Pressurised? how?

Hi folks,
A quick question. I'm wondering how one would go about 'pressurising' a container? Like the pepper spray cannisters? I dont have access to commercial equipment but I was wondering if there was a straighforward way to achieve a fairly strong spray/cone from a household bottle?
Strange question I know but surely no stranger than a few of the questions asked here /ubbthreads/images/graemlins/smile.gif

Ta

Ross

You could use Charles' Law. It states that temperature and volume are directly proportional. Charles' law is T1/T2=V1/V2.

T=Temperautre
V=Volume.

Example: Take an air tight container. Say it is at room temperature (70°F), and it's volume is 10 cubic inches. Now heat it up to 150°F. Using Charles' Law we can find the new volume.

70°/150°=10/V, where V is the new volume. If we solve it, V is about 21.428 cubic inches.

Charles' Law is an approximation, and is not always accurate. In your case, you might want to heat something up, seal it off, and then cool it down. Using temperature, you should be able to pressurize just about anything. I hope this makes sense.

Either I'm confused, or this is reversed. Take canning for instance: a jar and its contents are heated, the jar and its contents are sealed, then the jar is allowed to cool, resulting in a vacuum inside the jar, not a pressurized jar. /ubbthreads/images/graemlins/confused.gif

[ QUOTE ]
ewick said:
Either I'm confused, or this is reversed. Take canning for instance: a jar and its contents are heated, the jar and its contents are sealed, then the jar is allowed to cool, resulting in a vacuum inside the jar, not a pressurized jar. /ubbthreads/images/graemlins/confused.gif

[/ QUOTE ]

I agree with your example completely, and Charles's law backs it up. You can go the other way too though... you could cool the can, then cap it off and seal it up. When you warm it, the pressure inside would be higher than the outside air.

A canning jar is _not_ a sealed container. You heat the jar up. The gas inside expands, and the water inside boils. The pressure causes the lid to lift a bit and the gas escapes. That is why you _don't_ screw the lid down tight, but only loosely.

After you take the heat away, the gas inside cools down and contracts, and pulls the lid down. At this point the jar seals.

Regarding the initial question, the common techniques used for pressurizing containers are either to pump in gas using some sort of one way valve (much like a tire valve) or to include some sort of low boiling point material in the initial fill. For example, if you could mix your contents with liquid propane and then fill the container with this liquid and seal it, then at room temperature the container would be under considerable pressure.

-Jon

Do the canning process backwards and you get pressure. Seal the container cold, then heat it and it will be pressureized as long as it stays hot. Be aware that if it explodes, you may get sprayed with scalding hot stuff.

You could seal it _really_ cold....

For an undergrad research project I needed to use Xenon as a solvent. I had very heavy walled glass tubes, with a carefully designed valve on top. I would evacuate the tube with a vacuum pump. Then I would pressurize the system to a slight pressure with Xenon, and I would slowly dip the tube into a -100C slush. The Xenon would condense as a creamy solid in the bottom of the tube.

After the tube was properly loaded, I would seal the valve on top and then let the tube warm up. The pressure would steadily rise, the sample would dissolve, and then I could run my experiments. Only had one tube fail under pressure, normally we operated at about 750 psi.

-Jon

Illuminatingbikr,

That's right! I thought by reading your post that you were saying to follow the heating/sealing/cooling routine to wind up with a pressurized container...sorry about that.

Can't think straight...need to sleep...must find bed...

Can you give more details on what exactly you are trying to do? If there is any way you could install a valve stem from a car tire on the container, pressurizing it would be simple. What are you working with?

[ QUOTE ]
ewick said:
Illuminatingbikr,

That's right! I thought by reading your post that you were saying to follow the heating/sealing/cooling routine to wind up with a pressurized container...sorry about that.

Can't think straight...need to sleep...must find bed...

[/ QUOTE ]

That's okay. CPF is a nice relaxing place to be /ubbthreads/images/graemlins/sleepy.gif

Well.............
I live in a nice flat, in a nice part of Edinburgh. I have a wee garden to the front of my flat which my front lounge window (1st floor) looks onto.
I used to have nice grass until the local cats & dogs started to pee on it.
The plan is, to have a pressurised water container on the window ledge so that I can open the window and aim a stream onto the offending animal. I'm hoping that eventually they will get the message.
All I was wanting was basically a water pistol but in a pressurised plastic bottle type form (water pistols dont go far enough and arent strong enough)
NOTE: I have a cat myself - I am not being cruel to animals (they would get wetter in the rain) and I have tried other options - fences/speaking to owners/shouting etc etc.

Sounds like an interesting plan. Why not get a super-soaker? Those are always fun. /ubbthreads/images/graemlins/grin.gif

I know, but they always seem to leak and I'd rather have a bottle of water sitting on the windowsill rather than a Super-soaker!!

Two things:
First of all, it sounds like you want an old fashion seltzer bottle. I had one years ago that you would screw a CO2 cartridge onto the side to pressurize the bottle. Seems like it was about 1.5 liters or so. I bought it at Service Merchandise but I dont know if they are in business any more. Just like the 3 Stooges! Another thing to try is one of those hoses that you hook up to the kitchen faucet. I talking about the ones you can drag all over the house for watering plants while still connected. About 1/4" I.D.
Mouse trap land mines work very well too!
Second of all I have done a bit of home canning. The vacuum is achieved by placing HOT items in HOT jars. They are then boiled, pressure cooked, or steamed purely for sterilization (kill all of the bacteria in whatever you are canning). Nothing usually escapes from the container during this process. The vacuum developes when the contents cool.

maybe a pressurised water fire extinguisher with some ammonia as a deterrant?
i prefer to let mr.crossman do the talking here.

Have you tried washing the car more frequently? Animals usually use the same spot as a bathroom as a marker, and will continue to scent as long as they continue to feel they are "refreshing" their territorial boundaries, cats don't have very large territories, if you remove the scent from it quite a few times they may lose interest in marking it again. spraying water is ok, but you have to remove the reason the spot is a hot marking spot, which is the scent. You could also try spraying something offensive smelling to animals like hot pepper sauce (Might strip paint though!) on the area or a product from a pet supply meant to keep animals from doing their nasty business on furniture.

Shooting at domesticated animals especially someones possible pet is idiotic. If you did it to my pet and I found out, I'd likely crack your skull with a baseball bat. Cracking peoples skulls is idiotic too, but hey, if it gets that far it's tit for tat. /ubbthreads/images/graemlins/ooo.gif

TSG /ubbthreads/images/graemlins/grin.gif

Do they have garden sprayers where you live? Here you can go to the local home center or gardening center and get 1 or 2 gallon sprayers for applying liquid weed killers, insecticides, etc. They have an air pump attached to pressurize it. You could fill one with just plain water, pump it up and let it sit there until needed.

Bill, thanks for the suggestion, I think that may be the way to go. BTW, here in the UK, we have garden shops & everything....we're really quite cultured!

Ross

There are also commercial products meant for safely discouraging animals. Supposedly moth-balls work, too.

How about the small spray bottles used for spraying indoor plants that hold about 0.5 liters of water and work with a trigger lever that pumps out the water. If you set the nozzle to the stream setting, some of them produce a pretty good stream. A good one would reach 5 meters or more. It would look pretty normal on the window sill also. And handy for other things like your plants or quick clean-ups.