Looking for a vacuum

[NobbyNobbs]

I'm looking for some help in an engineering sort of way. My daughter, 9 years old, is fascinated with Mars. She wants to enter in the school's science fair, and her burning question is "Can things live on Mars?"

Well, short of tens of millions of dollars and a few dozen more probes, we can't really answer that. But I thought we might be able to simulate some of the individual conditions and see how plants do.

I thought we'd try to grow a plant in some dry sand, grow another in the freezer, grow another under dim light. These I can all, probably, figure out how to do.

The big problem is atmosphere. I explained about air density to my daughter and she (and I) thought it'd be neat to try to simulate that. Get a vacuum chamber and grow a plant in it.

So here's the question: anybody know how to build a small vacuum chamber? The requirements are:

--must be cheap. I'm not well off.
--must be simple. I'm a doofus when it comes to building things.
--must have at least one transparent wall/window.

Any ideas? Thanks.

[dasmiller]

So here's the question: anybody know how to build a small vacuum chamber? The requirements are:

--must be cheap. I'm not well off.
--must be simple. I'm a doofus when it comes to building things.
--must have at least one transparent wall/window.

Any ideas? Thanks.

Actually, I think any reasonably heavy glass jar will hold a vacuum. A mayonnaise jar, for example, would probably be strong enough but I'd be worried about the lid (either collapsing or simply leaking).

An old genuine glass Coke bottle might work quite nicely. No, it doesn't look like a terrarium, but . . .

Another option would be work in one atmosphere but use a neutral gas. Helium springs to mind - chemically, helium is a lot like a vacuum (yeah, there are convection effects, etc, it's not perfect). There's probably some clever trick involving one of those helium-filled mylar party balloons. I'll have to ponder it - interesting problem, thanks.

[Modified]

You could use a fat bottle and one of those vacuum stoppers they sell with a cheap pump for wine. I'm not sure how much vacuum one of those produces, or how you could find out.

ETA: If you could get a little balloon in a bottle and estimate how much bigger it gets when you pump out the air, you could estimate the pressure that way.

[wackyvorlon]

It really depends on how hard a vacuum you need. A professional roughing pump will create a surprisingly good vacuum, but things like mayonnaise jars are not at all safe. There are hand vacuum pumps and little bell jars sold for experimentation. Edmund Scientific has them. Alas, I'm too new here to link directly to them.

Only problem is that you will lose vacuum, and have to keep pumping. What is your budget for a vacuum pump?

[Dan O.]

They should be advertising a vacuum canning pump (http://www.viworld.net/pumpnseal/) in an infomercial near you. (check the second hand stores to find one at the price it's probably worth)

[wackyvorlon]

Ha, I was just thinking... You don't *really* need a vacuum. The effects of a vacuum on plant structures wouldn't be that fantastic, I don't think. So, it should be enough to enclose it in an inert atmosphere. Helium, nitrogen, argon, etc. Much easier to do, and can be done with much cheaper equipment.

[CardZeus]

Depends on your definition of 'vacuum'. I use an ultra high vacuum system which operates at 10-11Torr - but I'm not going to put any plants in it!!
For the purposes of your experiment a small bell jar and a small roughing pump will do the trick. The pump will be your biggest expense. I would imagine that you should be able to achieve 10-2 - 10-3 Torr (roughly the same in units of millibars. Or for S.I. units 750 Torr ~ 100 kPa).

[wackyvorlon]

Depends on your definition of 'vacuum'. I use an ultra high vacuum system which operates at 10-11Torr - but I'm not going to put any plants in it!!


Can I have your diff pump? Pretty please?:)

[CardZeus]

Can I have your diff pump? Pretty please?:)


Diff pumps are rarely used now, in my experience. We rough with a scroll pump (oil free), then we have 2 turbomolecular pumps (500 l/min) and 2 ion pumps. We also use 2 LN2 cooled titanium sublimation pumps.

[wackyvorlon]

That's... what? Roughly 50k worth of equipment?

[CardZeus]

That's... what? Roughly 50k worth of equipment?


