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principle of autoclaving


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#1 lyok

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Posted 18 December 2010 - 10:45 AM

Dear all, I have a question about the use of an old autoclave.

Its a an Stovetop autoclave and I was wondering how it works.

If I am right, then it works by heating the water you add in the stovetop autoclave, wich becomes gas and thus you also build up the pressure ? And then when you get the right pressure you can archieve the 121C and sterilise?

Or?

And why is that pressure needed? Is this because at normal room pressure the water will become gas at 100C and will not get any hotter? Or ?

#2 bob1

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Posted 18 December 2010 - 07:20 PM

Essentially you are correct, the temperature and time is important for autoclaving - some bacteria form spores that are not killed by 100 degrees celsius, but the increased pressure allows this temperature to be reached, thus sterilising the things you put into the autoclave. The increased pressure means that water boils at a higher temperature than it does at standard air pressure according to the triple-point diagram for water.

#3 lyok

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Posted 19 December 2010 - 09:44 AM

Ok thanks

another question:

what is the difference between "solid cycle" and "liquid cycle"?
I do not understand this, is there a difference between autoclaving a solid or liquid?

I did find this at the forum: solid cycle vents rapidly and the liquids will boil over while the liquid cycle vents gradually. This helps prevent boiling.

So is that the only difference?

Edited by lyok, 19 December 2010 - 10:06 AM.


#4 bob1

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Posted 19 December 2010 - 02:55 PM

Correct.

#5 mdfenko

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Posted 20 December 2010 - 07:18 AM

what a lot of people forget is that once water is gaseous you can raise its temperature a lot higher than the boiling point (superheated steam, anyone?).

increasing the pressure in the autoclave can only serve to maintain the transition point at a higher temperature and you won't have to completely convert the liquid to gas to reach the higher temperatures required.
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#6 bob1

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Posted 20 December 2010 - 01:11 PM

You explain it more eloquently than I did.

#7 lyok

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Posted 21 December 2010 - 07:41 AM

what a lot of people forget is that once water is gaseous you can raise its temperature a lot higher than the boiling point (superheated steam, anyone?).

increasing the pressure in the autoclave can only serve to maintain the transition point at a higher temperature and you won't have to completely convert the liquid to gas to reach the higher temperatures required.


I do not understand what you mean with: "increasing the pressure in the autoclave can only serve to maintain the transition point at a higher temperature and you won't have to completely convert the liquid to gas to reach the higher temperatures required"

By raising the pressure you can keep more water in the fluid phase in stead of the gasous fase. Is that what you mean or?

I am getting confused no to be honest.

#8 philman

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Posted 21 December 2010 - 07:48 AM

At atmospheric pressure, water boils at 100 degrees. Increasing the pressure that much means that water boils at a higher temperature. So the media you are autoclaving still rises in temperature to 120 degrees or so, but since it is at high pressure also, it doesn't all boil off. If you did not raise the pressure, when you opened the autoclave again afterwards then most of the liquids would have boiled off, defeating the point of autoclaving them!

#9 lyok

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Posted 21 December 2010 - 07:52 AM

At atmospheric pressure, water boils at 100 degrees. Increasing the pressure that much means that water boils at a higher temperature. So the media you are autoclaving still rises in temperature to 120 degrees or so, but since it is at high pressure also, it doesn't all boil off. If you did not raise the pressure, when you opened the autoclave again afterwards then most of the liquids would have boiled off, defeating the point of autoclaving them!


You mean if you would not lower the pressure?

The pressure of the autoclave is normally lowered at the end very slowly to prevent that all the media would boil "over".

#10 mdfenko

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Posted 21 December 2010 - 11:18 AM

i feel so stupid, i was so fixated by the temperature of steam that i forgot the actual reason for high pressure. philman is correct, the pressure is so that autoclaved liquids won't boil away during the process.

lyok, in answer to your last question, yes, the pressure is lowered slowly when autoclaving liquids to prevent them from boiling off.

it is not important to release pressure slowly when autoclaving solids but the quick drop in pressure may allow them to eliminate any liquid which may have condensed (this is speculation on my part).

Edited by mdfenko, 21 December 2010 - 11:19 AM.

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#11 lyok

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Posted 22 December 2010 - 06:12 AM

Ok

Just to check if I am understanding it: higher pressure means that you need a higher temperature to boil (change from water to gasous phase), thus when raising the pressure you also raise the temperature by wich water boils and in this particular case you assure that you reach a certain pressure that you can savely sterilise the products at 121 without losing all the contents of the products (so you wont boil away everything).


Anyway, this in my mind, I have to following questions:

- When you drop the pressure too rapidly, then I would think that you would turn too much of your product into a gasous phase and thus boil away the producht (when looking at the phasediagram of water). Is this correct then?
(I do not understand what you mean by: "but the quick drop in pressure may allow them to eliminate any liquid which may have condensed (this is speculation on my part" , allow to elimitate the liquid that has condensed? You mean that the condensed liquid would turn into gas and this be lost? (loss of medium?))?

