge that can help me. I'm trying to reach 10e-4 Torr or greater for the benefits of its' insulation properties. I've determined that increasing the heat in a bakeout will continue to decrease the gas density in my chamber in a proportional way and as such I was wondering if I would be able to skip the 2000$ vacuum machine and simply bake a metal box with firewood up to a temperature of 600 celcius ( as opposed to the 100-300 celcius most pro bakeouts use) and apply an aspirator that can normally draw about 30 cm hg. Is anyone able to determine a ball park figure for the final pressure after my box has been heated to 600C, pumped to what would normally be a negative pressure of 30 cm hg, then then allowed to cool? All sealed of course.

If my method here fails to achieve 10e-4 Torr and I still get a significant vacuum, will it still be worth the effort of a bakeout? Can anyone link me a graph of the insulation value or thermal conductance values as vacuum is increased? With values between atmospheric pressure to 10e-4 Torr. I'm trying to figure out if it's a proportional scale, linear, exponential etc.. to get an idea of whether or not it's still worth trying to create a vacuum if it can't be equal to or greater than 10e-4 Torr.

Thank you all in advance!

[Rich Feldman]

1. The usual admonishments to careless newbies have lately been posted by John Futter, who could use a break. Somebody gave them to me when I was a noob here. So without further ado:

The forum registration dialog has some rules that you apparently skimmed over. Your login name should be your real name, and you should post an introduction of yourself in the Please Introduce Yourself forum. Your questions do not belong in the Vacuum Technology forum, it says so right there: "Do not post simple questions here. Use the New User Chat area."

2. Now as to your questions. Here is a straight answer, with a hope that you are not a forum troll.

It sounds like you expect the pressure to drop if a hot, partly evacuated, sealed box is cooled down. The simplest formulation, "pressure is proportional to absolute temperature", was apparently discovered by Guillaume Amontons between 1700 and 1702, about the time Anders Celsius was born. Let's try applying that.

Say your chamber cools to 20°C from 600°C in the wood-fired furnace. The absolute temperature ratio is 293/873, or about 0.336. OK so far?

You said your aspirator normally draws a vacuum of 30 cm Hg. It ought to do just as well pumping on the hot chamber, unless a hose melts. That gets you an absolute pressure of (76-30)/76 atmosphere, or 460 torr. After cool-down, the chamber pressure would be 0.336 x 460 = 154 torr -- about 1/5 of an atm.

That's more than a million times higher than you asked for.
You could get a vacuum 10x better with no baking, simply by using the firewood money to buy a better aspirator.
You could get to 2e-3 torr by boiling some mercury in the vessel & letting its vapor displace all the air before sealing off.

geber_distillation.jpg (44.86 KiB) Viewed 5970 times

[Hadez411]

Thanks for the reply and for looking past my forum blundering. My name is so unique and known I could be found across the world, so I'm not putting it up on a public forum, thanks.

154 Torr? Shit, not even close.

Do elaborate about these better aspirators though, I have a commercial sized irrigation pump at my disposal if that's the kind of flow it'd require. The 30 cm HG aspirator is just a garden hose add-on.

Also, the question that was left unanswered about thermal conductance. Would the reduction of thermal conductance be significant at this level (154 torr or less) or is there a change in the thermal conduction characteristics of the vacuum once it is beyond a certain point? (as I would like little to no convection, which I've read is achieved at about 10e-4 Torr)

My original idea was trying to be cost effective. I'm in the country with wood galore and plenty of welding materials. I really appreciate the physics lesson, it makes sense now how I'd calculate it.

Thanks in advance for humoring me. I realize that I'm asking a question which seems complex to me but would be but a few moments for someone who is capable of crunching the numbers for fusor tech

Could boil elemental magnesium??

[Richard Hull]

Rules are rules. Real name please.

Richard Hull

[Rich Feldman]

You minimize convection by filling the insulating space with something fluffy, like goose down or Fiberfill (tm), or polystyrene foam beads, or perlite or fiberglass (depending on wall temperatures). The heat loss by conduction through air would, I think, go down in proportion to the absolute pressure.

Re. the aspirators, I thought your "-30 cm Hg" was a mistake and you meant to say -30 inches, which is surely a stretch. A few minutes of Internet searching found me ones like this, for a sink faucet, claiming 10 torr. http://www.flinnsci.com/store/Scripts/p ... duct=14079 I'm skeptical. Here are some user-measured values reported on another forum: http://www.sciencemadness.org/talk/view ... ?tid=17250

Funny story about aspirators here, halfway through the first significant block of text:
http://blogs.sciencemag.org/pipeline/ar ... aspirators

[Frank Sanns]

Thanks for the reply and for looking past my forum blundering. My name is so unique and known I could be found across the world, so I'm not putting it up on a public forum, thanks.

