SpudFiles

for my science project, I want to find the optimum air to propane ration when firing a spud gun, how can I go around finding how much propane do I put into the chamber when I want to use 1% propane, 2% propane and so on and so forth?

Your not going to get very broad results, as propane ingnites only at tricky ratios. You would need to make a propane meter, and use the calculator on burnt latke.com to get the pressure needed. good luck

"How do you tell how much propane you injected?"

Easy... measure it!

Two options:
1) Constant volume meter at variable pressure, such at that outlined at <a href="http://www.burntlatke.com/lp.html">burnt latke.com</a>.
2) Constant pressure meter at variable volume... such as a syringe. Check the wal*mart food isle (marinade injectors: ~2-3$) or your local farm supply store (I got a 60ml syringe, minus needle, for 1.60$ at tractor supply co., IIRC).
Filling the syringe with propane takes some innovation, and you may want to put a proboscis on the syringe so that you can inject the propane deep into the chamber and prevent propane from escaping before you put the endcap on.
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If you have a chronometer and a scale, you may want to measure the kinetic energy of the projectile rather than the velocity. (more accurate measure)

Whatever you do, DON'T try to calculate the muzzle velocity of the projectile from its apogee or hangtime. It's a dangerous trap I've seen many intelligent individuals fall into.
In this case, report your values as the values you measured. It's less than ideal (the hangtime and the apogee don't correlate linearly with any important metric), but it does indicate that shot A is faster that shot B.

alright, I already have a propane meter, it looks somewhat like this one.
http://www.advancedspuds.com/propane.htm

next,
I measured the volume of my meter, and my combustion chamber on my gun.

My meter is: 6.102374 cubic inches
Chamber: 170.866483 cubic inches

I want to start my tests with 1 percent propane to air. So (correct me if I am wrong) I would first multiply my chamber volume by .01. This gives my the volume of propane that I want. Then I substitute that along with several other numbers into P1*V1=P2*V2. Then solve for P1 which is the PSI reading that I want on my meter when doing the test.

Please correct anything I did wrong, or if I did everything wrong, let me know!

thanks

http://www.advancedspuds.com/SpudToolonline.htm

this might help you if you haven't already checked it out

BigGrib wrote:http://www.advancedspuds.com/SpudToolonline.htm

this might help you if you haven't already checked it out

yeah i have seen that, but are my calculations correct?

Well whats the sizes of your chamber and barrel?

why do you need my barrel?

I havnt measured it yet, but my chamber is: 170.866483 cubic inches
and my meter is 6.102374 cubic inches

Its ridiculous to show that much significant numbers behind the comma.
You can just say chamber is 171 and the meter is 6.1 cubic inches.
However, if you want to be exact, use water to check the volume.
It might differ 5% or more.

I meant meter sorry.

i did use water, and found the volume in mililters, then converted mililiters to cubic inches. Sorry for all the digits after the period.

All I want to know is if I did the calculations right in my second post in this thread

thanks!

Hate to tell you this but the methods you have choosen are really not adequate for doing even a half-ass job of measuring performance versus fuel load.

You fuel meter is extremely inaccurate for two reasons. (1) The pressure gauge (or regulator) is not all that accurate. (2) The schrader fill valve is very unreliable for making a seal without any leakage.

You really should go with a syringe. Much more accurate and much easier to make small, but accurate, changes in the fuel load.

How are you going to measure the gun's performance? Got a chrony?

Like boiling said, you also need to measure the mass of the spuds so you can calculate the KE from the velocity. The velocities will vary too much from spud to spud to get accurate results unless you can correct for the varying masses. Or, don't use spuds, make up single round that is used for all the firings (check out latke's gasketed wood round).