Exhaust Pressure sensing

[Bernd Felsche]

Eck, Joel tapped away at the keyboard with:
> Actually, it will. Check out this site: 
> 
> http://dbhs.wvusd.k12.ca.us/GasLaw/Gas-Gay-Lussac.html
> 
> It describes Gay-Lussac's gas law, which shows the direct
> correlation between temperature and pressure of a gas given
> contant amount and volume. As temperature increases, pressure
> increases, and, as temperature decreases, so does pressure.

That gas law assumes a constant _amount_ (mass) of gas. In this
case, it isn't constant as any drop in pressure due to gas cooling
in the side-tube will cause the amount of gas to increase as the
main exhaust pressure forces some more into the tube.

So, although there is cooling, the pressure doesn't change (much) as
the amount of gas in the tube increases to establish a pressure
equilibrium . Without pressure equilibrium, you have a flow which
means that mass moves.

Although the gas temperature at the opening of the tube will be
close to exhaust temperature and at the sensor end much cooler, the
volume occupied by a particular molecule of gas within the tube will
be compressed by adjacent, hotter molecules.

The pressure indicated at the end of the cool tube will be therefore
be indicative of the exhaust pressure. It won't be identical due to
gas dynamics, but it'll be a fair measure. Just watch out for
exhaust pressure pulses.

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