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Some of you might be interested in this experiment that I did today. The water in the pot had been boiling for at least ten min. with the bag of water in the center. The water in the bag never came to a rolling boil like that in the pot.
Who can tell us why the water in the bag does not come to a rolling boil.
http://www.youtube.com/watch?v=YMJE6x3JRHo
Was the bag open on the top, creating some pressure?
Mountain Mike
01-06-2013, 19:19
More pressure creates higher boiling temps.
OK. Just ran the video. The foil is acting as an insulator, however slight, and as a heat sink.
Mountain Mike
01-06-2013, 19:21
Same reason why at high altitudes (lower air pressure) water boils at lower temps. Put a pot of water in a vacuum chamber & you can make it boil at room temperature.
Was the bag open on the top, creating some pressure?
Yes, both the pot and bag were open since the gas was turned on. I waited and waited and the water in the bag never boiled. This making my brain hurt:)
My guess:
The boiling water in the pot is at exactly the b.p. for the atmospheric pressure, no more no less. Plastic is not a particularly good conductor of heat, thus the heat transfer is less than 100% which means the water in the bag can never reach the b.p. Try an Al pot "floating" with some water in it, I bet it doesn't boil either even though Al is good conductor, it is not 100%
Here's my crack at an answer. Anyone can feel free to disagree.
At normal atmospheric pressure water boils at 212F. Liquid water won't get any hotter. When water gets hotter, it turns to steam. The water in the pot is in contact with the metal, which conducts heat rather well from the heat source, the stove. The stove is much hotter than 212F so plenty of heat to conduct. The water in the plastic bag is in contact with plastic that is in contact with water. That means it is trying to get to 212F from water that is at most 212F. It should get there eventually, but it will take a while.
Recall from your 7th grade natural science class that heat is transferred by one of thre modes. radiation (sun, miscrowave ovens), conduction (direct contact), and convection (mixing). The water in the bottom of the pot is heated by high heat (stove)through conduction, then mixes(convection) as it rises (remember that heat rises). The plastic effectively prevents the convection of the hot water in the pot from the water in the plastic bag. Thus the water in the plastic bag must rely on conduction at relatively low heat (water at 212F) and then have it's own internal convection.
The concept is similar to a double boiler that can be used to keep food warm but not scorched.
Here's my crack at an answer. Anyone can feel free to disagree.
At normal atmospheric pressure water boils at 212F. Liquid water won't get any hotter. When water gets hotter, it turns to steam. The water in the pot is in contact with the metal, which conducts heat rather well from the heat source, the stove. The stove is much hotter than 212F so plenty of heat to conduct. The water in the plastic bag is in contact with plastic that is in contact with water. That means it is trying to get to 212F from water that is at most 212F. It should get there eventually, but it will take a while.
Recall from your 7th grade natural science class that heat is transferred by one of thre modes. radiation (sun, miscrowave ovens), conduction (direct contact), and convection (mixing). The water in the bottom of the pot is heated by high heat (stove)through conduction, then mixes(convection) as it rises (remember that heat rises). The plastic effectively prevents the convection of the hot water in the pot from the water in the plastic bag. Thus the water in the plastic bag must rely on conduction at relatively low heat (water at 212F) and then have it's own internal convection.
The pot water had been boiling at least 10 min. I gave up and shut it off. The thin layer of plastic disappoints me in the fact that it could not transfer the heat from the pot water. Humbug!!!! On the other hand I am well pleased to know that it is an excellent insulator.:D
So can we say this is an effective way to cook a meal. It gets to an almost boiling point and then "simmers" for the duration of the cooking. You prepackage your meals. When ready to use, add the water to the bag, zip it almost closed, place in pot of 2 cups water and boil till cooking is done.
Double boilers usually have an inch of water in the bottom pot and then a large air space and then the top pot. the top pot heated by steam.
Here's my crack at an answer. Anyone can feel free to disagree.
At normal atmospheric pressure water boils at 212F. Liquid water won't get any hotter. When water gets hotter, it turns to steam. The water in the pot is in contact with the metal, which conducts heat rather well from the heat source, the stove. The stove is much hotter than 212F so plenty of heat to conduct. The water in the plastic bag is in contact with plastic that is in contact with water. That means it is trying to get to 212F from water that is at most 212F. It should get there eventually, but it will take a while.
