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InvisibleSmushroom
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Basic PC physics question
    #7732074 - 12/08/07 06:42 AM (16 years, 1 month ago)

Water boils at a lower temp when under reduced pressure and boils at a higher temp when at an increased pressure. A PC works off of the fact that it pressurized the container to make the water boil at a higher temp, thus sterilizing the contents.

Now for the question. Is it possible to bring a PC up to 15 psi without the water actually boiling? or without requiring a rise in water temp at all?

I am pretty sure that if the PC gets up to pressure then the water will reach 257 degrees. I would imagine that if the water didn't reach that temp it would result in an equilibrium where the water would no longer produce any steam and then not increase the pressure anymore. I just want to make sure that is right though.


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OfflineRogerRabbitM
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Re: Basic PC physics question [Re: Smushroom]
    #7732167 - 12/08/07 07:46 AM (16 years, 1 month ago)

No. In fact, if you don't vent steam at the start of the cycle, the pc will reach 15 psi, but not be close to 250F.

Quote:

Now for the question. Is it possible to bring a PC up to 15 psi without the water actually boiling? or without requiring a rise in water temp at all?




Easily, with an air compressor. That would give 15 or even 100 psi, with no rise in water temp at all. It would also not sterilize anything.
RR


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OfflineYrat
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Re: Basic PC physics question [Re: RogerRabbit]
    #7732245 - 12/08/07 08:32 AM (16 years, 1 month ago)

this is more of a chemistry question

just compressing air into the PC will not result in any temperature increase, maybe a degree or two at most. imagine filling a propane tank or compressed gas cylinder, by your reasoning they should be many hundreds of degrees when they are filled to 2000PSI.

like you said, the pressure allows the water to reach a higher temperature before boiling. but you still need the heat source. i'm sure you've heard of the deep sea hydrothermal vents. in those places, liquid water reaches 400C before boiling (750 degrees F), but that's because it's under five miles of ocean. what's even more insane is that there are organisms that live in those plumes of superheated water.


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"There are a thousand hacking at the branches of evil
to one who is striking at the root."
-Henry David Thoreau
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OfflineNibin
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Re: Basic PC physics question [Re: Yrat]
    #7732314 - 12/08/07 09:07 AM (16 years, 1 month ago)

This is pure physics, nothing to do with chemistry.

Anyway. Each substance has a fusion temperature (solid to liquid) and a boiling point (liquid to gas).

The actual temperature at which this point is reached depends on the external pressure.

If you added compressed air into a PC you would modify the boiling point of that water, but unless you added heat it wouldn't boil. If you reduce the pressure sufficiently the boiling point will lower enough to be at room temperature and boil BUT, remember that it isn't the boiling we want for sterilization but the heat. Using a pressure cooker is our way of managing to increase the temperature inside a container using water.

You had your concepts right. If you don't add compressed air or heat, the pressure in your PC will not increase. As you add heat, steam forms, which cannot escape, so increasing the pressure in the PC a bit (increasing boiling point) and so on and so on until you get to the desired pressure and the cooker starts venting steam to maintain the pressure at the desired levels.


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OfflineNebula
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Re: Basic PC physics question [Re: Yrat]
    #7732368 - 12/08/07 09:27 AM (16 years, 1 month ago)

Quote:

Yrat said:
like you said, the pressure allows the water to reach a higher temperature before boiling. but you still need the heat source. i'm sure you've heard of the deep sea hydrothermal vents. in those places, liquid water reaches 400C before boiling (750 degrees F), but that's because it's under five miles of ocean. what's even more insane is that there are organisms that live in those plumes of superheated water.



Exactly. This also explains why water starts to boil at a temperature lower than 100C when you're high up in the mountains (so due to lower air pressure).
It amazes some organisms can live in superheated water of 400C! What organism are you talking about?