Yup - just for the pumps. Then add in the chambers, gate valves, gas leak valves, sample manipulator, scanning tunnelling microscope, control systems etc etc and the lab renovation for ultra-low vibration (and di-silane use), then your looking at around US$500,000 :)

[wackyvorlon]

Lucky rat:) I went to university briefly, and I used to bum around the physics labs. It was incredible. Millions of dollars of equipment all over the place. I would have moved in if they'd let me:)

[Giraffe107]

Have you considered a carbon dioxide atmosphere?
http://en.wikipedia.org/wiki/Atmosphere_of_Mars

The atmosphere of Mars is 95% carbon dioxide- you could get some dry ice, melt it in an air-tight container and try to grow a plant in that- perhaps as a backup experiment if you can't get the pressure one going.

[wackyvorlon]

You'd want to be careful about the expansion. If you used a piece of dry ice that was too large, the container would burst.

[NobbyNobbs]

Have you considered a carbon dioxide atmosphere?
http://en.wikipedia.org/wiki/Atmosphere_of_Mars

The atmosphere of Mars is 95% carbon dioxide- you could get some dry ice, melt it in an air-tight container and try to grow a plant in that- perhaps as a backup experiment if you can't get the pressure one going.

Now that I think I could handle. I used to do dry ice demonstrations for my students. I feel kinda stupid for not thinking of that.

[Scazon]

A trick my dad showed us was to get a hand bicycle tyre pump, open it up, and reverse the piston with its (then rubber) skirt valve. I doubt if you get a real hard vacuum, but it was good enough to collapse a tin oil can. We used to cut off a tyre valve (Wood's in those days- they are either Presta or Schraeder now) from an inner tube, and insert it, reversed, with a nut and washer either side, in a hole drilled in the can lid to attach the pump.

[skbuncks]

You'd want to be careful about the expansion. If you used a piece of dry ice that was too large, the container would burst.

You could do it in a demijohn fitted with a gas bubbler: CO2 could leave but air cant get in. This would also allow you to purge all the air out of the container

skb

[skbuncks]

For a weak vacuum (around 100mmHg) you might want to try a water aspirator. They can be attached to your household taps and cost around £13 from Sigma-Aldrich

From wiki
An aspirator, also called an eductor-jet pump or filter pump, is a device that produces vacuum by means of the Venturi effect. In an aspirator, fluid (liquid or gaseous) flows through a tube which then narrows. When the tube narrows, the fluid's speed increases, and because of the Venturi effect, its pressure decreases. Vacuum is taken from this point.

skb

[Dave Rogers]

Actually, what you're probably best off doing is filling a glass jar with CO2, then pumping it out again until the pressure is about 10 millibars. For that low a vacuum, you might get away with using a diaphragm pump - an airbrush compressor connected the wrong way round is a common example. Some sort of tap or valve in the pumping line will allow you to seal the jar when you've got enough of a vacuum.

The tricky bit will be measuring the pressure inside the jar. A water manometer should give the right range - you're looking for a pressure of about four inches of water, so you could put a lump of wax in the end of a 6" transparent plastic straw, fill it with water, and stand it stopped end upwards in a small pot of water - but water tends to evaporate, self-cool and freeze, so it may well not work. If it does (you can tell I've never tried this) you need to pump out the jar till the level of water in the straw is about 4" above the level of the top of the pot. If you can connect the manometer on the same side of the valve as the pump, that'll stop water vapour getting into the jar and raising the pressure.

Dave

[Dancing David]

Diff pumps are rarely used now, in my experience. We rough with a scroll pump (oil free), then we have 2 turbomolecular pumps (500 l/min) and 2 ion pumps. We also use 2 LN2 cooled titanium sublimation pumps.


Oh, ah, wow!

Are you doing microscopy? If that is not a stupid question.

[Freethinker]

Never mind

[Graham Jackman]

SAFETY WARNING. If you must pull a vacuum of any sort in a glass container, ensure that it is at least wrapped with adhesive tape, in case it breaks. That way, the fragments will be held together.
Assuming you intend to keep water in the container tg grow the plants, you ultimate vacuum will be limited by the vapour pressure of the water at the temperature of the vessel, and evaporation of the water will cause it to cool.

