6 replies to this topic

[richard2]

Hi guys,

I know this is not pyrotechnics but it is still possibly of interest to some of you.

I have recently been playing around with some liquid nitrogen doing some demos and trying to get friends interested in practical science. During which I have done a little research into the Leidenfrost effect (the reason you can dip your hand in LN2 and not burn/freeze yourself). While doing so I have come across a few accounts of people dipping 'WET' fingers into molten lead. For a few reasons this seems absolutely insane to me however it does demonstrate the same principle and would be fairly impressive.

I would like to know if anyone has been mad enough to try this and if so did you remain uninjured?

Another thought I have had about this is that some of the lower melting point alloys that have been heated enough not to solidify on contact with your skin but to above waters Leidenfrost point may provide more room for error. Any comments on this or suggestions of appropriate alloys/metals would be appreciated.


On a completely different note I have attempted to find the thread that this was posted in but have not been successful. I am considering purchasing the CR2 rock tumbler from Manchester Minerals however have a few questions. I know that a few members have one of these and would like to know the diameter of the jars and length of both the 1400g and 800g size jars. The reason I ask is I am a little concerned with the amount of media that these will hold (especially the larger one). If these are filled half way with lead media do they exceed the 2800g max recommended for the tumbler? I also realise that these run at a lower than optimal speed for the media I would be using (1/2 inch) so has anyone tried lift bars in these jars and how effective at reducing milling time were they? Also from what I have seen in the past the maximum capacity of similar tumblers tends to be rated on the jar size. Does anyone have a CR1 and if so what are the motor specs (CR2 specs would also be useful). I ask only because I would likely only run one jar at a time and if this was going to be a heavy jar then shorter rollers with the same motor would be more likely to last.

Thankyou in advance for any advice.

Regards


Rich

[fishy1]

you can dip your fingers in liquid nitrogen and not get burned?
i thought it only worked when you pour it on your hands and let it roll off?


i talked to a guy who claimed he had done it, and not burnt himself.
appartently, it won't work with cats unless you shave them first, as the hair insulates it.

that's his theory anyway.

also, where do you get your LN2, and at what price?

Edited by fishy1, 21 June 2005 - 09:26 PM.

[ProfHawking]

i can get loads of ln2 from uni
however i dunno how to store it

if i stick it in a thermos with an expansion hole, then it should last a while, but all the time boiling away right.

what pressure or type of clynder is required to keep it permanently?

[Andrew]

what pressure or type of clynder is required to keep it permanently?

not a chance

The vapour pressure is about 750 bar at room temp (almost 11,000 psi). Seems as it is dirt cheap, it does not make sense to invest in a pressure vessel capable of holding such a high pressure, your looking at a couple of grand for a 1L vessel.
You could apply a little refridgeration to a cylinder capable of holding 200/300 bar, these are only a hundred quid or so for a 1L cylinder. You would still have to get it below 220 K (-50 C) in order to be safe, that is still beyond your average home chest freezer though. Having said that, my 200 bar cylinder is tested at 480 bar, you might be able to get away with it if you used a 300bar cylinder, but you would be diceing with certain death.

The insulated way is definitely the way to go, a good thermos flask will hold liquid nitrogen for weeks. Plus it's a peice of piss to just tap off a little, from one of the 5' darlics (gloves permitting of course)

It's quite neat, the evaporation of a small amount keeps the rest of the liquid cool. The only variable is the amount of evaporation that occurs. This is regulated by how much energy you let get into the container and that is up to you, the better the thermal insulation, the longer the nitrogen will stay. You could keep it for years if you went over board with expanded polystyrene. The liquid will reach a state of equlibrium if you prevent energy from entering the system too fast. Once the liquid is cool inside your thermos flask, it will not boil, only evaporate. It will only boil if you supply enough energy. Eventually it will all piss off back to whence it came, but not as quick as you think. The rate of nitrogen lose will be determined by the rate at which it is heated, in a vacumn flask this is next to nil.

Edited by Andrew, 22 June 2005 - 11:46 AM.

[ProfHawking]

cool. i might try and mod a thermos with a vent and more insulation, and then get some.

I dunno what i want it for, just seems like a cool thing to play with and freeze stuff

[lord_dranack]

Another thought I have had about this is that some of the lower melting point alloys that have been heated enough not to solidify on contact with your skin but to above waters Leidenfrost point may provide more room for error. Any comments on this or suggestions of appropriate alloys/metals would be appreciated.

I've heard about this. I have tried this with liquid nitrogen once (while supervised by someone who knew what they were doing). You can actualy scoop some up and throwi it, vaporising before it hits the floor!

for low melting alloys, this website may be of interest:
Lowden metals

Thomas

[Andrew]

I've used nitrogen to seat bearings before, it's quite cool, no pun intended. cooling the bar so that it shrinks, and heating the inner raceway leaves you with loads of room to fit them together, and then when the temperature has normalised the two are mated together permanately (ish, one extreame friction fit).