Question

Today, I got my toaster malfunctioning. As such I decided to use my pressure cooker for the purpose of toasting my bread (just to see what happens). I oiled the breads separately, and then put them inside the pressure cooker. And waited expecting a whistle. But instead it just blew off.

I can't find any explanation for this observation. Why will this thing work for steam, but not for air. After all they are both supposed to create pressure to the whistle. And if the air have created enough pressure, then why couldn't it just blow the whistle. It apparently seems to violate pascals law.

Answer 1

By the term "blew up" I meant the safety valve blew up.

Answer 2

Hi Iam,

I can give you my thoughts on the matter later. I'm tied up for the rest of the day, unfortunately.

Cheers.

Answer 3

You have to consider that oil becomes a gas much easier... so the pressure would be raised really fast. Aside from that, can't think of anything...

Answer 4

I think I found out the answer. We generally use pressure cookers with watery food. And water has high heat capacity. But bread has very low heat capacity. While we supply heat to the pressure cooker (which has watery food inside) much of the heat is taken in by the food itself, thus not allowing the heat to get contributed to the body of the pressure cooker. On the other hand when it is just bread the body of the pressure cooker instead got heated up.
The safety valve is made of Pb which has very low melting point. As the temperature of the pressure cooker rose the safety valve reached its melting point,and melted away even before anything could escape through the whistle. Thus giving the wrong impression that the pressure cooker was about to explode. The safety valve blew up only because it reached its melting point, and not because there was extreme pressure to melt it.
Infact I can say that the system was very safe, and would have worked if there were no safety valve at all.
To prove this I hung a bottle with a hole in it, over the pressure cooker (arranging it in such a way, that the water fell in drops over the safety valve, and would not allow the safety valve to rise to its melting temperature). And I did the same thing. AND THIS TIME IT WORKED. The whistle blew!
And the only difference was it blew more like a punctured vehicle tyre.

I came to this conclusion few days back, but had to get my pressure cooker repaired from its last accident to actually put it to test.

Answer 5

Hi Iam, I did take a look at your problem, and I did conclude that the answer hinged upon knowing the mechanics of a steam whistle. You seem to have analysed the situation well.

As for the pressure cooker exploding, from the basic physics, you have a situation where the rate of energy input into the system is the same in both instances, but the fact that it explodes in the second suggests the rate of energy dissipation (through the nozzle) is not great enough to counter the rate at which air is thermalized. You get a net increase in energy, and therefore pressure, that the system can't handle.

As for why there is no warning before explosion, like I said, this may have to do with the whistle. I have heard that steam whistles essentially only work with steam. If air is applied, essentially nothing happens.

I have no idea if this is helpful or not. Sorry it took so long - I've been busy.

Good luck.