Edited by Nebula (12/08/07 09:30 AM)


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OfflineYrat
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Re: Basic PC physics question [Re: Nibin]
    #7732395 - 12/08/07 09:39 AM (16 years, 1 month ago)

Quote:

Nibin said:
This is pure physics, nothing to do with chemistry.





i remember doing a billion pressure/temperature/volume problems way back in general chemistry, but not too many in my intro physics course :tongue:


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-Henry David Thoreau
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OfflineCaribou_Lou
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Re: Basic PC physics question [Re: Yrat]
    #7732420 - 12/08/07 09:51 AM (16 years, 1 month ago)

Yeah me too.. but it's still physics.


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OfflineYrat
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Re: Basic PC physics question [Re: Nebula]
    #7732479 - 12/08/07 10:05 AM (16 years, 1 month ago)

Quote:

Nebula said:
Quote:

Yrat said:
like you said, the pressure allows the water to reach a higher temperature before boiling. but you still need the heat source. i'm sure you've heard of the deep sea hydrothermal vents. in those places, liquid water reaches 400C before boiling (750 degrees F), but that's because it's under five miles of ocean. what's even more insane is that there are organisms that live in those plumes of superheated water.



Exactly. This also explains why water starts to boil at a temperature lower than 100C when you're high up in the mountains (so due to lower air pressure).
It amazes some organisms can live in superheated water of 400C! What organism are you talking about?




unlike virtually every other ecosystem on the planet that receives energy input from the sun, there are complex ecosystems based around the deep sea hydrothermal vents that use chemosynthetic bacteria to produce energy from sulfur ejected in the water. bizarre creatures live there, including 6ft tubeworms with one end anchored in 100+C water and the other end in -100C (water also freezes at a much lower temp under so much pressure). some crazy stuff exists in the dark down there in some pretty insane conditions. there are many theories describing these places as likely areas for the origin of life on our planet.



--------------------
"There are a thousand hacking at the branches of evil
to one who is striking at the root."
-Henry David Thoreau
Strike The Root


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OfflineNibin
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Re: Basic PC physics question [Re: Yrat]
    #7732481 - 12/08/07 10:06 AM (16 years, 1 month ago)

Quote:

Yrat said:
Quote:

Nibin said:
This is pure physics, nothing to do with chemistry.





i remember doing a billion pressure/temperature/volume problems way back in general chemistry, but not too many in my intro physics course :tongue:




I also did lots of these in chem (and even more in physics) but it still is relative to the physical properties of the water, not it's chemical properties, and so, is Physics


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Invisiblederx
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Re: Basic PC physics question [Re: Nibin]
    #7732511 - 12/08/07 10:13 AM (16 years, 1 month ago)

Quote:

Nibin said:
Quote:

Yrat said:
Quote:

Nibin said:
This is pure physics, nothing to do with chemistry.





i remember doing a billion pressure/temperature/volume problems way back in general chemistry, but not too many in my intro physics course :tongue:




I also did lots of these in chem (and even more in physics) but it still is relative to the physical properties of the water, not it's chemical properties, and so, is Physics




if you think chemistry only includes chemical properties and not physical properties, you're a total dumbass.

What this is, is a physical chemistry question directly from thermodynamics.
It's the best of both worlds.
'Chemical thermodynamics is the study of the interrelation of heat with chemical reactions or with a physical change of state within the confines of the laws of thermodynamics'


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better living through chemistry

OVERGROW the government!!

it's not a war on drugs, it's a war on personal freedom, ok, thats what it is.


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OfflineYrat
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Re: Basic PC physics question [Re: Nibin]
    #7732524 - 12/08/07 10:16 AM (16 years, 1 month ago)

i'll agree with you there, although one could argue these labels are interchangeable for these fields of study

while we're on the topic of crazy physics and the deep sea, check out these videos

the first is bubbles of LIQUID carbon dioxide, kept from converting to a gas by the immense pressure under water. the second is an underwater "lake" of super dense salt water. the ALVIN operators stumbled upon this brine lake and tried to descend into it, but it was much too dense for the submersible. instead, they sent out a wave that you would think was crashing on a beach if you didn't know any better.
&feature=user
&feature=related


--------------------
"There are a thousand hacking at the branches of evil
to one who is striking at the root."
-Henry David Thoreau
Strike The Root


Edited by Yrat (12/08/07 10:19 AM)


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Invisiblederx
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Re: Basic PC physics question [Re: RogerRabbit]
    #7732561 - 12/08/07 10:25 AM (16 years, 1 month ago)

Quote:

RogerRabbit said:
No. In fact, if you don't vent steam at the start of the cycle, the pc will reach 15 psi, but not be close to 250F.



