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Manometric metering

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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Manometric metering

Unread postAuthor: 8tonsemi » Wed May 02, 2012 4:19 pm

Can someone help me more greatly understand manometric metering for a hybrid.
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Unread postAuthor: jackssmirkingrevenge » Wed May 02, 2012 4:33 pm

lets take a hypothetical chamber of 100mL which we will be fuelling with propane. We have a 10mL meter to do this with. a correct fuel to air ratio is 96 air:4 propane

The chamber needs 4mL of propane (4% of the chamber volume) per mix.

Say we want to fill it to 5x, so the total volume of propane we need is 4 x 5 = 20mL of propane at atmospheric pressure.

We need to factor in the volume of the meter too though, so total volume is 100mL + 10mL i= 110mL. 4% of this is 4.4mL of propane.

For a 5x mix therefore, we need 4.4 x 5 = 22mL of propane at atmospheric pressure.

We have a 10mL meter, which will hold 10mL of propane at atmospheric pressure. We want 22mL though, so 22/10 = 2.2

This means we need to pressurise the meter to 2.2 bar (32 psi). Once we do this, then open the valve between meter and chamber, we will have the correct amount of propane in the chamber.

Now, we need the right amount of air, so we pressurise the chamber to 5 bar (74 psi)

Voila, correct mix.

What's important to know is how much air and fuel you need in your chamber, then there are various ways to go about it.

Important to know that the pressure values I gave are absolute. Most pressure gauges will read zero at 1 bar/14.7 psi.

This means in our case that for the fuel metering, even though you want 2.2 bar in the chamber, you will actually fill it until the needle reads 1.2 bar (ie 2.2 bar - the 1 bar of atmospheric pressure it's already reading.

Assuming you have your dead volumes and venting sorted out, it's not a big deal.

Personally, I prefer syringes ;)
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Unread postAuthor: 8tonsemi » Wed May 02, 2012 4:49 pm

Thank you for the help and how do you fuel with a syringe.
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Unread postAuthor: jackssmirkingrevenge » Wed May 02, 2012 4:56 pm

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Unread postAuthor: Fnord » Wed May 02, 2012 6:35 pm

Waitwait.

You gave him very detailed instructions for volumetric metering. :wink:

Manometeric simply means attaching a very sensitive gauge to the chamber and pumping fuel in... judging how much fuel you've added by the pressure increase. Isolate the gauge when adding air/firing.

In this case,
14.7 psi = 1 chamber volume of fuel = ~ ~25x,
7.35 psi = 1/2 chamber of fuel = ~12.5x, etc.

Rough numbers. It's not too hard to figure out if you understand the basic concept.
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Unread postAuthor: jackssmirkingrevenge » Wed May 02, 2012 11:37 pm

Fnord wrote:You gave him very detailed instructions for volumetric metering. :wink:


There will always be some dead space down the line, which has to be taken into account ;)
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Unread postAuthor: Fnord » Thu May 03, 2012 5:00 am

I think you've got me confused now.

I guess I should have just posed this in the first place.

http://www.spudfiles.com/forums/hybrid-fueling-101-t13602.html

Volumetric is covered first, manometric is in the 'part 2' section,
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Unread postAuthor: MrCrowley » Thu May 03, 2012 5:08 am

jackssmirkingrevenge wrote:There will always be some dead space down the line, which has to be taken into account ;)
Fnord wrote:I think you've got me confused now.
Same here, can't tell if you're being sarcastic because I don't see why there would be any dead space in a manometric setup.

I'm a manometric fundamentalist, I can't see me going back to the poor-man's metering :D

In case no one realises, I am taking the piss. There's a time and a place for volumetric metering, neither of which I've yet encountered. It is good if you're using a syringe though.
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Unread postAuthor: 8tonsemi » Sun May 27, 2012 8:31 am

I was reading this post over today and I realized I don't know how to calculate dead space or vent. So more help would be Very appreciated. :lol:
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Unread postAuthor: ramses » Sun May 27, 2012 10:42 am

I think JSR is talking about dead space in the pump system used to fuel the gun. After you fill with propane (which works as expected). Once you hook your air pump to the chamber and open the valve, some of the propane/air mix flows back into the pump/hose, up to the check valve. You need to consider this in your metering.

I'm not sure if pumping several bar of air through this system would make the propane lost negligible. I would be inclined to think no; the chamber and pump hose would reach an equilibrium of concentration. the amount of propane that was correct for just the chamber is now divided between the pump hose and the chamber proportionally.

I think this is addressed in the syringe fueling link
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Unread postAuthor: DYI » Sun May 27, 2012 3:28 pm

Even if both lines are attached to the meter at all times, there is still a small problem: due to the geometry of the fueling lines, mixing to equilibrium will be a slow process. As such, it is desirable to make the chamber volume large in comparison to the total volume of the fuel meter in a manometric metering system.

I ran into substantial trouble with this when I started building small hybrids, and it still causes design headaches. Essentially, without very expensively rebuilding my meter, I can't decrease chamber volume like a want to, substantially increasing build cost (and time).
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Unread postAuthor: MrCrowley » Sun May 27, 2012 8:37 pm

ramses wrote:I think JSR is talking about dead space in the pump system used to fuel the gun. After you fill with propane (which works as expected). Once you hook your air pump to the chamber and open the valve, some of the propane/air mix flows back into the pump/hose, up to the check valve. You need to consider this in your metering.

I'm not sure if pumping several bar of air through this system would make the propane lost negligible. I would be inclined to think no; the chamber and pump hose would reach an equilibrium of concentration. the amount of propane that was correct for just the chamber is now divided between the pump hose and the chamber proportionally.

I think this is addressed in the syringe fueling link

Would a check valve after the chamber ball valve help? As DYI mentioned, I sort of relied on the large chamber volume and small meter volume to negate this problem and on my smaller hybrid I would fuel it a touch richer than normal.
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Unread postAuthor: DYI » Sun May 27, 2012 8:59 pm

Would a check valve after the chamber ball valve help? As DYI mentioned, I sort of relied on the large chamber volume and small meter volume to negate this problem and on my smaller hybrid I would fuel it a touch richer than normal.


This all depends on meter geometry. If you have a "cross" type meter with oxidizer, fuel, gauge, and chamber ports which is always open throughout, you may actually break even depending on the hose lengths.

If you have a "tee" type meter the mix would tend toward rich, unless you fueled "backwards" (oxidizer and buffer first).

Of course, there's the issue with fuel mixes as well. With hydrogen as a fuel (and possible buffer) it would be natural to fill "backwards". Given the tremendous assortment of possible propellant mixes (and my obstinate refusal to assume the use of air/propane :roll: ), it's not really possible to give a categorical answer here without going on for pages.

Either way though, the check valve you mention (regardless of what you mean by "after") would probably exacerbate the matter, owing to its cracking pressure.
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Unread postAuthor: Labtecpower » Mon May 28, 2012 4:54 am

but if you have a check valve BEFORE the meter, no dead volume would have to be calculated, right?
:roll:

like a check valve between the fuel and the metering chamber, and a check valve between the air/oxidizer supply.
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Unread postAuthor: wyz2285 » Mon May 28, 2012 5:45 am

As soon as you remember the check valve`s minimum opening pressure, a check valve will solve the dead space problem. In my case, I have never had any of these problem :lol:
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