Recall from your 7th grade natural science class that heat is transferred by one of thre modes. radiation (sun, miscrowave ovens), conduction (direct contact), and convection (mixing). The water in the bottom of the pot is heated by high heat (stove)through conduction, then mixes(convection) as it rises (remember that heat rises). The plastic effectively prevents the convection of the hot water in the pot from the water in the plastic bag. Thus the water in the plastic bag must rely on conduction at relatively low heat (water at 212F) and then have it's own internal convection.
The concept is similar to a double boiler that can be used to keep food warm but not scorched.
I think you are correct. For heat to transfer it needs a temperature differential. The answer involves "latent heat". Water not only needs to be 212 F at 29.92 HG, it also needs to absorb 970 BTUs per pound to convert it from a liquid to a gas. So although the water in the bag can reach 212 F, once it does, heat transfer stops (since the water in the pot is 212). That along with the heat loss to conduction, prevents a rapid change in state (boiling), becasue it cannot absorb the required latent heat.
Dr. Professor
01-06-2013, 21:54
You've just rediscovered the double boiler.
Odd Man Out
01-06-2013, 23:20
With that music, I felt like I was watching a Woody Allen movie - nice touch!
**Science Alert**
The pot water had been boiling at least 10 min. I gave up and shut it off. The thin layer of plastic disappoints me in the fact that it could not transfer the heat from the pot water. Humbug!!!! On the other hand I am well pleased to know that it is an excellent insulator.:D
So can we say this is an effective way to cook a meal. It gets to an almost boiling point and then "simmers" for the duration of the cooking. You prepackage your meals. When ready to use, add the water to the bag, zip it almost closed, place in pot of 2 cups water and boil till cooking is done.
Don't fret. The bag is not acting as a significant insulator. As pointed out above, the water in the pot is in contact with the pot which is much hotter than 212F so heat flows from the pot to the water. The water doesn't get above 212 however as that heat goes to causing the phase change from liquid to gas. Since the water in the bag can only get its heat from the water in the pot, it can only get up to 212F, but then no more heat will flow. If you put a thermometer in the bag and in the pot you will get the same temperature (or nearly so). Yes this would be a fine way to cook, if you want. You wouldn't even need to open the bag if it were vacuum sealed. A little pressure will build up in the bag, but mostly from air expanding (not from the water boiling). Once hot, you could reseal the bag and it won't expand any more (it will contract upon cooling).
Two other features could be at play. One is that the bag is acting like a radiator. Some heat flows up the bag (by conduction) and then radiates heat to the relatively cold air. This could account for a slightly colder temperature in the bag, but I don't think it would be significant. Another factor, (again, probably not significant) is that the phase change from liquid to gas is not always spontaneous. A catalyst is sometimes needed to make that phase change happen fast enough to keep the water at 212F. The most common catalyst is a rough surface. You may notice that the bubbles in a boiling pot often come from certain places on the bottom. This is where there is a scratch acting as a phase change nucleation site. If you have a very clean and very smooth surface, there will not be any nucleation catalysts and it is possible to heat the liquid water up to a temperature much above 212F (super heating). The water should boil, but it doesn't because there are no nucleation sites. When it does eventually boil, it boils all at once, causing an explosive release of super heated steam (can be quite dangerous). In a chemistry lab, we call this bumping and we add boiling chips (basically small rough rocks) to provide the nucleation sites and prevent this. I would assume any pack packing pot would be pretty beat up and not need any boiling chips. However, your plastic bag might be smooth enough to not provide phase change nucleation sites. But as mentioned above, as long as your pot is boiling (not super heating), you bag water can't heat above 212F anyway so it can't superheat. Super heating can be a significant hazard with glass containers, however. If you boil plain water in a china cup in a microwave, it can easily superheat. Agitation can also catalyze the phase change, so just touching the cup can be enough agitation to trigger and explosive "bump" of superheated steam and you can be severely burned by the blast. This is why you may hear not to microwave plain water, or if you do, put something in it like a tea bag or a plastic spoon. BTW, when you open a bottle of pop, you create a supersaturated solution of carbon dioxide in water. A huge amount of gas should come out of solution, but the phase change process is rather slow, which is good as this keeps your drink carbonated. A boiling chip will catalyze the release of CO2. Add a rough rock or salt crystal to the Coke and watch all the bubbles form around it. Perhaps you have seen the You Tube vids of people putting Mentos in Coke. I'm not sure why, buy they are really good at doing this. Agitation also catalyzes the phase change, as you know from shaking a Coke bottle before you open it. The agitation of drinking the Coke also releases the fizz (which is good). The most dramatic example of this is when you put an open Coke bottle in a sonicating water bath (used for cleaning jewelry). The phase change is so rapid, you create a Coke geyser.