[CardZeus]

Oh, ah, wow!

Are you doing microscopy? If that is not a stupid question.

Yes - scanning tunneling microscopy. We are part of a larger project in what is becoming known as "Atomically Precise Manufacturing".
We aim to use the STM tip to selectively remove H atoms from a H-terminated Silicon substrate in an atomically precise manner. Then grow a different atomic species only in those vacancies we've created - repeat the process for potential 3-D tailored growth.

[rjh01]

If you want a near vacuum done cheap the best way is to boil a small quantity of water in a container so that the container is full of steam and no air. Seal up the container and allow the steam to convert to water. Result - very low pressure as the only atmosphere is water vapor and the air you did not allow to escape because you are human.

[roger]

If you want a near vacuum done cheap the best way is to boil a small quantity of water in a container so that the container is full of steam and no air. Seal up the container and allow the steam to convert to water. Result - very low pressure as the only atmosphere is water vapor and the air you did not allow to escape because you are human.I don't think that will be good for the plant.

[roger]

I have a vacuum setup for my woodworking - to create veneers and such. These pumps will create around 26.6 Hg, or, if my calculations are correct, around 0.9 bar. Looking up the Mars atmosphere, it runs in the single digit millibars, so that is still pretty far off. If that kind of pressure is sufficient (after all, this is a school project), you can either ask around the local wood working community for somebody with the setup, or you can convert a pump from an abandoned refrigerator for the task.

Oh, and for the record, 27"Hg will create 1800 lbs/ft2 of pressure, so make sure your enclosure is strong, with a margin of safety. I've destroyed things in my vacuum press accidentally. Ask yourself if you'd be comfortable with your daughter working around the enclosure with a car parked on it.

[Lothian]

You know that enlarger you bought.....

[alfaniner]

Buy a used FoodSaver and one of the canisters.

[ben m]

Hi Nobby,

I have one idea for you, but you may find more poking around at http://www.belljar.net, the society of amateur vacuum system hobbyists.

If you didn't have the don't-kill-the-plant constraint, rjh01's steam idea is a good one. Here's another cheap idea: imagine putting the plant in an airtight jar, then filling the jar completely with water. (Most plants will be perfectly fine for a half hour or so.) If you can get the water out using a *water* pump, you'll leave a vacuum behind amounting to 0.001 atmospheres or so---the vapor pressure of water.

Two ways to pump the water: if you want to use a mechanical pump (an aquarium pump, for example) it needs to be able to drive a "head" of 33 feet---that's the ambient pressure at sea level. If your pump is pushing water up a 33-foot hill, it's also (modulo some complexities, cavitation, etc.) capable of pulling water out of a vacuum chamber against the 33-foot pressure of the atmosphere.

Even cheaper: take your water-filled plant chamber to the top of a 40-foot building. Fill a 40-foot garden hose with water (get all the trapped air out!). Plug one end of the hose into your chamber. Then drop the entire hose out the window. The 40-foot column of water wants to flow down the hose, and it'll do so with enough force to pull a vacuum in your chamber. (Make sure it's the stiffest hose you can find: it's going to want to flatten and collapse under the vacuum; you want the rubber's springiness to hold it in a round shape. Stiff plastic plumbing line would work better than most garden hose. And it's better to exceed the minimum 33 feet---extra hose will help prevent "gurgling" of air into your water column.)

That's how Torricelli got the world's first artificial hard vacuum. He did it with mercury, though, where due to the higher density you only need 27 inches of head, not 33 feet ...

[Earthborn]

Or you could use one of those vacuum storage bags that are meant to store clothes and pillows in. Comes with an somewhat airtight seal and a valve for putting sucking out the air with a vacuum cleaner. The only tricky part is getting the airtight seal closed enough that it is actually airtight. Or you could get her one of these "breast enhancement pumps (http://www.ramenterprisesinc.com/products/breast_pump.jpg)" (NSFW) :)

Maybe you can also experiment with Martian soil. You might go to a junkyard, get some rust and crush it and get a decent analogue. Or use a bunch of steel wool scouring pads and burn them in a gas flame.

