GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: rumbleUU on September 10, 2019, 07:33:42 AM
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Hi, always want to know and learn, for example, will there be an restriction by the 3mm hole threw the manometer-piece? also does the small volume(6cc) has something to do with it?
Having hard time imagane if that hole resicts or not, hmmm?
Thanks
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Yes, but whether that reduces the power depends on many factors.... The Benjamin Discovers is like that, with a 4mm exhaust port, a 6mm inlet on the valve, but only a 3mm hole in the gauge block.... In stock tune it makes little or no difference.... but if you start trying to increase the power, eventually you get to the point that the hole in the gauge block is enough of a restriction that increasing it to 6mm, like the inlet hole in the valve makes a difference....
Bob
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ok, but how does the volume between gauge and valve matter?
if small it matters more, right? cause if that volume was very big the hole in the gauge would not matter....
Just to have an example, pretend the gun is shooting .22 kaliber 40 ft/lbs, is there some math of how much more energy you would get with a hole of lest say 4mm instead of 3mm? would it matter.... Im guessing the air in the reservoar will fill that space with 6cc inbetween really quick....
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You are correct, the larger that volume (including the volume inside the valve), the less the pressure drops during the shot.... assuming the hole through the gauge block is small.... If it is larger than the valve inlet, then the volume doesn't matter....
If the hole in the gauge block was really small (eg. 1mm or less), then the space between the block and valve would act like the plenum in a regulated PCP.... that would be the only air available for the shot.... In that case, it would be good to have 1 cc for each FPE (eg. 40 cc for 40 FPE) to minimize the pressure drop during the shot.... At least 1/2 cc per FPE is required to prevent large pressure drop.... However, if the inlet hole through the gauge block is the same diameter (or larger) than the valve inlet, the volume of that space doesn't matter, as it can fill faster than it can empty.... I like to see the valve inlet, and any upstream restrictions (eg. the hole in the gauge block) at least as large as the diameter of the valve throat (the hole under the poppet seat)…. to prevent any pressure drop inside the valve....
Bob
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Great explaning, youre the man on stuff like this, it was about the same i was thinking..
Sometimes isnt easy to understand how air under pressure flows and behaves, especially if there is a chamber inbetween....
One thing more i did wunder, what pressure will be behind a poppet ? Is it the hole in the valveseat or the area of the delrin piece on the poppet that creates the force. Example, the valveseat hole is 5mm and the delrin piece on the poppet is 8mm, would it also matter if the valveseat is straight and plain or conical?
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better to draw :)
Is the above more difficult for the hammer to open?
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Sure the length of the passage through the gauge block matters as much (probably more) than the diameter?
If the passage is very short, like a restrictor plate, then pretty much only the area of the hole matters. If the passage is 10 times longer than the diameter of the port, then a small diameter is worse than if the passage was the same length as its diameter. No?
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Sure the length of the passage through the gauge block matters as much (probably more) than the diameter?
If the passage is very short, like a restrictor plate, then pretty much only the area of the hole matters. If the passage is 10 times longer than the diameter of the port, then a small diameter is worse than if the passage was the same length as its diameter. No?
thats a new one, didnt think of that, but have no idea if thats true?
Maybee Bob can
Thinking about it, it cant matter, think of it this way, you have a tube that is 4" long and one that is 10" long, blowing air threw must be about the same resistance? Of course, if one length is 1" and the other is 100", then maybee something else matters, but dont think length up to 1" should matters for holes like 3-5mm big..
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Sebastian,
It is not the absolute length of the passage or tube bore that matters. It is the ratio of length to diameter (area to be more exact) that matters. So, for a tube that has a 25 mm bore, the length for airgun applications matters very little.
Now, compare that to a 4.5 mm bore that is 900 mm long, and the drag of the air flow down the barrel matters. It matters more for high pressure air, as the effective viscosity is higher. So, a barrel is a bad example, as the pressure decays towards the muzzle.
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Sebastian,
It is not the absolute length of the passage or tube bore that matters. It is the ratio of length to diameter (area to be more exact) that matters. So, for a tube that has a 25 mm bore, the length for airgun applications matters very little.
Now, compare that to a 4.5 mm bore that is 900 mm long, and the drag of the air flow down the barrel matters. It matters more for high pressure air, as the effective viscosity is higher. So, a barrel is a bad example, as the pressure decays towards the muzzle.
ok, but we are talking lengh up to 1" and hole 3-5mm big, do you really think there will be any restriction of airflow threw that short distance?
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Making the hole diameter through the gauge block helps, not so much because a larger hole flows easier, but because if large enough, the air in that passage at the start of the shot cycle can be added to the plenum volume nearer the valve.
I would be confident that if you could open the gauge block "passage" to 10 mm diameter, the gains in power would come almost completely from the larger plenum effect; rather from easier flow.
It would also make the airgun ping louder when fired, as the air damping effect of the narrow channel through the gauge block would be removed.