In a dream:-? not really, just as I was laying in bed ready to doze off I remembered a thread here at WB of a stove that I made called a "Stiletto" The burner was made of a small tea lite candle tin. I placed a perforated cone over the top of it to concentrate the flame onto a small diameter pot. The water in the pot rose to 212 degees but only a 1" diameter of rolling boiling bubbles were visible in the center of the pot. I was able to pick up the pot slightly above the pinpoint flame and move it in a circle and the column of vigorously boiling bubbles moved within the pot.
So, I suspect the bottom of the bag has to be in full contact with the bottom of the pot to acheive the boil within. There are 2 types of bags sold. One type has an expanded base like the one I used, the other is boilable bag that looks like it would conform to the entire bottom of a pot. The bag needs to have close contact in order for the contents to boil. It need that intense heat to activate the rolling boil.
Those of you that said the bag was being insulated from the bottom of the pot were correct.
To be sure I'll order the boilable bag and repeat the test. The bag I used yesterday was a boil-in-bag type. I guess that means pour in boiling water.
I'll see if I can locate the Stilleto Stove thread and get a photo or video.
The seams of the bag held strong and did not leak water overnight.
In another bag of the same type I had 12 esbit cubes that had been removed from original packaging. It was successful in retaining all of the odor.
Snowleopard
01-07-2013, 12:49
Nice physics question!
bfayer and perrymk have it right. Boiling water can't go above the boiling point (212F), so the bag can't either. There is no temperature differential between pan water and bag water, so there is no way for the latent heat for boiling into the bag water.
The good news is that boiling or simmering anything in water is cooking it at 212F, so most things will cook as fast in the bag as in the pan water. The exception would be something where you need to boil off some of the water. At high altitude the boiling point is lower, so the cooking temperature is lower so it takes longer to cook; this is true of both the pan and bag. Mountaineers used to use pressure cookers because of this. A standard pressure cooker increases pressure by 15 psi and this raises the boiling temperature to 257F at sea level.
You can increase the boiling point by dissolving a lot of salt in the pan water. If you do this, you ought to be able to see the pure water in the bag boiling.
There is also a 'latent heat' involved in melting ice/snow. The amount of fuel needed to melt snow at 0C to water at 0C is roughly the same as the fuel needed to heat 0C water to 100C. This is why it takes a lot of fuel to melt snow.
This afternoon, I will go outside and apply salt to lower the melting point of the solid water on my steps.
To be sure I'll order the boilable bag and repeat the test.
I'll suggest you also use a metal cup or make a small bag out of aluminum foil. This will remove any insulator effects of the plastic, although in the context of this experiment I doubt there is much in the way of an insulator effect. Conductor and insulator are relative terms anyway.
DeerPath
01-07-2013, 13:33
I love WhiteBlaze because of these kind of questions and answers!
Odd Man Out
01-07-2013, 15:17
The bag needs to have close contact in order for the contents to boil. It need that intense heat to activate the rolling boil.
Those of you that said the bag was being insulated from the bottom of the pot were correct.
I don't think this is quite right. It is misleading to say that "the bag was being insulated from the bottom of the pot". The only thing between the bag and the pot is boiling water and it seems misleading to say that boiling water is a heat insulator. Also insulators won't stop the flow of heat, they only slow it down. Eventually, the temperature of the water inside the bag will be the same as the same as the temperature of the water in the pot since the only thing separating them is a thin sheet of plastic. Since that is a pretty bad insulator, you reach that equilibrium pretty quickly.
