This is what a pitting by a pedantic, overeducated mechanical engineer looks like.
My wife found a YouTube video last night that purports to demostrate the cooling associated with adiabatic expansion of gases. The problem is that it doesn’t actually demonstrate what the instructor in the video claims it demonstrates.
Video is here. I’m guessing it was made perhaps 20-30 years ago, because the instructor - a middle-aged Paul G. Hewitt - is now about 82 years old. That means a lot of people have seen it by now, and have probably shared these misconceptions with their friends.
What’s the problem?
First, he demonstrates that pursing your lips and blowing forcefully on your hand feels cooler than an open-mouthed exhale. Then he claims that this is because pursing your lips elevates the pressure in your mouth/lungs, so that when this air leaves your mouth, it expands to atmospheric pressure and cools. While this is technically true, the pressure rise is tiny - a fraction of a PSI if you’re blowing lightly, perhaps as much as 1.5 psi if you’re really pushing as hard as you can - and so the temperature drop when you exhale is correspondingly small, just a few degrees. The cooling effect you feel when blowing on your hand is almost entirely due to your breath mixing with cooler ambient air, and the increased convective cooling due to the higher air velocity.
Next he turns to a water-filled pressure cooker operating at full temperature, as evidenced by the jiggling relief valve - probably 15 psi, and a temperature of 250 degrees F. He removes the weight from the valve, and the pressure cooker produces a steam jet. He claims that adiabatic expansion of the 250-degree steam as it exits the pressure cooker results in a temperature drop. Then he demonstrates this by holding his bare hand in the steam jet - about 12 inches above the pressure cooker. He refuses to hold his hand lower for fear of burning it, because yes, that steam jet is damn hot right when it comes out of the pressure cooker. In fact, it’s at exactly 212 degrees. Yes, it’s undergone adiabatic expansion/cooling at that point, but most of the temperature drop (from 212 to whatever temp his hand can tolerate) happens after that expansion, and is because of mixing of the 212-degree steam with cool ambient air.
If you don’t have a background in the physical sciences, maybe this all seems trivial to you. But it bugs the shit out of me to think that people - a lot of people - are being miseducated about the phenomena seen in the video. We all hold our heads in frustration at the state of science education in this country, and this guy is part of the problem. No doubt people believe him because he’s in a position of authority (by golly, he’s a physics instructor!). Goddamit, my wife watched the video, and now I’m struggling to convince her that he’s wrong. If I stick to my guns, it’s a fair bet that she’ll get pissed off at me and accuse me of thinking she’s stupid. If it comes to that, well then I’m blaming Hewitt.