The flow through the gauge block occurs mostly after the valve has closed, to refill the plenum. So, a 25 mm long gauge body with 2 mm diameter hole in it won't affect power by restricting flow. It will reduce power because the plenum is "too small". Making the passage in the gauge block larger will increase the effective plenum volume; boosting power potential.
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Making the hole diameter through the gauge block helps, not so much because a larger hole flows easier, but because if large enough, the air in that passage at the start of the shot cycle can be added to the plenum volume nearer the valve.
I would be confident that if you could open the gauge block "passage" to 10 mm diameter, the gains in power would come almost completely from the larger plenum effect; rather from easier flow.
It would also make the airgun ping louder when fired, as the air damping effect of the narrow channel through the gauge block would be removed.
The flow through the gauge block occurs mostly after the valve has closed, to refill the plenum. So, a 25 mm long gauge body with 2 mm diameter hole in it won't affect power by restricting flow. It will reduce power because the plenum is "too small". Making the passage in the gauge block larger will increase the effective plenum volume; boosting power potential.
ok, so if the hole is 3mm , making it 4mm wouldnt add nothing to power, as the small 6cc "plenum" volume is what restricts the power anyway, is that right?
So the rule of making every hole the same all the way to the pellet, does not apply the hole threw the gauge, am I right?
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but does the 6cc space in between gauge and valve really matter , ? Im not trying to understand how regultaed plenum works with a small tiny hole to fill it up works, that I know of..
In other words, is there no befinit AT ALL making the hole threw gauge from 3mm to 4mm?
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The 6 CC plenum is almost all that matters in terms of potential power and efficiency (at a given starting air pressure; other than the valve size and action). It is a portion of the air available in the plenum that drives the pellet down the barrel. The valve opens briefly on firing. As it does this, the pressure in the plenum drops; and is then refilled from the rest of the air tanks via the 3 mm channel in the gauge; after the valve is closed.
The degree to which the plenum pressure drops depends on the volume of the plenum and the bore volume behind the pellet when the valve closes. The smaller the plenum, the more the pressure drops while the valve is still open. The less the pressure drops while the valve is open, the more power and efficiency you can get.
If the effective plenum volume is only 6 CC and you want more than 12 ft.lb from this airgun, then anything you can do to enlarge the effective plenum will increase power potential and efficiency. This includes making the air channel through the gauge larger.
If it were me, I would ask if 4 mm is the maximum you could go? Remember, the hole does not have to be the same size all the way through the gauge body, end to end. If the end of the hole facing the valve was drilled 12 mm in diameter by 10 mm deep, you have just increased your plenum volume by about 17%. A little large and a little deeper and the gains could double.
Drilling the hole in the gauge body on the opposite side of the valve, while leaving a 4 mm restriction in the middle would not help power or efficiency; as it cannot be considered part of the plenum volume.
The fact that the rest of the air channel through the gauge body is only 3 or 4 mm in diameter would only matter if this airgun fired fully automatic, at such a rate that the air pressure could not be fully restored in the valve before the next shot was fired.
The devil is in the details. Bob will no doubt be along shortly to give you the most accurate input about this.
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Ok , lets skip the fiction, my first post was just something I made up to understand how the air flows and all works, I want to learn....
Lets take a real gun and discuss, as fiction can be wrong.
lets take the Artemis PR900W .22 as an example, its pretty similar to my first example.
Airtube 100CC big, gauge with 3mm hole, valve with 5mm hole intake, 3.5mm holes all the way to the pellet, transferport is 3.5mm in FAC version making 30Ft/Lbs and 0.7 to 1.0mm here where I live producing 7Ft/Lbs..
I think it has a "plenum" volume between gauge and valve that is not much bigger then 6CC.
From what Ive read the gun is not bad in efficiency, even in FAC version, so HOW can this PR900W produce 30Ft/Lbs efficient with only a 6CC plenum if thats the only thing that matters? you said a small "plenum" of 6CC is only good for 12Ft/Lbs...... Im not arguing against what you said, Im sure you know what you are saying, Im just trying to understand....
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Fair questions. Perhaps the answer is a 3000+ PSI or 200+ BAR operating pressure. The energy in the compressed air depends on its volume and pressure.
6 CC at 3000 PSI is more than equivalent to 18 CC at 1000 PSI. More than, because higher pressure usually produces greater efficiency.
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one thing: isnt the air "filling up" the "plenum" throw that smaller 3mm hole even in the start where the hammer opens the valve up?
And also, if all holes where 3.5mm all the way threw from the tube to the pellet, would there really be any restrictions even with a small size plenum like 6CC?
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this is a simple way to look at it, if its wrong, well, the I will learn again :)
lest think of the volumes, 1,2 and 3, where 1 is airtube, 2 is the "plenum" and 3 is the air goin OUT from Valve that drives the pellet in the barrel forward... if the hole is the same IN and OUT from volume 2, I cant see how there would be a restriction? No matter how fast the air is sucked out from volume 2 it should be filled up again as fast as the hole IN and OUT are the same size, Am i thinking right?
