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meter pressure and flexible hose pressure rating?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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meter pressure and flexible hose pressure rating?

Unread postAuthor: JDP12 » Tue May 01, 2007 9:20 pm

Hey all...Well I posted this in a different thread...But no one answered..So I just want to check with you guys... It just seems like such a low pressure compared to the link I found in my propane meter thread...

Here's the math for my new and updated meter..

152.550(Chamber volume)*.042=6.4071(amount of propane needed.)

Atmospheric pressure=14.43

Meter volume=3.7072

(6.4071*14.43)/3.7072=24.9 psi??

I'm almost sure that that's right.....But it just seems so low...Here's the link I used..

http://www.launchpotatoes.com/forums/viewtopic.php?t=7

Also...What should my hose that comes from my propane tank to my meter be pressure rated to??? I went to fleet farm and got everything for my meter but that...They only had 1/2" hose that was rated to 45 PSI...So I didn't want to get it..What should it be rated to?
thanks guys..

ilovetoblowthingsup
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Unread postAuthor: hi » Tue May 01, 2007 9:50 pm

You will probably want it rated a little higher than what you need it for because if you get liquid propane it can freeze the hose and make it brital. i got mine from a auto shop and its rated at 300 something psi at like 380 degrees or something like that. if its 150 psi, then i would say 200 psi. one thing you could do to get more propane at a lower presssure is make the hose longer.
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Unread postAuthor: JDP12 » Tue May 01, 2007 9:52 pm

Kthanks hi...anyone know if my calculations are right?
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Unread postAuthor: jimmy101 » Wed May 02, 2007 11:31 am

You might want to take a look at Latke's meter calculators;
http://www.burntlatke.com/ft_splash.html
(see the help at http://www.burntlatke.com/ft_info.html)
or download the Excel version from http://www.burntlatke.com/calc.html

Using the Excel version;
<pre> chamber volume of 152.5in<sup>3</sup>,
4.2% target propane concentration
0.47" ID meter pipe (1/2" nominal?)
24.9 PSI tank pressure (actually that is the regulator's pressure)

Fuel-tool says the meter pipe needs to be 21.9" long.</pre>

I suspect your calculated regulator pressure is off by about 50%. You have to remember that you leave one meter's volume worth of propane in the meter. So, if you need 6.4in<sup>3</sup> in the chamber and the meters volume is 3.7in<sup>3</sup> then you need to include an extra 3.7in<sup>3</sup> in your calculation.
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Unread postAuthor: Pete Zaria » Wed May 02, 2007 2:32 pm

@ilovetoblowthingsup ,

I think Jimmy101 got your fuel meter problem dead-on, but check out ProMeterES for more info: http://www.spudfiles.com/forums/viewtop ... html#15862 .

As to the hose, I had the same problem here. I couldn't find hose rated to over about 60psi in half or 3/8ths" anywhere. I used some of the clear plastic hose a few times, but if you leave it pressurized for more than about 10 min, it'll rip and POP, which is AMAZINGLY loud when you're right next to it :-p
I ended up finding a coil of garden hose that fit perfectly on 3/8ths" hose barbs. I pressure tested it to 125psi (as high as my compressor goes) with no problems. I've used that same hose for the propane system on two guns now with no problem. I still have 10 or 15 ft of it. If you want to pay shipping, I'll give you a few feet for free... I don't need it.
Your other options are to find some tubing that'll work (try automotive stores, look for radiator tubing or etc... that fits on your barbs, also try sprinkler/plumbing shops?) or to use a hose-less system by tapping threads onto the propane torch head and using fittings from there.

Peace,
Pete Zaria.
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Unread postAuthor: JDP12 » Thu May 03, 2007 8:16 am

Okay...But I can't have 21 inches of meter pipe...So are you saying that I would need to double the pressure and make it like 48 psi?? Is that what you are saying??

I'm all confused now..All I want to know is what pressure I need to set my 3.9cu. in. (Yea I know I remeasured it) meter to to get 6.4071 cu. in. of propane in my chamber...


Jimmy101..A) I'm not using a regulator. I'm using a ball valve to let propane into the meter, then a pressure gauge monitors the pressure inside the meter and I turn off the ball valve when it gets to the desired pressure. B) What direction am I 50% off in?? Would I have to have 48 psi or 12 psi?? C)What do you mean when you say I have to include that extra 3.7 in my calculation?

All I really want is a simple explanation of what pressure I would have to have in my meter to get 6.4071 cu.in. of propane in my chamber


Sorry if I seem ticked off..I'm just getting frustrated..
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Unread postAuthor: FeLeX » Thu May 03, 2007 8:32 am

Hey man have you read this?
http://www.burntlatke.com/lp.html
It helped me a whole lot. Hope it helps you too.

By the way to make the pipe shorter try bigger diameter. As for the pressure all I did was put 100psi guage on my pipe and at frist I tried 25psi, my chemistry teacher told me thats what I should start with and keep on going up by 5psi untill I hit something around 50psi and if it works with no trouble just use 50psi.

What I am trying to say is I wouldnt worry about pressure that much. Just figure out your pipe to be the right length and then figure out the pressure needed by trial and error.

