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shirley knott
not my real name

Registered: 11/11/02
Posts: 9,105
Loc: London
Last seen: 8 years, 16 days
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that's all you can do, yep
-------------------- buh
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Mykey
spectraltraveler

Registered: 04/07/04
Posts: 542
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Re: Altitude [Re: ATWAR]
#3084431 - 09/02/04 07:58 PM (20 years, 4 months ago) |
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Oh boy do I love a good debate!!
Ok,let me try and explain my position on this debate with a little more detail.
First of all let my say that we are both right and wrong at the same time.
I fully understand the basic physics of the effects of atmospheric pressure on the amount of kinetic energy required to cause water to boil. What I dont understand is how this is the total explanation of why cooking times or pressures have to be adjusted due to increases increases in altitude and lower atm. pressure. If one were canning using the old open type canners which do not use pressure this would be a totally viable explanation,but we are talking about thermodynamics within a semi-enclosed system which takes things to a new level. Let me say that I am absolutely sure that there is a constant and direct relation between water steam pressure and temperature. A vessel which has been vented properly and pressurized to an absolute pressure of around 30psi will always be 250deg/f no matter what planet you are on. lol So,let me explain why we are both correct!
There are two way in which pressure canners are designed in respect to pressure control. The first is a standard pressure gauge that gives a you variable pressure capability without changing the weight of the little wobbler. This type of gauge gives you a reading which actually takes into account the atmospheric pressure. Zero on one of these gauges ,when properly callibrated,is equal to one atmosphere or 14.7 psi. This number is somewhat arbitrary since the actual pressure at any given point on earth at sea level can fluctuate to some degree,so this number is a static pressure number which does not take into account temp wind ect. So when your pc gauge reads 15psi the absolute pressure within is actually 29.7psi which will always give you a temperature of 249.8deg/f or 121deg/c. This explains why you have to adjust your pc'ing time,or preferably pressure to compensate for the innaccurate representation of the internal absolute pressure given by the gauge. The gauge is just simply not designed to read accurately when the external pressure drops below 14.7psi.
The second type of regulating mechanism is a simple weight. This type depends almost solely upon the force of gravity to regulate the internal pressure therefore there would be no reason to change cooking times at all since the internal temp will be the same no matter where you are.
Here is a simple chart I put together to help everyone who wonders about their elevation and pc'ing effectively.
Elevation (feet)...........Std. Pressure Dial Type...... Weighted Type
0------------------------------10-------------------------10
1000---------------------------10.5-----------------------15
2000---------------------------11-------------------------15
3000---------------------------11.5-----------------------15
4000---------------------------12-------------------------15
5000---------------------------12.5-----------------------15
6000---------------------------13-------------------------15
7000---------------------------13.5-----------------------15
8000---------------------------14-------------------------15
9000---------------------------14.5-----------------------15
10000--------------------------15-------------------------15
**NOTE** These figures are given such that the internal temp of the pc reaches 240deg/f. This is the temp recommended to kill everything including bacterial endospores.
**ONE MORE NOTE** To those of you who skip the 10min venting at the beginning of your pc'ing process. Due to the relationship between volume,pressure,and temps you may not be reaching the temps you think you are if you do not vent all of the air out of the pc before you begin to build pressure. For the temp/pressure relationship to be constant and true the interior of the pc must be airless.
I hope this helps and clears things up on this subject.
*EDIT* woops,I took a break to get something to eat and didnt realize there so many replies while I was gone.
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Edited by Mykey (09/02/04 08:10 PM)
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ATWAR
Connoisseur

Registered: 01/26/03
Posts: 1,640
Loc: #108768 in line...
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Re: Altitude [Re: Mykey]
#3084736 - 09/02/04 09:17 PM (20 years, 4 months ago) |
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Quote:
Mykey said: The second type of regulating mechanism is a simple weight. This type depends almost solely upon the force of gravity to regulate the internal pressure therefore there would be no reason to change cooking times at all since the internal temp will be the same no matter where you are.
You also must understand that not only is gravity acting upon the weight, but the atmospheric pressure is also. Not only is gravity holding the weight down, air pressure is as well. When the air pressure is reduced, the pressure upon the weight is not the same as it is at sea level...
-------------------- To give is to live...
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Mykey
spectraltraveler

Registered: 04/07/04
Posts: 542
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Re: Altitude [Re: ATWAR]
#3084854 - 09/02/04 09:45 PM (20 years, 4 months ago) |
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You are absolutely correct (that is why I said almost soley) ,but the fact that steam pressure and temp relationships doesnt change due to altitude is a fact. If you have absolute steam pressure of 29.7psi you will always have a temp of 249.8deg. I dont think the difference in downward force is going to make enough of a difference in the absolute pressure to be of any consequence.
I paid attention in physics class too! lol
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fastfred
Old Hand



Registered: 05/17/04
Posts: 6,899
Loc: Dark side of the moon
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Re: Altitude [Re: Mykey]
#3085807 - 09/03/04 01:41 AM (20 years, 4 months ago) |
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What is "absolute steam pressure"?
-FF
Edited by fastfred (09/03/04 02:38 AM)
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fastfred
Old Hand