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Sebastian,
In theory, the 3 mm hole through the gauge will start to refill the plenum as soon as the valve starts to open. It is a matter of degree. The amount of air flowed then is small compared to the air leaving the valve, because the valve area is larger than the gauge port flow area. There simply is not enough time while the valve is open to refill the plenum to a meaningful degree with a 3 mm tube feeding the plenum.
The idea that the restriction depends only on the tube diameter; and that if the diameters match there is no restriction, is incorrect.
Yes, steps in the flow path create flow losses, so a near constant diameter path is better. However, if that path is narrow and long, then there is a "restriction" in the form of friction. It is like the pressure loss seen at the end of a 50 m garden hose; or the electrical resistance of a 50 m extension cord.
If all the holes through gauge, valve port and barrel all match, then the actual plenum exists only on the air tank side of the gauge. As in, very far away. Now, all the air leaving the valve has to accelerate through a very long narrow path. That will cause a significant pressure drop and reduce the acceleration of the pellet.
If you can get it, you want a fat pipe all the way to the valve. The port between the valve and barrel should be as short as possible. It should match the bore area where it meets the barrel (if possible). The effective valve area (considering the valve stem is in the way) should be a little larger than the port area. Very much larger and it starts reducing efficiency.
"Restriction" is measured as pressure loss. The higher the flow velocity of the air, the greater the friction losses. The longer and narrower the path, the greater the losses, or restriction. Wide and short are better, all the way to the front of the valve. After the valve, match the bore area and keep the path short.
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I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm on mine PR900W 5.5mm ;)
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...
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I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm on mine PR900W 5.5mm ;)
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...
and where did you put the manometer? with 6mm hole threw there will be no material left to stop the manometer to tighten it..
Also, did you enlarged the valve-seat hole? cause it is only the same effective area as a 3.5mm hole when you subtract the area of the poppet...... otherwise this will be the place where airflow will choke....
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Also, did you enlarged the valve-seat hole? cause it is only the same effective area as a 3.5mm hole when you subtract the area of the poppet...... otherwise this will be the place where airflow will choke....
OK. It is not only length to area ratio of a flow passage that matters. If the air flow reaches the speed of sound through the narrowest section, no further increase in flow can take place, despite higher input pressure. This is known as choked flow. It can certainly happen at an orifice plate. Or a nozzle shaped section.
Perhaps that is what you have been alluding to all along. In which case you are correct.
As for the idea that an air flow path must always be parallel or at least constant area section; this is clearly not the case with the inlet and exhaust valves of internal combustion engines. Here, pressure in the duct is converted to flow, and flow converted to pressure, via nozzle shaped paths leading to or from the valves.
The same rule applies for avoiding supersonic flow in the convergent section or throat of the nozzle; else the nozzle is flow limited due to chocking:
https://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html (https://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html)
https://aeronotes.weebly.com/nozzle-choking.html (https://aeronotes.weebly.com/nozzle-choking.html)
https://www.sciencedirect.com/topics/engineering/sonic-flow (https://www.sciencedirect.com/topics/engineering/sonic-flow)
https://en.wikipedia.org/wiki/Choked_flow (https://en.wikipedia.org/wiki/Choked_flow)
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I disagree that very little air can flow through the gauge block to refill the "plenum" during the shot.... It all depends on the diameter of the passage through it....
A Disco has almost no "plenum", the space between the gauge block and valve is about 1/16" long, or less than 1/2 cc.... The valve is roughly about 3 cc inside.... The hole through the gauge block is 1/8" and through the front of the valve it is 1/4".... The valve throat, stock, is 13/64" with a 5/32" stem in the way (throat area is the equivalent of a hole 0.130"), and the barrel port is 0.134".... The hole through the gauge port is almost as large as the throat area, and likely flows just as much air, so on a stock gun it makes little difference it is so small....
When you start hogging out the ports, that is no longer the case.... Go to a 3/16" barrel and transfer port and a 1/4" throat, and if you don't increase the hole in the gauge port to 1/4" or a bit larger, you will be giving up a significant amount of power.... I know, because I hogged out the ports, and initially ended up disappointed in the FPE.... Increasing the hole in the gauge port to 5/16" made a huge difference in the FPE.... because it makes the entire 135 cc reservoir act like a plenum.... The drastically increases the pressure available at the valve seat during the shot....
If you are dealing with a regulated PCP, where the regulator is in between the HPA reservoir and the plenum.... and the passage between the plenum and valve are large, the pressure in the valve and plenum remain essentially the same, all the time throughout the shot.... The regulator can't keep up with the flow requirements of the shot, so the plenum doesn't get topped up during the shot.... That is why larger plenums increase FPE, because they keep the pressure higher during the shot.... Take the regulator out, and the reservoir becomes part of the plenum, unless you have a gauge port (or tank block) in the way.... If the hole in the gauge port is smaller than the ports leading out of the valve, the pressure inside the valve drops.... If the hole is larger than the throat and valve inlet, then the air in the reservoir passes easily through the gauge port, keeping the pressure in the valve topped up....