- I see why you need the pressure system for autoclaving medium, but what for solids?
If I would simply heat the autoclave untill I reach 121C , then why wouldnt this work without the pressure? Is this because without the pressure all the water that turned into gas would be away? (escape , since you do not keep it under pressure, you do not close the autoclave)?
Or is it really physically impossble to heat the inside of the autoclave to 121C at normal pressure?


- Why do people wrap so much in aluminumfoil? I would think that this foil would prevent a very good sterilisation? Since the steam might not be able to penetrate the foil as good as without foil?
Whats the idea behind the use of foil?

#12 mdfenko

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Posted 22 December 2010 - 08:32 AM

Just to check if I am understanding it: higher pressure means that you need a higher temperature to boil (change from water to gasous phase), thus when raising the pressure you also raise the temperature by which water boils and in this particular case you assure that you reach a certain pressure that you can safely sterilize the products at 121 without losing all the contents of the products (so you wont boil away everything).


Anyway, this in my mind, I have to following questions:

- When you drop the pressure too rapidly, then I would think that you would turn too much of your product into a gaseous phase and thus boil away the product (when looking at the phase diagram of water). Is this correct then?
(I do not understand what you mean by: "but the quick drop in pressure may allow them to eliminate any liquid which may have condensed (this is speculation on my part" , allow to elimitate the liquid that has condensed? You mean that the condensed liquid would turn into gas and this be lost? (loss of medium?))?

i was talking about autoclaving solids when i said that. by reducing the pressure rapidly the temperature will be high enough to allow any liquid which had condensed from the steam to boil away.

- I see why you need the pressure system for autoclaving medium, but what for solids?
If I would simply heat the autoclave until I reach 121C , then why wouldn't this work without the pressure? Is this because without the pressure all the water that turned into gas would be away? (escape , since you do not keep it under pressure, you do not close the autoclave)?
Or is it really physically impossible to heat the inside of the autoclave to 121C at normal pressure?

solids, such as surgical implements, used to be sterilized by placing them into boiling water for a period of time (remember sterilizers in doctors' offices?). by using the higher temperatures obtained with steam it takes less time and is more effective than simply boiling. if you don't close the vessel (autoclave) then the steam will cool too rapidly causing the solids to get wet and sterilization may not be efficient. of course you can heat without pressure (look at ovens) but steam gives you more thermal mass and may allow more even heating of the material being autoclaved (this is all supposition on my part, reality may differ from what i say).

- Why do people wrap so much in aluminum foil? I would think that this foil would prevent a very good sterilization? Since the steam might not be able to penetrate the foil as good as without foil?
Whats the idea behind the use of foil?

foil (and autoclave bags, wrapping paper, vessels) are used to ensure that what you are autoclaving remains sterile until you remove it from the package. the steam doesn't really need to touch the item, it just needs to heat it.
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#13 lyok

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Posted 22 December 2010 - 08:42 AM

solids, such as surgical implements, used to be sterilized by placing them into boiling water for a period of time (remember sterilizers in doctors' offices?). by using the higher temperatures obtained with steam it takes less time and is more effective than simply boiling. if you don't close the vessel (autoclave) then the steam will cool too rapidly causing the solids to get wet and sterilization may not be efficient. of course you can heat without pressure (look at ovens) but steam gives you more thermal mass and may allow more even heating of the material being autoclaved (this is all supposition on my part, reality may differ from what i say).


And also because you need a higher temperature then 100C to kill the bacteria (I think? or not?)

Or can you kill bacteria at 100C if you just let it boil long enough in stead of going up to 121C?


foil (and autoclave bags, wrapping paper, vessels) are used to ensure that what you are autoclaving remains sterile until you remove it from the package. the steam doesn't really need to touch the item, it just needs to heat it.


Ok, but I have noticed that they put aluminum foil around boxes containing pipettetips? Seems rahter strange since they remove this foil right after autoclaving and they put the boxes without the foil in the cabinets or drycabinets..

I can understand you put foil around pieces of paper or material (needles or something) in order to keep it sterile, but putting it around closed boxes?

#14 mdfenko

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Posted 22 December 2010 - 08:45 AM

they're just wasting aluminum foil. i sterilize tip boxes held closed with autoclave tape.
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#15 lyok

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Posted 22 December 2010 - 08:54 AM

they're just wasting aluminum foil. i sterilize tip boxes held closed with autoclave tape.


you wrap the tape all around or just in the middle of the boxes to keep it closed?

I have seen them do the following thing: wrap tape all around and then aluminim foil.. and then when its done, they remove the foil, put it in the drychamber.. after a while take it out, use it and put it in a cabinet next to the flow for using it the next time..
For me this seems rather strange.. and not logical.. but heck, I am not a microbiologist...




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