My prediction is that most of what you are up to with these poor plants is going to kill them. May Mother Nature have mercy on your soul.

[esquel]

Anything you use to pull a vacuum will also suck all the moisture out of the soil and the plant -- think of how water boils at a much lower temperature at higher altitudes. You may be able to get around this by having a chunk of frozen CO2 to sublimate off, but I still predict your plant will not be long for (Mars) this world.

[GreyICE]

Will a glass mayo jar hold a vacuum?

Well lets do some simple calculations. We can assume a small mayo jar to be 4 inches acrossed. Assume that it is 8 inches long, and a cylinder.

That gives it a surface area of 113 inches.

Atmospheric pressure is 14.7 pounds per square inch. Assuming you get to 50% of atmospheric pressure (much higher than Mars) your Mayo jar is suffering from roughly 1,000 lbs. of force across the surface.

Please do not do this near children. Frankly, if an adult chooses to see what happens when they apply a thousand pounds of force to a glass jar, I don't particularly mind (sliced fingers are nature's way of teaching you about basic forces). Little kids are a different issue.

Do not simulate a vacuum this way. The danger is not that the jar might shatter, it's that it might not IMMEDIATELY shatter. Then you will have what looks like a stable jar system. Until someone goes to put it down on the table, and it vibrates hard, then it shatters in your hands. That's a lot of ow.


Now how to simulate a vacuum in reality? Well, plants survive on Carbon Dioxide. Therefore, anything that limits the amount of CO2 in their environment is a simulated vacuum.

Place the plant in a large, sealed fish tank. Purchase pure nitrogen or Argon, and purge the fish tank with it.

Using a compressed CO2 canister and a simple seltzer bottle, introduce CO2 into the fish tank with a calculated volume equal to the atmospheric pressure you are aiming for.

This will not FULLY replicate the effects of a partial vacuum, but it will replicate the effects of CO2 starvation. And, no glass shards.

Cost is probably several hundred dollars. Sorry :p

[Earthborn]

Now how to simulate a vacuum in reality? Well, plants survive on Carbon Dioxide.Or oxygen. Plants can survive quite well without carbon dioxide, they'll just won't be able to photosynthesise. But they don't do that at night either, which means they are just breathing in oxygen.

[ben m]

Will a glass mayo jar hold a vacuum?

Well lets do some simple calculations. We can assume a small mayo jar to be 4 inches acrossed. Assume that it is 8 inches long, and a cylinder.

That gives it a surface area of 113 inches.

Atmospheric pressure is 14.7 pounds per square inch. Assuming you get to 50% of atmospheric pressure (much higher than Mars) your Mayo jar is suffering from roughly 1,000 lbs. of force across the surface.


Hi GreyICE,

I agree that *caution is warranted*, but your calculation doesn't particularly bear on whether or not a glass jar can survive 14.7 psi vacuum. "1000 pounds of force total" sounds scary but doesn't carry much information about whether the glass will break. It's 1000 pounds of nice uniform compression stress, not a 1000-pound barbell sitting on top of the jar. Keep in mind that light bulbs---with their paper-thin envelopes---are under a fairly hard vacuum and hold up just fine.

I'm pretty sure that a *home canning jar* is explicitly built to be evacuated down to H20-vapor-pressure. Your grandmother's standard home canning setup (mmm, plum jam!) is very similar to to rjh01's boiling-water vacuum proposal. I would not trust a random empty mayo jar but real Ball jars should be fine.