While the length of the hole through the gauge port may make some difference to how quickly the air can get from the reservoir in to the valve, I think the area is the key.... I make sure that the inlet into the valve, and the passage through the gauge port, are at least as large in diameter as the valve throat diameter.... Since the actual throat area is reduced by the area of the stem, that insures that the entire HPA reservoir acts as plenum volume in an unregulated PCP.... I even do this on my unregulated bottle guns, by using a large passage through the tank block, to make sure that the bottle keeps the pressure in the main tube topped up during the shot.... If all I was adding to the plenum was (part of) the volume of the larger passage through the tank block, I would only be increasing the plenum volume by 1% or less.... In reality, by having large enough passages between the two through the tank block, my plenum is increasing by the entire volume of the bottle (up to 400%).... and it shows in performance, compared to having a small passage between them....
Bob
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Note, choked flow can only occur in the very last part of the shot cycle, when the pellet velocity is high.... At the beginning of the shot cycle, the pellet is stationary (or nearly so), and even though the valve is cracked just a few thou the flow velocity is very low, and no choking can occur.... Only once the pellet gets up a good head of steam (and with it the column of air behind it), and the valve starts to close, can the flow at the seat choke, and in fact it WILL do that just before the valve closes, because the curtain area under the poppet is so small.... The only effect that has is to make the flow think the valve has already closed, a few micro-Sec earlier than when the poppet actually hits the seat....
Also note, you can't just use a speed of sound of 1126 fps to calculate the choking velocity, you must use the speed of sound at the pressure involved.... At 3000 psi in air, Mach 1 is about 1370 fps.... The larger a passage, and therefore the slower the flow, the less likely it is to choke, so using large diameter passages on the inlet side of the valve, so that the smallest area is the valve throat, is the simplest way to insure that the flow can never choke at the valve inlet, gauge port, or tank block passages....
The column of air inside a barrel is expanding and accelerating as the pellet travels down the bore.... If the velocity of the column was the same everywhere, then shots of over 2000 fps would be impossible, because the flow out of the valve would choke long before that velocity was reached.... Far from being the stuff of legend, I have produced repeatable velocities of 2092 fps by using a 1.8 gr. airsoft BB in my 6mm PCP instead of the 75 gr. lead bullet at just over 1000 fps.... Interestingly, the mass of air expelled in the shot is much greater than even that 75 gr. bullet, over 100 gr. of air at 4200 psi....
Bob
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Thanks Bob.
The stock Disco is severely power limited because it has to flow so much air through the gauge body passage. If it had a large plenum at the valve, the gauge block passage could be tiny and it would work fine. The shot cycle would be more like a regulated plenum refilling after the shot.
Increasing the passage diameter leading up to the valve, such that air flow velocity that occurs while the valve is open, remains well below super sonic at the narrowest section, is key. Else, any narrow portion, no matter how short risks choking due to supersonic flow.
Certainly, increasing the passage diameter to over bore diameter should reduce flow velocity to a very useful degree. The OP is talking about increasing a 3 mm flow path to 4 mm. As area increases with the square of the diameter, he is going from minuscule to tiny :) . Not sure what caliber he is considering. I did suggest he open the flow path as wide as he could...
Now, to put things into perspective about long narrow flow paths being "bad"; consider the flow path down the barrel once the pellet is almost half way to the muzzle. That long "flow path" does not stop the airgun from working (it can't be avoided). And neither does a long skinny flow path leading up to the valve prevent an airgun from working. It just limits maximum power and reduces efficiency at high power.
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Very good points about choked flow only being a "risk" once the pellet is moving fast, Bob. Also, the reminder that the speed of sound in air is higher at high pressure.
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In fact, with typical airgun velocities of under 1000 fps, if you build a gun with near bore-size porting (which you would normally do if in search of high power)…. there is virtually no risk of the flow choking except at the seat, just as the valve shuts.... In reality, if the valve is closing before the pellet gets half way down the barrel (typical if you are looking for even reasonable efficiency)…. then the 80% barrel ports we drill (if matched to similar size transfer ports) are unlikely to choke either.... Any calculations predicting the choking of the flow at the transfer port must be done using the pellet velocity at valve close, NOT the muzzle velocity....
Bob
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Thanks, Bob
I was hoping you would weigh in sooner (hence the PM this morning :) ). Certainly, you have a deep understanding, based extensive experience tuning and making from scratch, all manner of airguns. This, while I can easily get bogged down in over emphasizing one theoretical aspect at the expense of more important ones.
This reminds me of my first year Strength of Materials class: The professor asked the class, "Now that you understand the concept of yield strength, would you trust a product where the material yield strength had been exceeded?"
3/4" class raised their hands to indicate "yes". The others smelled a trick question.
The prof then said, "For those of you who said "yes"; the chairs you are sitting on were formed by exceeding the yield strength the steel tubing. And there is nothing unsafe about them. The devil is in the details".
The point of that story, is the mistaken idea that choked flow is always bad. As you pointed out, the flow always chokes in a PCP as the valve closes. But reducing the flow is kinda the point of closing the valve, so no harm done :)
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I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm on mine PR900W 5.5mm ;)
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...
