As I have forgotten most of what I might or might not have learned in high school…
…why does the valve get cold when releasing air?
…there is a lot of pressure in the tube… then air passes through the valve fast…and then…???
Thanks.
Pressure is related to temperature. When you release air from the tube, the free expansion of the compressed air cools it.
Ric
It’s called adiabatic cooling.
See: http://en.wikipedia.org/wiki/Adiabatic_process
PV=nRT can explain it as well.
I’m sure I’ll get in trouble for saying the wrong thing…but hey.
It’s one of the gas laws. The air coming out of the tire is expanding rapidly, going from 100-150psi to atompshere, 10 or so PSI I think. At the same time the expansion is causing the gas to absorb energy. Heat is one form of energy. So basically the expanding gas is sucking the heat out of the stem.
Now can someone explain that in actual scientific terms?
~Matt
Thanks. Just reading up on thermodynamics…
from wikipedia:
In thermodynamics, an adiabatic process or an isocaloric process is a process in which no heat is transferred to or from the working fluid. The term “adiabatic” literally means an absence of heat transfer.
…
Adiabatic heating and cooling are processes that commonly occur due to a change in the pressure of a gas. Adiabatic heating occurs when the pressure of a gas is increased. An example of this is what goes on in a bicycle pump. After using a pump to inflate a pneumatic tire or ball, the barrel of the pump is found to have heated up as a result of adiabatic heating.
…
Adiabatic cooling occurs when the pressure of a substance is decreased, such as when it expands into a larger volume. An example of this is when the air is released from a pneumatic tire; the outlet air will be noticeably cooler than the tire.
10 or so PSI I think
14.7…it’s one of the few things I remember.
Damn I should have gone with my first reaction of 15PSI. I can argue that 14.7 round to 15, but rounding 10 to 15 is a bit of a stretch.
~Matt