Boiler/autoclave/pressure cooker question

[Buchan]

Knowing the great mix of folk on here, someone may be able to help me. I'm doing a bit of research on sterilisation of equipment. An autoclave is standard for cleaning theatre kit and works like a domestic pressure cooker - set volume, increase the temperature, pressure builds up so more heat is needed to get the water to boil - ideal gas equation.
The settings have me puzzled, with the lowest used one being 121C at 15psi. But that is atmospheric pressure so it should be 100C. I feel I'm missing something. If I do the PV/T calculation I get 18psi.
Any thoughts. I am, as usual, overthinking this as it really is a question of switch on and let it work as someone else has done the calculations, but I'd like to know.

 

[GlennUK]

Knowing the great mix of folk on here, someone may be able to help me. I'm doing a bit of research on sterilisation of equipment. An autoclave is standard for cleaning theatre kit and works like a domestic pressure cooker - set volume, increase the temperature, pressure builds up so more heat is needed to get the water to boil - ideal gas equation.
The settings have me puzzled, with the lowest used one being 121C at 15psi. But that is atmospheric pressure so it should be 100C. I feel I'm missing something. If I do the PV/T calculation I get 18psi.
Any thoughts. I am, as usual, overthinking this as it really is a question of switch on and let it work as someone else has done the calculations, but I'd like to know.

I think you may be getting confused with absolute pressure and gauge pressure.

Typically gauges read zero at atmospheric pressure, when in fact the actual pressure is ~1 bar.

Pressure cookers (and presumably autoclaves) uses a measure which indicates the increase in pressure. So the 15lb on the pressure cooker actually refers to atmospheric pressure plus the 15lbs.

One other thing, the temperature should be in Kelvins not Centigrade, this accounts for the relatively small temp difference versus your expectations i think.

 

[Freeforester]

Mind the prions.

 

[Rafter]

Read your steam tables, grasshopper.

The answer you seek is within.

But remember gauges start at atmospheric pressure, and read 0 at that point.

120°c is at 1.9848 bar - or close enough to 1 bar gauge.

Fuggedabout ideal gas laws, saturated steam is a vapour - and it is constantly trying to turn back into water via energy transfer.

 

[Buchan]

Awesome! You have defeated a whole bunch of vets who couldn't answer this and it makes perfect sense. Thanks

Mind the prions - indeed, little gits

 

[Selous]

@Buchan Don’t forget that you need a vacuum autoclave to adequately sterilise wrapped packs. Non vacuum units or pressure cookers are only suitable for unwrapped instruments!!

 

[GlennUK]

Awesome! You have defeated a whole bunch of vets who couldn't answer this and it makes perfect sense. Thanks

Mind the prions - indeed, little gits

In my experience of working with vets they're usually v.good at putting their hands/arms inside animals, not so good when comes to mechanical things though!!!

 

[Buchan]

In my experience of working with vets they're usually v.good at putting their hands/arms inside animals, not so good when comes to mechanical things though!!!

I'll have you know I stripped and reassembled an MGB and it went vroom second turn of the key

 

[vitalspark]

I remember coming home from work to the cooker boiling away. I asked hopefully what was for tea. My wife replied "Nothing, we're spaying the cat.". I then had the pleasant job of manning the headtorch as she set to work on the poor beast, on the kitchen table.