Hope that helped man, good luck.
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Unread postAuthor: JDP12 » Thu May 03, 2007 8:35 am

I did read it and it at first helped me..But now all this propane stuff is massively confusing me..I cant go get all new pipe....Don't have the budget..But doesn't the pressure determine how much propane is inside the meter?? So wouldn't I have to worry about it?? Seeing as I only have about half of the necessary propane needed?
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Unread postAuthor: FeLeX » Thu May 03, 2007 8:40 am

Yea man your right. But if you figure out the right length of your pipe for your chamber then you can figure out the pressure by trial and error.

If you want give me all the important dimensions and I will calculate all of this for you. I got Tech Ed at the moment and nothing to do since my rocket is done =(
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Unread postAuthor: JDP12 » Thu May 03, 2007 8:41 am

God I would love that...What dimensions do you need??

ANd why would the length of the meter pipe even matter if I can just pressurize the propane and put more in it?
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Unread postAuthor: FeLeX » Thu May 03, 2007 8:46 am

Okay here we go man. I need:
1. Diameter of your chamber
2. Length of your chamber
3. Diameter of your meter pipe

After I do the math crap Ill explain why the length of the pipe matters, deal?
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Unread postAuthor: FeLeX » Thu May 03, 2007 8:59 am

Alright man my class is about to be over in 4 minutes and then I got algebra and lunch at 11:45am (its 9:57am right now). Ill check for your post during lunch and then calculate all that for you and post back during my 4th block, at around 2pm. Sorrys cant do it right now. But still post them measurements and Ill get back to you at arround 2pm.

Edit: Sorry for double post, I just didnt think the edit of the previous post would be very noticable.
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Unread postAuthor: jimmy101 » Thu May 03, 2007 10:11 am

ilovetoblowthingsup wrote:Jimmy101..A) I'm not using a regulator. I'm using a ball valve to let propane into the meter, then a pressure gauge monitors the pressure inside the meter and I turn off the ball valve when it gets to the desired pressure. B) What direction am I 50% off in?? Would I have to have 48 psi or 12 psi?? C)What do you mean when you say I have to include that extra 3.7 in my calculation?

All I really want is a simple explanation of what pressure I would have to have in my meter to get 6.4071 cu.in. of propane in my chamber

Sorry if I seem ticked off..I'm just getting frustrated..


Don't sweat it too much, once you get it figured out it'll be obvious (like many things in life).

Whether you are using a regulator or valve+pressure gauge, it is the same thing. Basically you are just trying to get a known pressure in the meter. A regulator is a little easier to use but costs more. Either method works fine, though with a valve+gauge you run a slightly greater risk of accidentally over-pressurizing the meter and having it blow up on you. (A propane tank is what, ~100 PSI? A black tank that has been sitting in the sun for a while might hit 200 PSI.)

My comment on the amount of propane left in the meter wasn't correct. Just trying to remember that you leave propane in the meter when you open the valve to the chamber. Now that I think about it, the actual correction for the propane in the meter is a very small factor and can be ignored. (You really do leave the meter's volume in the meter but it was there to start with so the affect is very minor and is only related to changes in temperature and pressure when the gun is fueled. When you first use your meter you need to "burp" it a few times to get the air out of it.)

A practical approach, similar to what FeLex posted, is to design the meter so that it has about the right volume when pressurized to somewhere in the middle of the range that you can read on your gauge. WAG it as, say, 40 PSI (for a 100 PSI gauge). Then fine tune the actual pressure by trial and error. If you hosed your math and the meter is only half as big as it should be then you can run it at 80 PSI and get the right amount of propane. If you were off in the other direction you can run it at 20 PSI and get half as much propane. So, you have a pretty broad range over which you can tweak the actual amount of propane the meter delivers. Folks that have made meters that are easily transferred from gun to gun use this to their advantage. A mid-size meter can be used on guns with chamber volumes that vary over a range of perhaps 1 to 10. For example, for a 50in<sup>3</sup> chamber you might use 10 PSI in the meter, for a 500in<sup>3</sup> you would use 100 PSI in the meter.

Looking back at your numbers, 25 PSI looks to be pretty close.

And like FeLex said, if you post your dimensions (chamber L and D, meter L and D) then folks can do a quick check.
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Unread postAuthor: JDP12 » Thu May 03, 2007 10:23 am

Okay well guys..The chamber isn't just 4" pipe because I had to use a 4x2 reducer then a 2x1.5 reducer, then a 1.5" male thread..All with 2"and 1.5" Piping in between..But the majority of the chamber is 4" pipe..So I'll give you those dimensions.

4"x8" Main chamber

My meter pipe diameter is 1/2" I believe...That's what it said when I bought it..Sorry I can't check I'm at school right now.

And the meter pipe length..Including the 1/2" nipple because that will be containing propane is probably around 6"

Like I said..I'm at school right now...So when I get home I'll check better...That'll probably be around 4:30 US central time...

But I believe the meter is around those dimensions...I do know for a fact that the propane "storage" area in the meter is 56.2 mL because I measured it yesterday, which equals about 3.9 cubic inches..
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Unread postAuthor: Freefall » Thu May 03, 2007 11:08 am

Assuming your measurements are correct, you're right on with 24.9 psig in the meter.
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