Registered: 05/17/04
Posts: 6,899
Loc: Dark side of the moon
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Re: Altitude [Re: fastfred]
#3086231 - 09/03/04 05:01 AM (20 years, 4 months ago) |
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From:USDA Complete Guide to Home Canning
Process Adjustments at High Altitudes
Using the process time for canning food at sea level may result in spoilage if you live at altitudes of 1,000 feet or more. Water boils at lower temperatures as altitude increases. Lower boiling temperatures are less effective for killing bacteria. Increasing the process time or canner pressure compensates for lower boiling temperatures. Therefore, when you use the guides, select the proper processing time or canner pressure for the altitude where you live. If you do not know the altitude, contact your local county Extension agent.
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ATWAR
Connoisseur

Registered: 01/26/03
Posts: 1,640
Loc: #108768 in line...
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Re: Altitude [Re: Mykey]
#3087399 - 09/03/04 12:31 PM (20 years, 4 months ago) |
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Quote:
Mykey said: You are absolutely correct (that is why I said almost soley) ,but the fact that steam pressure and temp relationships doesnt change due to altitude is a fact. If you have absolute steam pressure of 29.7psi you will always have a temp of 249.8deg. I dont think the difference in downward force is going to make enough of a difference in the absolute pressure to be of any consequence.
I paid attention in physics class too! lol

The fact of the matter is, at high altitude you will not have the same absolute pressure as that at sea level. Pressure cookers maintain their pressure in direct proportion to the outside pressure. While the weight will maintain a 15psi difference at any altitude, the absolute pressure does not stay the same. While you would have an absolute pressure of 29.7psi at sea level, the same pressure cooker operating at 5000 feet will only have an absolute internal pressure of 27.2psi (barometric pressure differences aside)...
This is why pressure/times must be increased for canning, cooking, sterilizing, etc. Because the difference in atmospheric pressure DOES affect the action of the weights ability to maintain pressure. This difference in pressure does not result in less corrected pressure (I.E. - it will still be 15psi), but results in less absolute pressure.
This is the reason why pressure/cooking times must be increased at higher altitudes when canning or sterilizing. A google search should provide you with altitude adjustment tables. My pressure cooker even has them in the manual...
-------------------- To give is to live...
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fastfred
Old Hand



Registered: 05/17/04
Posts: 6,899
Loc: Dark side of the moon
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Re: Altitude [Re: ATWAR]
#3087502 - 09/03/04 12:56 PM (20 years, 4 months ago) |
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All I've been able to find is the 5% per 1000 ft rule...
-FF
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Fucknuckle
Dog Lover

Registered: 04/24/04
Posts: 6,762
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Re: Altitude [Re: ATWAR]
#3087511 - 09/03/04 12:59 PM (20 years, 4 months ago) |
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I think the entire two sideness to this thread is because of the simple answer. You know one of those things people will debate about until somebody goes HUhhhhhhhhh I get it now!!
That happened to me.
People are confusing the fact that if you set the pressure in an inclosed container.( Just for disscusion lets say 15 psi..) That no matter what pressures you subject the outside of the container to the pressure inside remains the same, Very much a true statement. There by wining the point of " you do not need to change Pcing time in realton to the atmosphereic pressures. " Well if a PC were an enclosed vessel you would might be right.
But even if it were sealed you would still be wrong. When you close the lid on your pc at 1,000 feet above sea level you have a base psi Which is now the starting air pressure inside the container.
Same at 15,000 feet when you closed the lid. You start at that air pressure level when the lid goes on. But now you are starting from a different air pressure then when you were down in the valley. Which means different cooking times and cooking pressures.
The air pressure on the weight deff will effect how it operates.
But even after finnaly going thru a big, Aa HA I got it!! I still ain't sure.
-------------------- What it is, is what it is my Brother.
It is as it is, so suffer thru it.
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fastfred
Old Hand



Registered: 05/17/04
Posts: 6,899
Loc: Dark side of the moon
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Well you can probably trust the USDA... I don't think they would lie about something involving safety of foods.
From:USDA Complete Guide to Home Canning
"Process Adjustments at High Altitudes Using the process time for canning food at sea level may result in spoilage if you live at altitudes of 1,000 feet or more."
-FF
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Mykey
spectraltraveler

Registered: 04/07/04
Posts: 542
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Re: Altitude [Re: ATWAR]
#3088009 - 09/03/04 03:17 PM (20 years, 4 months ago) |
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Quote:
ATWAR said:
Quote:
Mykey said: You are absolutely correct (that is why I said almost soley) ,but the fact that steam pressure and temp relationships doesnt change due to altitude is a fact. If you have absolute steam pressure of 29.7psi you will always have a temp of 249.8deg. I dont think the difference in downward force is going to make enough of a difference in the absolute pressure to be of any consequence.
I paid attention in physics class too! lol