The fact that the water in the pot is boiling and the water in the bag isn't is irrelevant. So when you say "the bag needs to have close contact in ordre for the contents to boil", I ask, why do you feel you need the contents of the bag to boil? If you are trying to cook in the bag, you only need it to reach the boiling point (not even that, really), and in fact you have done that, even though the water in the bag is not "boiling", i.e. going through the phase change from liquid to gas. It will cook the same way because it is at the same temperature. Water that is boiling and the water vapor that forms the gas bubbles in a pot that is boiling are not hotter than water at the boiling point that is not boiling. You can also look at it this way, the only place in a pot of boiling water where the phase change from liquid to gas is taking place is at the surface of the pot or at top where it is evaporating. In a pot of boiling water, 99.99% of the water is liquid is not at a surface where a phase change is taking place. Yet all the water is at the same temperature.
You have a similar situation in a glass of ice water. In a glass of ice water, once the condition is allowed to reach equilibrium, the temperature of solid water (ice) and the temperature of the liquid water are exactly the same (the freezing point).
FarmerChef
01-07-2013, 15:33
mmmm science. I like this thread.
Liquid water can get above 212 if nothing is present to break the surface tension of the water. This is dangerous so do not do this but here are the instructions anyway.
Materials Needed
Brand new glass container without any scratches or seams
Non-Rotating microwave oven
Cord to attach to microwave door handle
Water
Directions
Fill the container halfway with water
Place in microwave and heat for 5 or 6 minutes
Pull on string to open door (Make sure it's more then a couple of feet of string)
See what happens maybe 25% of the time. I took 4 tries with different containers to make this work.
As a side note in comparing plastic to metal as an insulator, we chill chicken stock in an ice bath to cool. If 4 gallons of stock is inside a plastic container it would take about 5 hours to cool to 80 from 180. If inside a metal container that time is cut to about 1-1/2 hours. If the pot is aggitated and stirred then the same four gallons can be cooled in under 20 minutes.
...Water that is boiling and the water vapor that forms the gas bubbles in a pot that is boiling are not hotter than water at the boiling point that is not boiling. You can also look at it this way, the only place in a pot of boiling water where the phase change from liquid to gas is taking place is at the surface of the pot or at top where it is evaporating. In a pot of boiling water, 99.99% of the water is liquid is not at a surface where a phase change is taking place. Yet all the water is at the same temperature.
You have a similar situation in a glass of ice water. In a glass of ice water, once the condition is allowed to reach equilibrium, the temperature of solid water (ice) and the temperature of the liquid water are exactly the same (the freezing point).
Actually the change in state from liquid to gas is occurring at the bottom of the pot (nearest to the heat source) and the gas bubbles rise to the top to break the surface. The gas bubbles (Steam) in the water can be and usually is significantly hotter than the water itself. This is because water conducts heat so well, as soon as the gas bubble at 212 F gives up it's latent heat, it converts back to liquid, this happens as the bubble floats upward in the pot. The bubble is giving up heat all the way to the top. The bubbles that are hotter than 212 F actually make it to the top and break the surface before the latent heat is absorbed by the water.
The latent heat of steam or water at 29.92HG is 970 BTUs per pound, the area of a pound of steam is 1600 times the area of the pound of water that created the steam, so a small bubble of steam does not need much heat to be removed from it to change back to water at 212 F. In the inverse situation the same area of water (of equal weight) needs 1600 times more heat to change state to steam.
This by the way is why Jetboil uses a tall pot, and why a beer can pot is more efficient than a low wide pot. The heat of the steam is reabsorbed into the water on the trip to the top. This means you need less fuel to maintain a boil.
I thought for sure if I had the bottom of a plastic bag compressed against the bottom of the pot I was going to get a boil. No, never got the liquid to a gas change over in the bag. I was not able to get a good reading on the thermometer to see if the water was at 212.
Everything that bfayer (http://www.whiteblaze.net/forum/member.php?27761-bfayer) said sounds the most logical. Odd Man Out (http://www.whiteblaze.net/forum/member.php?14450-Odd-Man-Out) was convincing also.