I didnt see yesterday when I looked quick, but it is a tapered hole right threw the gauge now where the manometer is going in, how do you make that leakfree, teflontape?
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I disagree that very little air can flow through the gauge block to refill the "plenum" during the shot.... It all depends on the diameter of the passage through it....
A Disco has almost no "plenum", the space between the gauge block and valve is about 1/16" long, or less than 1/2 cc.... The valve is roughly about 3 cc inside.... The hole through the gauge block is 1/8" and through the front of the valve it is 1/4".... The valve throat, stock, is 13/64" with a 5/32" stem in the way (throat area is the equivalent of a hole 0.130"), and the barrel port is 0.134".... The hole through the gauge port is almost as large as the throat area, and likely flows just as much air, so on a stock gun it makes little difference it is so small....
When you start hogging out the ports, that is no longer the case.... Go to a 3/16" barrel and transfer port and a 1/4" throat, and if you don't increase the hole in the gauge port to 1/4" or a bit larger, you will be giving up a significant amount of power.... I know, because I hogged out the ports, and initially ended up disappointed in the FPE.... Increasing the hole in the gauge port to 5/16" made a huge difference in the FPE.... because it makes the entire 135 cc reservoir act like a plenum.... The drastically increases the pressure available at the valve seat during the shot....
If you are dealing with a regulated PCP, where the regulator is in between the HPA reservoir and the plenum.... and the passage between the plenum and valve are large, the pressure in the valve and plenum remain essentially the same, all the time throughout the shot.... The regulator can't keep up with the flow requirements of the shot, so the plenum doesn't get topped up during the shot.... That is why larger plenums increase FPE, because they keep the pressure higher during the shot.... Take the regulator out, and the reservoir becomes part of the plenum, unless you have a gauge port (or tank block) in the way.... If the hole in the gauge port is smaller than the ports leading out of the valve, the pressure inside the valve drops.... If the hole is larger than the throat and valve inlet, then the air in the reservoir passes easily through the gauge port, keeping the pressure in the valve topped up....
While the length of the hole through the gauge port may make some difference to how quickly the air can get from the reservoir in to the valve, I think the area is the key.... I make sure that the inlet into the valve, and the passage through the gauge port, are at least as large in diameter as the valve throat diameter.... Since the actual throat area is reduced by the area of the stem, that insures that the entire HPA reservoir acts as plenum volume in an unregulated PCP.... I even do this on my unregulated bottle guns, by using a large passage through the tank block, to make sure that the bottle keeps the pressure in the main tube topped up during the shot.... If all I was adding to the plenum was (part of) the volume of the larger passage through the tank block, I would only be increasing the plenum volume by 1% or less.... In reality, by having large enough passages between the two through the tank block, my plenum is increasing by the entire volume of the bottle (up to 400%).... and it shows in performance, compared to having a small passage between them....
Bob
Thanks Bob, in other words: the hole threw gauge should be at least the same size as the inlet to the valve to prevent powerlose, so for any typical pcp gun with a gauge inbetween and a small "plenum": Gauge hole=inlet valve, the rest including valvethroat-stem should match for the power you want.... ?
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Im having hard time understanding this: If you have a gauge inside a tube, 3mm hole threw and a small tiny hole(1mm) that goes into the threaded manometer, there is also a white delrin washer in between the manometer and the gauge-block with a tiny 1mm hole in the middle... How much force is acting on the manometer? is the force only applied by the tiny 1mm hole, or by the whole M8 threaded area? cant understand that.....
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Looking at your diagram suggests the delrin washer is shielded from air pressure; except inside the 1 mm diameter hole. So, really very little force on the washer from air pressure. Essentially none on the 8 mm faces. The only forces of consequence acting on the washer faces are due to tightening the thread on the valve stem.
If the end face of the valve stem has a narrow raised sealing ring machined into it, then the delrin washer will see air pressure on one face, up to inside edge of that sealing ring.
The 1 mm channel is of no consequence here, as the pressure gauge does not flow air. The hole could be even smaller, except that punching or drilling very small holes is difficult and expensive.
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Looking at your diagram suggests the delrin washer is shielded from air pressure; except inside the 1 mm diameter hole. So, really very little force on the washer from air pressure. Essentially none on the 8 mm faces. The only forces of consequence acting on the washer faces are due to tightening the thread on the valve stem.
The 1 mm channel is of no consequence here, as the pressure gauge does not flow air. The hole could be even smaller, except that punching or drilling very small holes is difficult and expensive.
ok, look at Reply #19 by Atom, how much force will be acting on the manometer the way he did,
Will the forces pushing the manometer be the same in his case?
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I looked at post 19" https://www.gatewaytoairguns.org/GTA/index.php?topic=162493.msg155811577#msg155811577 (https://www.gatewaytoairguns.org/GTA/index.php?topic=162493.msg155811577#msg155811577)
I assume the manometer will screw into the threaded hole at 90 degrees to the air tube axis. I am not sure where the sealing surfaces are, but if I understand correctly, then the air pressure will act on nearly the thread outside diameter. That would potentially increase the force pulling on the threads. Somewhere on this forum Bob has a calculator for checking thread strength; if that is your concern.
