principle of autoclaving - pressure ? (Dec/18/2010 )

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 121°C and sterilise?

Or?

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

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.

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?

Correct.

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.

You explain it more eloquently than I did.

mdfenko on Mon Dec 20 15:18:55 2010 said:

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.

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!

philman on Tue Dec 21 15:48:37 2010 said:

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".

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).