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Comment by pazimzadeh | Hacker News Reader

spores are the reasons that labs use autoclaves instead of boiling to sterilize things. autoclaves are pressurized so can reach much higher temperatures

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I didn't know pressure is used in sterilisation. Trying to look up why that helps, the Wikipedia article on autoclaves does not explain why but says it uses 1 bar, which would be equivalent to ambient pressure (atmosphere at sea level is ~1.01 bar). Does it maybe mean an additional bar, so twice atmospheric pressure?

Wikipedia also says it uses steam, not just hot air, and again doesn't say anything as to why

Do you know why pressure and steam help?

Edit: I may have found it while looking at what I thought was an unrelated part of the rabbit hole. Gravity displacement sounds like antigravity technology but it's apparently the cheapest autoclave type and means "displace using gravity" (by letting gravity do its thing; not adding in a vacuum pump). What it's displacing is air. This matters because

> Steam at 134 °C can achieve a desired level of sterility in three minutes, while achieving the same level of sterility in hot air requires two hours at 160 °C. (—Wikipedia)

I'm now assuming that pressure must help push the air out, and steam is used for having better heat conductivity. Neither is quite spelled out but that's my best guess currently

I'm pretty sure it is because when there is high pressure the boiling point of water is higher.

How would that help? It never says water, always steam, and it needs to get in all crevices evenly which afaik is easier with a gas than a liquid. If the steam condenses to water (for example due to raising its boiling point above the autoclave's temperature of ~125°C) that'd probably be bad

Apparently at 2 bars, water boils at 120°C. The first temperature mentioned on Wikipedia is 121°C. That's awfully close; but elsewhere the article is quite clear about using higher-than-atmospheric atmospheric pressure. I don't see an explanation that fits all the mentioned facts

> If the steam condenses to water (for example due to raising its boiling point above the autoclave's temperature of ~125°C) that'd probably be bad

If water boils at 100C then steam condenses at 99C. If water boils at 120C then steam condenses at 119C.

> higher-than-atmospheric atmospheric pressure.

What does that mean?

> I don't see an explanation that fits all the mentioned facts

Is it really a difficult concept? If the boiling point rises so does the condensation point.

Yes, pressure raises the boiling point.

But if we want it to be steam, isn't that a bad thing?

Don't we want a nice big safety margin between our active steam temperature and the temperature it condenses?

> But if we want it to be steam, isn't that a bad thing?

Not if we want the steam to be hotter.

> Don't we want a nice big safety margin between our active steam temperature and the temperature it condenses?

Water is in one of three states: liquid, gas, solid. It has to be one of those. Where does the margin come from?

You're phrasing that like you're unaware you can keep heating stean after boiling it. Steam can be 100C at 1atm, it can be 125C at 1atm, it can be 500C at 1atm.

The autoclave does not superheat steam.

Okay. Now we're back at the original unanswered question, which is why it's doing it that way.

Is the pressure helping directly? Why not use a heater; is dangerous levels of pressure easier than an extra heating element?

When you raise the pressure the boiling point of water increases which makes steam which is hotter than 100C. This is not 'superheated' because it is still at the boiling point (which is raised by pressure). This will condense on things inside the autoclave and dump the its heat into the thing killing anything on it. If you superheat the steam it is just as effective as air, which is to say not very. If you want to know more than that, you need to ask someone else, because that's about the extent of my knowledge.

Okay so the answer to my first question of "if we want it to be steam" is "not particularly, letting it condense is good".

It took some roundabouts but we got there. Thanks for the information.

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