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HYBRID FUELING 101

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Tue Mar 03, 2009 6:03 pm

fogus wrote:It almost seems like a mistake. He might have meant to say, "precise to within 0.1 psi".

I'd ask.

No, the meter is accurate to 1 PSI. For digital displays "precision" simply describes the resolution of the display. +/- 0.2 or 0.1 PSI for this digital display.

There is no relationship between "accuracy" and "precision". Usually a measurement tool can be no more accurate than it is precise. But the converse is not true. It can be less, even much less, accurate than it is precise.

What a user would really care about is "reproducability" which is different than "accuracy" and "precision".

For a meter, reproducability is probably the most important consideration.
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Unread postAuthor: ramses » Tue Mar 03, 2009 6:06 pm

I don't think so; the website he linked so had the same mistake. Unless, of course, he just took the data off the website and the website had made that mistake.

edit: they didn't link to the website, they gave a part number and I looked it up. here

further edit: do digital gauges like this come out of calibration easily?
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Unread postAuthor: fogus » Tue Mar 03, 2009 7:01 pm

jimmy101 wrote:
fogus wrote:It almost seems like a mistake. He might have meant to say, "precise to within 0.1 psi".

I'd ask.

No, the meter is accurate to 1 PSI. For digital displays "precision" simply describes the resolution of the display. +/- 0.2 or 0.1 PSI for this digital display.

There is no relationship between "accuracy" and "precision". Usually a measurement tool can be no more accurate than it is precise. But the converse is not true. It can be less, even much less, accurate than it is precise.

What a user would really care about is "reproducability" which is different than "accuracy" and "precision".

For a meter, reproducability is probably the most important consideration.


You are very correct in your understanding of accuracy and precision. Other people, not so much.

I maintain that the seller might have made a mistake and that it would be best to ask him.
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Unread postAuthor: ramses » Tue Mar 03, 2009 7:13 pm

I understand that, but I checked for myself that it is accurate to 1.0psi (so now we can rant on their sig figs, too).

I am now wondering if these gauges fall out of calibration easily. are they precise (if I fill to 6.2psi today, then fill to 6.2psi in 3 months, is it the same pressure?)

if so, then I will buy it, because it would be REALLY convenient to have a precise gauge with a wide range with a built in regulator for the air.

and BTW, jimmy101, reproducibility=precision linky
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Unread postAuthor: Cannibal Corpse » Fri Apr 10, 2009 6:16 pm

Did anybody buy that gauge? I'm trying to find a gauge for about 50$ that will work for manometric metering any ideas?
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Unread postAuthor: ramses » Fri Apr 10, 2009 6:50 pm

I bought some off eBay for $2.5 each. Still untested, as my bench vice can't accommodate 2" pipe. :(

hopefully I will gain access to a big ass bench vice, a massive pipe wrench, and a lathe in the next few weeks. :twisted:
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Unread postAuthor: Cannibal Corpse » Fri Apr 10, 2009 7:06 pm

2.5$ were they new or used? If new was it a ebay store or single seller?
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Unread postAuthor: c11man » Sun Sep 27, 2009 10:56 am

i am wondering about the chamber pressure method of metering. my proposed hybrid would be a piston hybrid so it would require a chamber pressure of about 25psi assuming the worst. how i am thinking to fuel is to add 25psi air, propane to a specific pressure, then top off the chamber with air. my promblem is if more starting pressure will affect the amout of pressure adding the would cause. would add the same psi or would it be something else?
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Unread postAuthor: D_Hall » Sun Sep 27, 2009 1:19 pm

Just a quick question to those who use pressure-based fueling... Do you take barometric pressure differences and temperatures into account?
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Unread postAuthor: Technician1002 » Sun Sep 27, 2009 2:03 pm

D_Hall wrote:Just a quick question to those who use pressure-based fueling... Do you take barometric pressure differences and temperatures into account?