The fact of the matter is, at high altitude you will not have the same absolute pressure as that at sea level. Pressure cookers maintain their pressure in direct proportion to the outside pressure. While the weight will maintain a 15psi difference at any altitude, the absolute pressure does not stay the same. While you would have an absolute pressure of 29.7psi at sea level, the same pressure cooker operating at 5000 feet will only have an absolute internal pressure of 27.2psi (barometric pressure differences aside)...
This is why pressure/times must be increased for canning, cooking, sterilizing, etc. Because the difference in atmospheric pressure DOES affect the action of the weights ability to maintain pressure. This difference in pressure does not result in less corrected pressure (I.E. - it will still be 15psi), but results in less absolute pressure.
This is the reason why pressure/cooking times must be increased at higher altitudes when canning or sterilizing. A google search should provide you with altitude adjustment tables. My pressure cooker even has them in the manual...
Look at what I wrote in the post that you quoted ATWAR! I said that you were right about the static pressure of the atmosphere having an effect on the weight,but what I am saying is that it doesnt have enough of an effect to warrant any change in cooking times in a weighted gauge cooker. You said that at 5000ft the absolute pressure would be reduced to 27.2psi,which is still well above the 240deg mark needed for proper sterilization. This is the point I am trying to make. Yes I am sure that you can put pen to paper and come up with the altitude at which the absolute pressure will reach around 24.8psi and from there up you will certainly need to compensate. The chart that I posted for pressure compensation in dial gauge and for weighted gauge is absolutely correct,I assure you! It only goes to 10000ft above sea level so if you live in Nepal or something you are SOL. I might also add that everyone who thinks that adding time is the way to fix the problem vs. upping the pressure ,think again!
By the way,you are one smart cookie ATWAR, I am just trying to enjoy a little constructive arguement,no need to get agitated and use all the bold type and start rolling your eyes at me!
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ATWAR
Connoisseur

Registered: 01/26/03
Posts: 1,640
Loc: #108768 in line...
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Re: Altitude [Re: Mykey]
#3088133 - 09/03/04 03:53 PM (20 years, 4 months ago) |
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The reduced temperature will have an effect on sterilization. This is why altitude adjustments need to be made. Now this thread is changing direction towards what temperature/time proper sterilization occurs... If you do not increase the pressure, then the effectiveness of your sterilization cycle will decline...
I rolled my eyes because you said the difference in pressure is not enough to make a difference. I believe it is, and thus the reason why it is recommended (by Stamets and other sources) that altitude adjustments be made. I believe these recommendations have their merit, where as you are saying it does not matter. I believe they would not make such recommendations without proper cause. I am not agitated, it takes much more than that... 
To quote an online source: When using a pressure cooker, the pressure must be increased by one pound per 2,000 feet of increased elevation to reach the same temperature as 15 pounds of pressure at sea level.
This is just getting dumb now. I give up...
-------------------- To give is to live...
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fastfred
Old Hand



Registered: 05/17/04
Posts: 6,899
Loc: Dark side of the moon
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Re: Altitude [Re: Mykey]
#3088150 - 09/03/04 04:00 PM (20 years, 4 months ago) |
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Quote:
Mykey said: Look at what I wrote in the post that you quoted ATWAR! I said that you were right about the static pressure of the atmosphere having an effect on the weight,but what I am saying is that it doesnt have enough of an effect to warrant any change in cooking times in a weighted gauge cooker.
That would seem to dispute the USDAs recommendation, and all the literature I've read on pressure cooking.
Quote:
You said that at 5000ft the absolute pressure would be reduced to 27.2psi,which is still well above the 240deg mark needed for proper sterilization. This is the point I am trying to make.
Wrong. If the media or substrate you are using calls for 15min @ 121 C, then no you are not getting that at all if you are at 5000ft. You are getting the equivalent of 15min @ 115 C. Obviously this is not the same ammount of sterilization.
Quote:
Yes I am sure that you can put pen to paper and come up with the altitude at which the absolute pressure will reach around 24.8psi and from there up you will certainly need to compensate.
Even as little as 1000ft can negatively affect sterilization according to the USDA and every site I've seen suggests adjusting for elevation at 2000ft or better.
Since the average elevation of North America is 2,362 feet most people should be applying an adjustment.
Quote:
I might also add that everyone who thinks that adding time is the way to fix the problem vs. upping the pressure ,think again!
In fact, adding time is exactly what the USDA recommends.
You need to be checking your facts better.
-FF
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Fucknuckle
Dog Lover

Registered: 04/24/04
Posts: 6,762
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Re: Altitude [Re: Mykey]
#3088406 - 09/03/04 05:09 PM (20 years, 4 months ago) |
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Hey I think the atmosphereic pressure at sea level is close to 14 lbs per sq inch.
and
Most weights on the pressures cookers i've seen are just about 1 sq inch in surface area.
So
I would conclude that moving up in the atmosphere 10,000 feet or more would effect the way a weight would work on a PC ,by a large margin.
-------------------- What it is, is what it is my Brother.
It is as it is, so suffer thru it.
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hyphae
born to grow


Registered: 12/13/02
Posts: 6,228
Loc: the rain forests
Last seen: 13 years, 8 months
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Quote:
No, 15 PSI as measured in a pressure cooker is in relation to the air pressure outside of the pressure cooker, it's actually a 15 PSI difference.
Best answer yet  Your right Fucknuckle 14.7
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