Quote:
Originally Posted by cr45
This looks very promising indeed and stagnation pressure is now where it would be expected to be.
I decided to have a look at the sensitivity of pressure to temperature change for a pressure box.
Using Gay-Lussac's law for a closed container then P2/T2 = P1/T1
Hence P2 = (T2/T1) * P1
Lets assume that the atmos pressure is 101325 Pa and the temperature is 10 C.
Also lets assume that the temperature of the air rises by 0.1 C by the time you take your aero measurements.
The new value for the pressure in the box will be
P2 = ((10.1 +273.15) / (10.0 + 273.15)) * 101325 = 101361
That is a difference in pressure of 36 Pa which is very significant considering the magnitude of the pressure differentials you are trying to measure.
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That much!
With the reference tank you can get it so that there's very little drift, but you need to first let the car's interior settle a lot in temperature, reset the tank pressure to atmospheric and then quickly do the test. Before then repeating the process for subsequent tests. (I also ended up insulating the tank.)
But I think the pitot tube ref pressure is better in every way - cheaper and easier for people to put together, too.
As I have shown above, the stability of the reading also gives me enough confidence to measure very small changes (eg changes in wake pressures).
I think your suggestion genuinely takes on-road aero pressure measurement to the next level. I am very excited by the possibilities that are now open.