RogerRabbit, How do you justify this concept? Where do you think the pressure or increase in entropy is being generated.
If you have a sealed vessel with some amount of air and an inch of water. Upon heating, pressure is created because the system is increasing in entropy BY water (l) =====> water (g), successfully pressurizing the chamber. Now if PV=nRT holds to be true, which I am pretty confident it does, a sealed vessel with a given amount of water will boil at the elevated temperature, regardless of whether you vent the system before capping the PC.


--------------------
better living through chemistry

OVERGROW the government!!

it's not a war on drugs, it's a war on personal freedom, ok, thats what it is.


Edited by derx (12/08/07 10:30 AM)


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OfflineYrat
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Re: Basic PC physics question [Re: derx]
    #7732604 - 12/08/07 10:34 AM (16 years, 1 month ago)

i've always wondered this myself. whether you vent or not, all the air (steam) will reach the same temperature. the pot doesn't start venting steam until a certain pressure. so venting may displace the colder air and allow you to reach the boiling point faster, but even if you don't vent it's all going to get up there eventually, it might just take 3 min longer.


Edited by Yrat (12/08/07 10:35 AM)


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OfflineRogerRabbitM
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Re: Basic PC physics question [Re: Yrat]
    #7732679 - 12/08/07 10:53 AM (16 years, 1 month ago)

If you don't vent first, part of your pressure is air pressure from the expanding liquid. With my AA, I can shoot an infrared thermometer at the black blow-out plug and get a reading of the temperature inside the PC. Without venting, when 15 psi is reached, the blowout plug reads 205F, below the boiling point of water. If the PC is vented for 5 minutes, when 15 PSI is reached, the blow-out plug reads 245F.

Fifteen minutes later, the unvented PC blow-out plug temperature has increased to 225F, while the vented PC blow-out plug still reads 245F. After 45 minutes, the temperature is the same, whether the PC was vented first or not. If you fail to vent your PC before closing the toggle valve or placing the weight on, you should add at least 1/2 hour to the cooking time to make sure your grains are at the proper temperature for the desired time.
RR


--------------------
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OfflineYrat
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Re: Basic PC physics question [Re: RogerRabbit]
    #7732728 - 12/08/07 11:09 AM (16 years, 1 month ago)

ah that makes more sense.  so it will indeed reach the same temp eventually, but just needs an additional half-hour, not my predicted 3 min :laugh:.

unvented, the initial pressure is created by the cooler, more slowly warming and expanding air already in the PC.  vented, you fill it up with hot steam right off the bat.  thanks for clearing that up.


--------------------
"There are a thousand hacking at the branches of evil
to one who is striking at the root."
-Henry David Thoreau
Strike The Root


Edited by Yrat (12/08/07 11:18 AM)


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OfflineNibin
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Re: Basic PC physics question [Re: derx]
    #7733800 - 12/08/07 03:44 PM (16 years, 1 month ago)

Quote:

derx said:
Quote:

Nibin said:
Quote:

Yrat said:
Quote:

Nibin said:
This is pure physics, nothing to do with chemistry.





i remember doing a billion pressure/temperature/volume problems way back in general chemistry, but not too many in my intro physics course :tongue:




I also did lots of these in chem (and even more in physics) but it still is relative to the physical properties of the water, not it's chemical properties, and so, is Physics




if you think chemistry only includes chemical properties and not physical properties, you're a total dumbass.

