SpudFiles

Well this weekend I took my metere propane combustion up itno the mountains and here in vegas my gun worked perfect with not one misfire but up in th mountains my gun had frequent misfires with pisspoor performance with most shots,with a few exceptions.

I could not get combustion on most shots even though I have a my mter properly set and I was getting ignition.

With a tennis ball I would sometimes click the igniter and the gun would combust and the tennis ball would travel half way up the barrel and the gas would escape through my endcap theads.

My question is whether altitude can affect performance negatively

I did have a few succesful shots using golfballs with one shot clearing a small mountain

The oxygen content is much lower at higher altitudes, which screws with the fuel air mixtures in combustion launchers.

yea, oxygen is lower at higher altitude. i think at 25,000 feet you need to take 7 breaths to get the same amount of oxygen at sea level that you get with one breath. so the answer is yes, thats why.

what altitude were you at by the way?

You may need to add an oxygen bottle to the mix,if your going to be in the mountains frequently.Just don't blow yourself up Ahh, oxygen pure hazard.
Come to think of it,would lowering the propane amount change the mix enough for it to combust?Might ought to try different fuel ratios.

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ahah with my sch 80 chamber I should play with oxygen.
I have no idea what altitude I was at.
Wasn't too high though.

Well... the air is less dense as you get higher, leading to a lower molar count in your chamber of fuel and air. 1

Atmospheric pressure is lower, meaning you generally need to compensate for your fueling equation for the difference from the sea level value of 14.7 PSIA. 2

For example, a liter of gas at sea level and ST, (0* C), would contain 1/22.4 moles of gas. A liter of gas at 1/2 SP, (.5 atm), and ST, (0* C), would only contain 1/44.8 moles of gas, meaning less energy for each shot, (chamber volume of reactants).

Final example. Calculated percentage of fuel/air, (by molar count), at sea level using 1 atm as your atmospheric pressure is 4.2%. Calculated percentage of fuel/air at 1/2 atm still using 1 atm is 8.4%.

NOTE: Although it may look it, the above numbers weren't pulled out of my ass saying, "well, 1 atm is 4.2%, so 1/2 atm should be 8.4%". I used a combination of Boyle's law with the ideal gas law to calculate these values, using a 1/2 cm diameter meter pipe 3 cm long, (volume = 3/4pi milliliters), and a one liter chamber. Below are the obligatory calculations. From this, it would be conclusive to say that atmospheric pressure, (in atmopsheres), is inversely proportional to fuel/air percentage if you are still using 1 atm for your fueling calculations. Written: %₁P₁ = %₂P₂. From this as well, it would be analytical to say that you should cut your meter pressure according to the direct proportion P₁A₂ = P₂A₁, where P is you meter pressure and A is atmospheric pressure.

So, if you were to go, say, 10,000 feet up into the mountains, where the atmospheric pressure is around .69 atm, then you would need to adjust you meter pressure using the above proportion.

My pencil and paper wrote: <table><tr><td><pre>
(3/4pi)p = (.042(1000))1
p = 56pi^-1

n = <U>56pi^-1 atm * 3/4pi * 1000^-1 L</U>
<U>atm * L</U>
.0821 mol * K * 273 K

n = .042/22.4

<U>.042/22.4</U>
1/22.4
<B>.042</B>

<br>

<U>.042/22.4</U>
1/44.8
<B>.084</B>
</pre></td></tR></table>

EDIT: stupid algebra.

I would doubt that you were any higher then 10K feet, probably more like 5K feet. So i'm surprised that the change in barometric pressure was enough to get the fuel ratio completely out of the combustion range. (Unless you really were up at 10K ft or more.) To make a significant change in the pressure that your gauge should be set at you have to go way up high. The correction for 5,000 ft is not enough to make the gun not fire (though it won't perform nearly as well). At 10,000 I would think you are in the altitude range where the gun will not fire. But, there aren't that many places in the US where you can easily get to 10K feet. Denver is at ~5K feet, Pikes Peak ~12K feet. Most mountain passes are well below 10K feet so there aren't many places where you can drive and get above about ~8K feet. (There is noplace in the eastern US where where you can get above what, 3K feet?)

Of course, if you meter is off at your normal altidude (i.e., if it is fueling too rich) then it will be easier to get the mixture out of the combustable range at higher altitudes.

If you normally fire at 2,000 feet then going up to 6,000 feet has a smaller affect, but if you calculated the fuel ratio for sea level then it might well be way out of wack at 6K.

The fact that the gun fired sometimes suggests to me that it is not an altitude affect on the amount of fuel needed. Something else is going on.

With a tennis ball I would sometimes click the igniter and the gun would combust and the tennis ball would travel half way up the barrel and the gas would escape through my endcap theads.

Something wrong with the endcap? Tennis balls should expand at higher altitude so I wouldn't think it is an altitude affect on the tennis ball.

Of course, the altitude affect on a golfball should be pretty much zero.

Do you have a chamber fan?

One advantage of fueling with a syring at atmospheric pressure is that the volume of fuel is the same at all altitudes, and at all barometric pressures.

Thanks for that last bit of info " One advantage"
It seems most people don't understand atmospheric pressure!

Peace
Scotty

jimmy101 wrote:I would doubt that you were any higher then 10K feet, probably more like 5K feet. So i'm surprised that the change in barometric pressure was enough to get the fuel ratio completely out of the combustion range. (Unless you really were up at 10K ft or more.)

Code: Select all

 Altitude  ---------Pressure---------
    ft	       kPa       PSIA       ATM
       0     101.3       14.7	     1
  1000      97.7       14.2      0.964
  2500      92.5       13.4      0.913
  5000      84.3       12.2      0.832
10000      69.7       10.1      0.688
20000      46.6         6.8      0.460

To make a significant change in the pressure that your gauge should be set at you have to go way up high. The correction for 5,000 ft is not enough to make the gun not fire (though it won't perform nearly as well). At 10,000 I would think you are in the altitude range where the gun will not fire. But, there aren't that many places in the US where you can easily get to 10K feet. Denver is at ~5K feet, Pikes Peak ~12K feet. Most mountain passes are well below 10K feet so there aren't many places where you can drive and get above about ~8K feet. (There is noplace in the eastern US where where you can get above what, 3K feet?)

Of course, if you meter is off at your normal altidude (i.e., if it is fueling too rich) then it will be easier to get the mixture out of the combustable range at higher altitudes.

If you normally fire at 2,000 feet then going up to 6,000 feet has a smaller affect, but if you calculated the fuel ratio for sea level then it might well be way out of wack at 6K.

The fact that the gun fired sometimes suggests to me that it is not an altitude affect on the amount of fuel needed. Something else is going on.

With a tennis ball I would sometimes click the igniter and the gun would combust and the tennis ball would travel half way up the barrel and the gas would escape through my endcap theads.

Something wrong with the endcap? Tennis balls should expand at higher altitude so I wouldn't think it is an altitude affect on the tennis ball.

Of course, the altitude affect on a golfball should be pretty much zero.

Do you have a chamber fan?

One advantage of fueling with a syring at atmospheric pressure is that the volume of fuel is the same at all altitudes, and at all barometric pressures.

I doubt I was at more than 5000ft.
I did have a chamber fan.
and I shot the cannon back at home after the trip and the performance was phenomenal with no adjustment from when I tryed to shoot it on the trip.