Definitely good to calculate that the margin of safety has not been compromised when modifying parts.
An 8 mm diameter plug that sees 3000 PSI will only have 230 lb acting on it. That should easily be handled by an 8 mm thread of sufficient depth.
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I looked at post 19" https://www.gatewaytoairguns.org/GTA/index.php?topic=162493.msg155811577#msg155811577 (https://www.gatewaytoairguns.org/GTA/index.php?topic=162493.msg155811577#msg155811577)
I assume the manometer will screw into the threaded hole at 90 degrees to the air tube axis. I am not sure where the sealing surfaces are, but if I understand correctly, then the air pressure will act on nearly the thread outside diameter. That would potentially increase the force pulling on the threads. Somewhere on this forum Bob has a calculator for checking thread strength; if that is your concern.
Definitely good to calculate that the margin of safety has not been compromised when modifying parts.
An 8 mm diameter plug that sees 3000 PSI will only have 230 lb acting on it. That should easily be handled by an 8 mm thread of sufficient depth.
Im just concerned if Atom dont know that the forces will be much more in his case then the stock one was, We will see when he replies to my questions how he seals the manometer the way he did it...
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Pertinent posts:
https://www.gatewaytoairguns.org/GTA/index.php?topic=138842.msg1395806#msg1395806 (https://www.gatewaytoairguns.org/GTA/index.php?topic=138842.msg1395806#msg1395806)
https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.msg1373003#msg1373003 (https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.msg1373003#msg1373003)
https://www.gatewaytoairguns.org/GTA/index.php?topic=127914.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=127914.0)
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Pertinent posts:
https://www.gatewaytoairguns.org/GTA/index.php?topic=138842.msg1395806#msg1395806 (https://www.gatewaytoairguns.org/GTA/index.php?topic=138842.msg1395806#msg1395806)
https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.msg1373003#msg1373003 (https://www.gatewaytoairguns.org/GTA/index.php?topic=136973.msg1373003#msg1373003)
https://www.gatewaytoairguns.org/GTA/index.php?topic=127914.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=127914.0)
dont have problems calculating forces mechanicly, the hard part as I see it to know wich material all the stuff is from, the manometer is brass, thats easy, the gauge is some kind of aluminum.....
yes I like also to calculate the forces so you are in the margin at least 3 times before the material starts to get weaker.. even better 4-5 times the margin, but that cant be expected for all pcp:s.....
Sometimes when someone else is doing a mod, im not sure they understand that the forces could get 20-30 times higher, not all people understand this.... Im very carefull of knowing before doing anything..
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in cross-section the manometer looks this way....
The H is the meat that is between the 3mm hole and the delrin washer, its 1.7mm thick at the lowest point.
Would it be safe to make the hole threw gauge like 5mm only leaving H=0.7mm ???
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Im very carefull of knowing before doing anything..
Good. Lazy people say pounds when they mean pounds per square inch; or the other way around. Then, effective area can make huge difference.
Bob and Lloyd have a gate on this forum. Lloyd runs Airgun Lab. He has a bunch of interesting videos testing airgun parts to destruction. Note that pressure tests use liquid, not gas. This minimizes the energy released when parts fail: https://www.youtube.com/user/1227air500/videos (https://www.youtube.com/user/1227air500/videos)
https://www.youtube.com/watch?v=RZ9BJbaPlFY (https://www.youtube.com/watch?v=RZ9BJbaPlFY)
https://www.youtube.com/watch?v=syT6yoT3TX8 (https://www.youtube.com/watch?v=syT6yoT3TX8)
https://www.youtube.com/watch?v=cuShgXYktq0 (https://www.youtube.com/watch?v=cuShgXYktq0)
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in cross-section the manometer looks this way....
The H is the meat that is between the 3mm hole and the delrin washer, its 1.7mm thick at the lowest point.
Would it be safe to make the hole threw gauge like 5mm only leaving H=0.7mm ???
I don't want to answer that without calculating a few things. Calculation is impossible without knowing the max pressure and the material type and state. I assume 3000 PSI and 6061 T6...
EDIT: I see 200 BAR pressure; or 2940 PSI.
You could make a test piece of similar material and destroy it with a hydraulic pump. This is ultimately better evidence than any calculation.
Remember, the passage feeding the manometer is a cylindrical hole, even though it appears as a cut in your image. As such, the loss in strength due to the 1 mm wall at the hole edge is not nearly as weak as it looks in your diagram.
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Sebastian,
According to my calculations, reducing "H" to 1 mm should be safe considering a safety factor of four:
My calculation assumes 200 BAR acting across the weak point. In reality, that 200 BAR will also act inside the 1 mm "tube" to the manometer. As such, there is no net force trying to burst that 1 mm thick wall. Anyway, I calculated as if there was a 200 BAR pressure differential across the weakest section; and it is safe, even then. Also, the manometer stem is threaded in and offers support to this this section via the delrin washer.