Nobby, I would again point to www.belljar.net for bits of safety advice. As for me, as a professional experimental physicist:


I'm personally intolerant of risk, and I like engineering safeguards
I have enough experience to recognize a risk when I see it, which you may or may not (and your daughter certainly doesn't)
I want safety issues to govern *how* I do an experiment. They won't govern *whether* I do an experiment. (With exceptions, of course---no dimethylmercury for me, thank you)


So, that's a big grain of salt: there are safety issues in this experiment, but IMO they're not as categorically forbidding as GreyICE suggests. Here's what I would do myself, which may or may not be good advice for you, depending on your comfort level:


Don't work alone---that goes for you and doubly for your daughter.
Anything within 5 feet of the jar is a STRICT SAFETY GOGGLES AREA.
Take Graham's advice about the tape (smart!).
Use a real Mason jar, not a random applesauce or mayo jar from the supermarket. Use as small a jar as possible; I'd be personally comfortable with a pint, a bit touchy with a quart, basically afraid of a half-gallon.
Consider "double-bagging", with the evacuated Mason jar nested inside of (e.g.) a normal plastic container (I'm picturing those barrels they sell sourdough pretzels in) which would fully contain any implosion. This would be a good idea all the time, but consider it essential for the very first time you evacuate a jar.
I haven't tried this and it's possible I am totally wrong.

[roger]

I will repeat what I typed above - I've broken stuff under vacuum. Further, I know people who've had pretty violent explosions. This is not idle speculation.

[Dan O.]

How much energy is in a vacuum? I'm calculating 10 joules per litter. That's only about 2.4mg of TNT or about 1mg of flash powder. This just doesn't sound very scary.

[GreyICE]

I will repeat what I typed above - I've broken stuff under vacuum. Further, I know people who've had pretty violent explosions. This is not idle speculation.

I'm seconding this. The problem isn't the pressure. The problem is you don't have any clue about what additional stress points do to a jar under compressive forces (this is not to say you don't personally know what additional stress does to an object under pressure. It's that no one knows. It's based on crack mechanics, and thus totally unpredictable). I once watched a plexiglass test specimen that should have stood up to at least 4000 pounds of force literally shatter at 50.

Why? Someone tightened the screws on the strain gage too hard.

You're going to be moving the jar around a lot. Someone is going to bang it, then there'll be glass shards everywhere.

As I said, if it was just grownups, I'd be all for it. College students, sure. High school? Fine. Age 9? No.

[RecoveringYuppy]

must be simple. I'm a doofus when it comes to building things.
--must have at least one transparent wall/window.


Have to agree with some other advice in this thread. Don't simultaneously attempt a hard vacuum and a specific mix of gases. It's not hard to build a small object that can hold a hold vacuum, but if you're not sure, don't do it. There are simple household items that can stand it, but don't practice on the public.

The simplest and cheapest way to simulate low oxygen, low CO2, is to find a gas station that will pump nitrogen.

[Dave Rogers]

Consider "double-bagging", with the evacuated Mason jar nested inside of (e.g.) a normal plastic container (I'm picturing those barrels they sell sourdough pretzels in) which would fully contain any implosion. This would be a good idea all the time, but consider it essential for the very first time you evacuate a jar.


This is standard laboratory practice for any evacuated glass vessel - a good example is the Edwards evaporator we use for putting down metal films. This has a bell jar about a foot and a half across by two feet high, sitting on top of a baseplate that has a port to a vacuum pump in it. Whenever the bell jar is under vacuum, even during pumpdown, there has to be a perspex cylinder placed over it to contain fragments if there's an implosion. The cylinder is about an inch oversized, has a fixed top on it (broken glass can go upwards as well as outwards), and has a ring of half-inch holes around it so that there's no possibility of the perspex being evacuated as well by a leak in the seal at the bottom of the bell jar. Safety glasses are a good idea as well, but the perspex shield is vital.

Dave

[ben m]

This is standard laboratory practice for any evacuated glass vessel - a good example is the Edwards evaporator we use for putting down metal films. This has a bell jar about a foot and a half across by two feet high, sitting on top of a baseplate that has a port to a vacuum pump in it. Whenever the bell jar is under vacuum, even during pumpdown, there has to be a perspex cylinder placed over it to contain fragments if there's an implosion. The cylinder is about an inch oversized, has a fixed top on it (broken glass can go upwards as well as outwards), and has a ring of half-inch holes around it so that there's no possibility of the perspex being evacuated as well by a leak in the seal at the bottom of the bell jar. Safety glasses are a good idea as well, but the perspex shield is vital.