What this is, is a physical chemistry question directly from thermodynamics.
It's the best of both worlds.
'Chemical thermodynamics is the study of the interrelation of heat with chemical reactions or with a physical change of state within the confines of the laws of thermodynamics'




Physics is the science of matter and its motion, as well as space and time — the science that deals with concepts such as force, energy, mass, and charge.

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions.

Thermodynamics are a part of Physics, even though it is used in Chemistry (the same as both of these sciences use Maths), as they concern the motion of matter (i.e. temperature and its effects).


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InvisibleSmushroom
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Re: Basic PC physics question [Re: Nibin]
    #7737854 - 12/09/07 07:09 PM (16 years, 1 month ago)

Nibin's last response is a good answer to the physics vs chemistry arguement started here.

So basically my original assumption is right as long as there is no increase in pressure from an outside source and as long as the original air is allowed to escape, either after reaching temp or before.


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InvisibleSillicybin
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Re: Basic PC physics question [Re: Yrat]
    #7739629 - 12/10/07 08:44 AM (16 years, 1 month ago)

Quote:

Yrat said:
just compressing air into the PC will not result in any temperature increase, maybe a degree or two at most. imagine filling a propane tank or compressed gas cylinder, by your reasoning they should be many hundreds of degrees when they are filled to 2000PSI.




Umm - that's exactly what happens! This is why you have to submerge air tanks under water when you fill them, and you have to fill them SLOWLY. If you fill them too fast, they will boil the water the air tank is sitting in. Compressing air creates heat, and releasing pressure absorbs heat. Think of those cans of compressed air for dusting computer equipment - as you release the pressure in the can, they become very cold, even cold enough to freeze the water vapor in the air and accumulate frost on the outside of the can.

Since heated air is less dense than cooled air, if you fill the tank up too quickly it'll get to 2000PSI, but then as you let the tank cool it'll drop back down to 1200-1700 PSI (depending on the temperature drop) so you end up getting shorted on air.

Compressing air from ambient to 15PSI does not create much heat, though (certainly not enough to boil water, anyway.)

To the OP:
If you pressurized a vessel to 30PSI with an air compressor and then heated the water to 257 degrees, then yes - you can sterilize without the water boiling, as long as the pressure on the water is enough to increase the boiling point above the heat you're subjecting the water to.


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OfflineFraggin
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Re: Basic PC physics question [Re: Sillicybin]
    #7741084 - 12/10/07 02:58 PM (16 years, 1 month ago)

Quote:

Sillicybin said:
Quote:

Yrat said:
imagine filling a propane tank or compressed gas cylinder, by your reasoning they should be many hundreds of degrees when they are filled to 2000PSI.




In this example, you are moving liquified gas from one canister to another. No energy exchange is taking place.

If you were converting a gas to a liquid, then you would have quite an amount of heat generated. Liquid to gas gives the opposite effect. It's the basis of how a house is cooled. Any compression has a side result of heat. Ever done the physics experiment where you light a piece of paper with a turkey baster?


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OfflineDem_Bones
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Re: Basic PC physics question [Re: Fraggin]
    #7741964 - 12/10/07 06:10 PM (16 years, 1 month ago)

Most all autoclaves used in the medical field for sterlizeing med equpment have an vacume pump on them and draw a vacume on the chamber before starting the boiler, Venting the presser cooker is very important if you dont the temp will not get much past 212 even with 15 psi , for more info look in A medicale stearlaztion book any one of them will have tons of info on this fact


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InvisibleSillicybin
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Re: Basic PC physics question [Re: Fraggin]
    #7744119 - 12/11/07 08:22 AM (16 years, 1 month ago)

Quote:

Fraggin said:
In this example, you are moving liquified gas from one canister to another. No energy exchange is taking place.




I'm talking about when then original compression takes place. It has to be run through a compressor somewhere...

Also, if you're filling propane tanks to 2000PSI, I hope that's in an industrial environment and not the tank you're using for your grill. Tanks should have relief valves that blow off at 375PSI.


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