I did a simple shell calculation, as if your weakest area was a thin walled tube (see crude image explanation below). I considered the projected internal area of an inch long section of this thin walled tube, multiplied by 200 BAR, to generate a force of 579 pounds, trying to split (burst that tube). I then assumed aluminum with a yield strength of 35,000 PSI (from and engineering source).
For the same inch long section, that 579 lb force has to be borne by two walls, trying to resist splitting of the shell. Using the yield strength and factor of safety, I calculated the minimum area that would resist that 579 lb. This was then reduced to a wall thickness of 0.033".
As 1 mm is 0.039", you are safe. Especially considering that wall in your design is thicker than 1 mm everywhere except right at the edge of the 1 mm diameter vent hole.
The very crude sketches below show the projected area, trying to split the shell due to the 200 BAR (red area). And how the aluminum wall area (yellow) has to resist the force generated by the air pressure. As the metal area is much smaller, the stress (also load/area) is much higher than the air pressure (load over area).
I hope this helps, as I need to go do something else. If you want to see how I calculated this in detail, PM me your email address for the Excel spreadsheet...
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=162493.0;attach=284070;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=162493.0;attach=284072;image)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=162493.0;attach=284074;image)
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Im having hard time following calulations in your units, I understand how you do it, just have to put my units in, then I can follow :D
I guess the safest way is to drill a hole next to the 3mm, lest say one on each side 2mm big, this way there will never be any thinner material then 1.7mm..... I have to calculate that first, will post it here then..
I have not considered the grooves for the orings, they are pretty deep to, have to measure and see it the material is thinner on that side....
This is both fun and edjucational 8)
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on the other side of the 3mm hole near the o-ring grooves, there is 1.9mm material left..
I guess it seems the engineers have thought this threw ;)
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Does the mechanical rule also apply high air pressure where it says something like you need to have the length of threads same as diameter of the bolt to get 100% safe to hold up... I think it is 1:1 ratio with steel bolt into a threaded steel hole, and steel into a threaded hole of aluminum its 1:1.5 ratio.....
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RE the question about the force holding the poppet closed.... The area used in the calculation is the outside diameter of the sealing surface.... If the poppet is flat or concave, that would be the OD of the poppet.... If the poppet face is convex or tapered, it would usually be the OD of the valve seat.... In the case of a raised seat in the valve body, and a flat poppet, you would use the OD of the seat....
If you use a tapered poppet and seat, and the poppet material is rather soft, it can wedge into the throat and make the valve more difficult to open.... I general, the softer the poppet material, the more hammer energy is required to open the valve....
Bob
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I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm on mine PR900W 5.5mm ;)
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...
I didnt see yesterday when I looked quick, but it is a tapered hole right threw the gauge now where the manometer is going in, how do you make that leakfree, teflontape?
Yes teflontape I use for sealing, thread depth is 8.5 mm and thread is M8x1mm the material is aluminum I suppose for high pressure.
My pr900w 5.5mm it will be intended for slugs (Altaros slug 0.13 BC which is likely to be produced by JSB soon) ;) :D
And of course I'll put SSG in it ;D
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I widened that hole to 6mm, and valve inlet at 6 mm and outlet at 5 mm on mine PR900W 5.5mm ;)
Now the plenum is the whole cylinder, and I'm going to put a 0.3 l 300 bar bottle and a regulator outside...
I didnt see yesterday when I looked quick, but it is a tapered hole right threw the gauge now where the manometer is going in, how do you make that leakfree, teflontape?
Yes teflontape I use for sealing, thread depth is 8.5 mm and thread is M8x1mm the material is aluminum I suppose for high pressure.
My pr900w 5.5mm it will be intended for slugs (Altaros slug 0.13 BC which is likely to be produced by JSB soon) ;) :D
And of course I'll put SSG in it ;D
but the valve-seat inside the valve where the stem goes into is only 4.5mm hole, and the steam is 3mm in diameter wich goes inside that hole, thats less then 3.5mm effective hole area.... Did you enlarged that valveseat hole also? otherwise there is no point of making the other holes bigger...
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RE the question about the force holding the poppet closed.... The area used in the calculation is the outside diameter of the sealing surface.... If the poppet is flat or concave, that would be the OD of the poppet.... If the poppet face is convex or tapered, it would usually be the OD of the valve seat.... In the case of a raised seat in the valve body, and a flat poppet, you would use the OD of the seat....
If you use a tapered poppet and seat, and the poppet material is rather soft, it can wedge into the throat and make the valve more difficult to open.... I general, the softer the poppet material, the more hammer energy is required to open the valve....
Bob
got it, thanks
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Valve inlet at 6 mm and outlet at 5 mm...
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Valve inlet at 6 mm and outlet at 5 mm...
:o
but how does the stem(valvepin) keep airtight? did you use same valvepin with the white delrin poppet?
valveseat is 4.5mm hole, if you have 6mm? then the valvepin will fall threw, no stop
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Poppet is 8 mm...