This is because water conducts heat so well, as soon as the gas bubble at 212 F gives up it's latent heat, it converts back to liquid, this happens as the bubble floats upward in the pot. The bubble is giving up heat all the way to the top. The bubbles that are hotter than 212 F actually make it to the top and break the surface before the latent heat is absorbed by the water.
That's really informative and says a lot for tall pots.
This by the way is why Jetboil uses a tall pot, and why a beer can pot is more efficient than a low wide pot. The heat of the steam is reabsorbed into the water on the trip to the top. This means you need less fuel to maintain a boil.
But that goes against all that we've been led to believe:confused:
This video shows that I at least tried:D
http://www.youtube.com/watch?v=VCUR0jAJh20
soulrebel
01-08-2013, 09:50
A double boiler, also known as a bain-marie (http://en.wikipedia.org/wiki/Bain-marie), is a stove top apparatus used to cook delicate sauces such as beurre blanc (http://en.wikipedia.org/wiki/Beurre_blanc), to melt chocolate (http://en.wikipedia.org/wiki/Chocolate) without burning or seizing, or cook any other thick liquid or porridge (http://en.wikipedia.org/wiki/Porridge) that would normally burn if not stirred constantly. It consists of an upper vessel containing the substance to be cooked that is situated above a lower pot of water. When brought to a boil, the steam produced in the lower pot transfers heat to the upper pot.
This apparatus utilizes the properties of water to establish a constant temperature. The phase transition (http://en.wikipedia.org/wiki/Phase_transition) of water from liquid to vapor occurs at 100 °C (212 °F). Therefore, as long as the lower pot does not become pressurized or boiled dry, the maximum temperature contacted by the upper vessel will be the boiling point (http://en.wikipedia.org/wiki/Boiling_point) of water, and scalding or uneven heat is avoided. The steam will either condense on the upper vessel or escape, but the temperature of the vapor phase will remain constant. In order to maximize the efficiency of the heat transfer (http://en.wikipedia.org/wiki/Heat_transfer) process, the base of the upper vessel is constructed of a thinner, lower-gauge metal than the lower pot.
The lid on the upper vessel must fit tightly, or else steam may enter the upper vessel and affect the cooking substance. Manufacturers usually design the upper vessel and the lower pot as a set, a single lid fitting both.
Agree with the above, what I was going to say.
You, in effect, created a double boiler. The purpose of the double boiler is to keep what is in the top portion from actually boiling.
My comment above was about bfayor's explanation.
Pedaling Fool
01-08-2013, 10:24
This thread is a big disappointment. When I read the title I was fully expecting to see you cooking in your sleeping bag.:(
Let me qualify my statement about tall pots. They are more efficient when matched with an appropriate stove.
For the average backpacking stove, a wide pot is more efficient because it captures more of the stoves heat. However, if a tall pot is matched to a burner that is designed to put all its energy into a small diameter, it will be more efficient, because more of the heat is retained by the water in the pot while it boils.
Remember the vast majority of the water in the pot is not actually boiling (changing state), it is just sitting there at 212F absorbing latent heat from the hotter surface of the pot or the steam bubbles.
The more heat the water can absorb without actually changing state, the more efficient the process is. Once a change of state occurs, the steam bubble rises, breaks the surface and all that latent heat is wasted to the atmosphere. If you could design a stove to take the water to 212F and keep it there without boiling, it would be much more efficient (steam is wasted heat in an open cooking pot).
Jetboil does not miss a trick, small diameter burner matched to a tall pot, heat sink built into the pot, built in wind screen, insulating pot cozy with a temp indicator. Its like the Prius of backpacking stoves.
But just like a Prius, there are tradeoffs. The same can be said for a beer can stove and pot. The question is which tradeoffs are important to the user?
Sent from my Nexus 7 using Tapatalk 2
I had mentioned the Stilleto stove. Here are photos of what it looked like. It had a narrow 1" flame tip that concentrated heat onto a small diameter pot. I used a 5" diameter one just to see the difference. The narrow flame boiled 2 cups and caused a 1" diameter column of boil bubbles up the center of the pot. The rolling bubbles occured only where the flame touched the pot.
http://img.photobucket.com/albums/v228/obijiwa/stilettostove012.jpg
http://img.photobucket.com/albums/v228/obijiwa/stilettostove005.jpg