I haven't built a combustion yet, but I wouldn't worry about either much as it would change the mix very little unless a long fill time subjected the cannon to a large temperature swing during filling. Day to day barometric changes are pretty much in the background noise level and can be ignored much like you don't make barometric compensation changes to your lawnmower.
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Unread postAuthor: D_Hall » Sun Sep 27, 2009 2:16 pm

Technician1002 wrote:I haven't built a combustion yet, but I wouldn't worry about either much as it would change the mix very little unless a long fill time subjected the cannon to a large temperature swing during filling. Day to day barometric changes are pretty much in the background noise level and can be ignored much like you don't make barometric compensation changes to your lawnmower.

I actually wasn't too concerned with day to day barometric changes as much as I was concerned with altitude-based barometric changes. I mean, just as an example I live at 2300 ft which is still pretty damned low altitude, but ambient pressure is nominally down 10%. Thus, if I use sea level calculations, I'm going to be fuel rich to the tune of about 10% more fuel than I'd planned on.
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Last edited by D_Hall on Sun Sep 27, 2009 7:57 pm, edited 2 times in total.
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Unread postAuthor: c11man » Sun Sep 27, 2009 7:35 pm

ok so i dont have to worry about tempature, but what about a chamber starting pressure of 25psi?
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Unread postAuthor: Technician1002 » Sun Sep 27, 2009 11:42 pm

D_Hall wrote:I actually wasn't too concerned with day to day barometric changes as much as I was concerned with altitude-based barometric changes. I mean, just as an example I live at 2300 ft which is still pretty damned low altitude, but ambient pressure is nominally down 10%. Thus, if I use sea level calculations, I'm going to be fuel rich to the tune of about 10% more fuel than I'd planned on.


That is a valid concern. Knowing the base pressure, you can calculate the gas mass for your elevation. The calculation based on pressure rise would cause the most variation due to elevation at low chamber mix. At higher mix pressures, the base atmospheric pressure drops in significance.

A 4% rise for a 1X mix would have the 10%, but at a 5X mix it would drop to about 1/5th that or aprox 2%.

The compensation can be done with a calculator. The metering you have is gauge. The mix is on absolute. 1 ATM is the " base pressure, you calculat the pressure rise due to the addition of the fuel. At elevation if your absolute base pressure is down 10%, then the new pressure is the new gauge zero. The rise will simply be recalculated based on the new mass in the chamber, so yes for a proper calculated mix, the rise from adding fuel will need to be less.

A solution is to purchase a guage that reads absolute and work from there. Lacking the money for that, a regular guage can be installed in a bell jar to read absolute instead of guage. Working from a known zero refrence (vacuum) for both air and fuel makes metering more accurate. Using this method, the cannon total gas mass can be repeated with high accuracy for even combustion power at any elevation. Use a manometer on the bell jar to ensure it is pumped down to less than 0.05 atm. Going further is in the noise level.

Further reading on absolute pressure measurements is here.
http://en.wikipedia.org/wiki/Pressure_measurement
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Unread postAuthor: D_Hall » Mon Sep 28, 2009 12:07 am

Technician1002 wrote:That is a valid concern. Knowing the base pressure, you can calculate the gas mass for your elevation. The calculation based on pressure rise would cause the most variation due to elevation at low chamber mix. At higher mix pressures, the base atmospheric pressure drops in significance.

I realize. Going back to my original question.... Does anybody here actually take this into account? I will be doing so with Vera (I'll also be taking temperature into account to get shot to shot, season to season repeatability), but I was curious who else did it (and if they noticed any differences).

And yes, I have high accuracy absolute pressure transducers in 20 psi (add the fuel), 50 psi (add the air), and 500 psi (monitor combustion) ranges.
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Last edited by D_Hall on Mon Sep 28, 2009 12:12 am, edited 2 times in total.
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Unread postAuthor: MrCrowley » Mon Sep 28, 2009 12:11 am

Does anybody here actually take this into account?

I'm about 100 feet above sea level, so no. Wouldn't make a noticeable difference, I'm sure my volume measuring (using water) left far bigger inaccuracies.
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