154 Torr? Shit, not even close.

Do elaborate about these better aspirators though, I have a commercial sized irrigation pump at my disposal if that's the kind of flow it'd require. The 30 cm HG aspirator is just a garden hose add-on.

Also, the question that was left unanswered about thermal conductance. Would the reduction of thermal conductance be significant at this level (154 torr or less) or is there a change in the thermal conduction characteristics of the vacuum once it is beyond a certain point? (as I would like little to no convection, which I've read is achieved at about 10e-4 Torr)

My original idea was trying to be cost effective. I'm in the country with wood galore and plenty of welding materials. I really appreciate the physics lesson, it makes sense now how I'd calculate it.

Thanks in advance for humoring me. I realize that I'm asking a question which seems complex to me but would be but a few moments for someone who is capable of crunching the numbers for fusor tech

Could boil elemental magnesium??

Convection is the transfer of energy by gas molecules but it is only one of the three modes of energy transfer. As the vacuums get better, the other two begin to dominate. There is a vacuum that is the point of diminished returns. That number is dependent on the distance of gap and what gas is in the gap.

When there are fewer gas molecules there is less energy (heat) transfer. As pressure is reduced, there is a point where where the gas has become so rarified that molecule to molecule convection is no longer the primary mode of energy transfer. This occurs because the mean free path of a molecule gets longer and longer as the pressure is reduced. At a point, the major energy transfer is directly from one surface to the other across the gap without the intermediate molecules in the space being the major transfer media. This of course is dependent upon absolute pressure (vacuum) but also on the gap between the surfaces and the gas that is between the surfaces. Heavier gases have much shorter mean free paths than lighter gasses. Helium would transfer more energy than air would transfer more energy than argon.

The absolute pressure from an aspirator is dependent on velocity of the water flow and not the volume. A bigger aspirator will not draw a better vacuum that a small one. A bigger one will evacuate faster and would be more appropriate to evacuate larger volumes. All aspirators are limited by the vapor pressure of the water ( or other fluid ) at the temperature that they are being operated. For water at 23 c that means no vacuum better than 22 torr can be attained no matter how big the aspirator is and how fast the water is moving.

We welcome you to the forum and hope that you will contribute the results of your work for all to learn. We are after all both a technical site and a mentoring site.

Now, as Forum Administrator, I have to repeat Richard Hull's restatement of the real names rule to you. You must realize that you cannot come and solicit the expertise of some super intelligent and accomplished people of the world who ARE using their real names and then not give anything back and not use your real name.

I am sure you will realize the value of what we do here and be happy to contribute and most certainly use your real name. Consider this a friendly but firm re-statement of the Fusor.net policy.

[Dennis P Brown]

You are wrong about the need for 10^-4 torr and there are methods to get well below a torr without electricity but frankly, without your name, I will not answer your question more fully. Your issues with the rule is ridiculous - this is a professional forum and frankly, read any paper or any place people who hold both professional standards and stand behind their work do research, and you will ALWAYS find that they publish their name with the work. In any case, once you fix this issue, do ask your question so I can give you the answer.

[prestonbarrows]

In regards of vacuum insulation as a function of vacuum pressure, read up on Paschen's law. Basically, it is all very well defined but also very non-linear.

Vacuum pumping is all governed by differential equations relating the addition of atoms from leaks or outgassing and the removal of atoms through pumps.Raising the vacuum chamber pressure, i.e. baking out, actually increases the baseline pressure of a chamber for a given pump. The idea is that this generally increases the pumping rate at higher pressures. Assuming you do not have leaks introducing new atoms into the chamber, the higher pressure and increased pumping speed means you exhaust the supply of atoms sooner and when you cool back down to room temperature you will be at a lower pressure than if you had not baked out.

Get some heating tape elements and aluminum foil instead of a damn fire pit to heat the chamber. These are widely available as used surplus. Amptek is the most common brand.

In regards to aspirator pumps, you are fundamentally limited to the vapor pressure of any fluids in your pump. If you are using a water aspirator, you can't get below the pressure/temperature where the water begins to spontaneously evaporate back into your chamber. This is a good exercise for you to look up what the minimum pressure that corresponds to, but I'll give you a hint that it is many orders of magnitude over 1E-4 Torr.

That pressure range is at the bottom end of standard 'rough pumps'. You will want something like a good used rotary vane pump to come anywhere close to this range and even those will most likely bottom out in the 1E-2 or 1E-3 Torr range unless you have an exceptional pump and vacuum chamber.

[Dan Knapp]

This has gone on too long. If this guy can't comply with the name rule, the whole thread should be erased; and he should be banned.