One of my undergrad labs had an Edwards evaporator with just a wire-mesh cage around it. Hmm. Glad I sat on the other side of the room.

This may be standard practice for evaporators, which are usually these big 12"+ bell jars. But this is *not* standard lab practice for small vessels; we have glass ion gauges dangling off vacuum chambers here and there. Breaking one is about as dangerous as breaking a light bulb of the same volume. The dangers are nonzero (flying glass) and must be planned for (basic shielding, gear). If your nine-year-old was curious about light bulbs and wanted to look at and handle a one, you'd stand over their shoulder as they did so and you should do the same here. But the danger of an evacuated one-pint Ball jar is not in the "I am never allowing that object in my home" category. It's in the category of things like knives and bleach and pressure cookers; things you can use safely because you're careful, things your nine-year-old can safely watch you use, things the nine-year old can manipulate somewhat under close supervision.

One other thing about bell jar shields: if the bell jar implodes, all of a sudden there's an unsupported vacuum inside your shield, and 14.7 psi outside. If those air holes aren't big enough to vent that vacuum REALLY FAST, the shield itself can collapse. (This is why I suggested a flimsy plastic barrel, which would crumple under such a stress, rather than glass or perspex which could itself shatter.) This can go to rather extreme lengths: for example, the KATRIN spectrometer has a UHV vacuum chamber with a volume of something like 1500 m^3. This huge chamber lives in a building whose volume is maybe six times that. If the vacuum were ever to fail catastrophically AND the building wasn't well-vented ... well, there'd be a 2 psi crushing force on the walls and the place would collapse. So the building has engineered "blast panels" which, in the case of a vacuum loss, would fail very early, admit lots of air, and save the rest of the building. Neat stuff.

[roger]

Keep in mind that light bulbs---with their paper-thin envelopes---are under a fairly hard vacuum and hold up just fine. Light bulbs are filled with an inert gas, such as argon.

[Dave Rogers]

One of my undergrad labs had an Edwards evaporator with just a wire-mesh cage around it. Hmm. Glad I sat on the other side of the room.

A wire mesh cage is fine. It'll stop any decent sized fragments, and anything small enough to pass through the mesh will be slowed very quickly by air resistance because its mass to cross-sectional area ratio is low.

This may be standard practice for evaporators, which are usually these big 12"+ bell jars. But this is *not* standard lab practice for small vessels; we have glass ion gauges dangling off vacuum chambers here and there. Breaking one is about as dangerous as breaking a light bulb of the same volume. The dangers are nonzero (flying glass) and must be planned for (basic shielding, gear).

I'd check that carefully if I were you; evacuated glass vessels in the open lab with no protective surround? Are you required to wear safety glasses in the room, or is there some protective film around them that you're not aware of? If I were doing a risk assessment on your lab, I'd require the latter at the very least; personal protective equipment is never the measure of choice.

We have large and small evaporators; the small one is about six inches across. We use a perspex shield for that one too, with a wire mesh top.

One other thing about bell jar shields: if the bell jar implodes, all of a sudden there's an unsupported vacuum inside your shield, and 14.7 psi outside. If those air holes aren't big enough to vent that vacuum REALLY FAST, the shield itself can collapse.

There's a significant volume of air inside the shield that mitigates things somewhat, but you're right, there is that possibility to consider. That's where a wire mesh shield scores over perspex.

Dave

[ben m]

Light bulbs are filled with an inert gas, such as argon.

You're right, I was mistaken. Apparently only the small ones (< 40 W) are evacuated.

[Dr H]

Now that I think I could handle. I used to do dry ice demonstrations for my students. I feel kinda stupid for not thinking of that.

Dry ice could be a little risky for reasons given by other posters. Why not just use CO2 from a beverage-dispensing rig? This gear is available at any homebrew shop, and not all that expensive for a 5# cylinder of gas. Or if you know any homebrewers that keg their beer, they're bound to have the gas, regulator, and valve setup necessary. I routinely use such a setup to purge air from my glass fermenters, and if you set the regulator to approximately atmospheric pressure it's a low-stress operation on the container.