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Poppet is 8 mm...
yes . but the steel pin and the brass part is 6mm, that will fall right threw, no stop, the brass and steel pin isnt attached to the white delrin poppet,,,, Im sure in your case when the valve is assambled together you can move the steelpin forward, when the gun is cocked then the brass and steel pin is free to move away from the delrin poppet.... not good. Maybee not plausible to happen, but it could..
if the hole was 5.5mm instead of 6mm wich you have then the brass part wich is 6mm could not be pulled/drop forward threw that hole..
But maybee you got lucky and the brasspart i 6.1mm and cant fall threw that 6.0mm hole??
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Poppet is 8 mm...
yes . but the steel pin and the brass part is 6mm, that will fall right threw, no stop, the brass and steel pin isnt attached to the white delrin poppet,,,, Im sure in your case when the valve is assambled together you can move the steelpin forward, when the gun is cocked then the brass and steel pin is free to move away from the delrin poppet.... not good. Maybee not plausible to happen, but it could..
if the hole was 5.5mm instead of 6mm wich you have then the brass part wich is 6mm could not be pulled/drop forward threw that hole..
But maybee you got lucky and the brasspart i 6.1mm and cant fall threw that 6.0mm hole??
Brass is 6.5 mm ;)
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Poppet is 8 mm...
yes . but the steel pin and the brass part is 6mm, that will fall right threw, no stop, the brass and steel pin isnt attached to the white delrin poppet,,,, Im sure in your case when the valve is assambled together you can move the steelpin forward, when the gun is cocked then the brass and steel pin is free to move away from the delrin poppet.... not good. Maybee not plausible to happen, but it could..
if the hole was 5.5mm instead of 6mm wich you have then the brass part wich is 6mm could not be pulled/drop forward threw that hole..
But maybee you got lucky and the brasspart i 6.1mm and cant fall threw that 6.0mm hole??
Brass is 6.5 mm ;)
ok , so its 6.5mm, understand...
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I have noticed the same thing in my .177 storm rider (same gun).
The gauge port is the restriction with the current valve system. I opened mine up slightly larger than bore diameter and experienced a slight increase in power and efficiency.
There is a solution that I am waiting for to be available. It is a fill port with gauge assembly. My plans are to build a new gauge port without the hole for the gauge to act as a plug and install the new fill port with gauge.
https://www.pyramydair.com/product/diana-fill-valve-and-gauge-assembly-for-bandit-stormrider?a=9034 (https://www.pyramydair.com/product/diana-fill-valve-and-gauge-assembly-for-bandit-stormrider?a=9034)
The valve throat is another thing:
Very hard to open up to be successful with the current valve stem design. My solution was to reduce the valve stem diameter just behind the poppet. This is basically the same thing as opening up the throat without increasing the cracking force of the poppet off the seat.
My next plans are to just a make a new valve the way i want it.
The gun has alot of potential being powerful....
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I have noticed the same thing in my .177 storm rider (same gun).
The gauge port is the restriction with the current valve system. I opened mine up slightly larger than bore diameter and experienced a slight increase in power and efficiency.
There is a solution that I am waiting for to be available. It is a fill port with gauge assembly. My plans are to build a new gauge port without the hole for the gauge to act as a plug and install the new fill port with gauge.
https://www.pyramydair.com/product/diana-fill-valve-and-gauge-assembly-for-bandit-stormrider?a=9034 (https://www.pyramydair.com/product/diana-fill-valve-and-gauge-assembly-for-bandit-stormrider?a=9034)
The valve throat is another thing:
Very hard to open up to be successful with the current valve stem design. My solution was to reduce the valve stem diameter just behind the poppet. This is basically the same thing as opening up the throat without increasing the cracking force of the poppet off the seat.
My next plans are to just a make a new valve the way i want it.
The gun has alot of potential being powerful....
how big hole on the gauge hole did you make, stock is 3mm and there is only 1.7mm "meat" to either the hole where the manometer goes or to the grooves where the oring is.... what is "safe" to drill, if you make the hole 4mm, then there will be only 1.2mm meat left
How much extra power did you gain only open up that gauge hole?
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Sebastian, why not make another two 3 mm diameter through holes, like you suggested already?
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Just under 4mm (3.85mm).
Not much more power added but some. I am just under 25fpe with 10.5 grain maxed out but, I run 20fpe on it using a different hammer spring.
The reason for not power modding more right now, is that the gun on the top half is not rigid enough to keep accuracy (even with an additional barrel band).
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Sebastian, why not make another two 3 mm diameter through holes, like you suggested already?
There is only room for 2 holes 1.5mm big(if you dont want to make the wall thinner where they allready are thin), not sure the extra holes would anything , Im only looking for more efficiency, power isnt my goal, its just interesting knowing how it all works inside a pcp...... And im not sure that even with a 8mm hole the efficiency would go up?
@ shorty, I did ask wrong last time, what I meant more, did you notice any efficency gain when increasing that hole, maybee you didnt do a full shotstring and compared to before? Also why is your gun not rigid enough?
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This is my theory: practicly on any unregulated pcp, If im wrong, correct me: I think there is a big chance of gaining efficiency by making all the ports the same size, so the air can flow smooth, then you regulate power by only the transfer port AND hammer tension, you can also play with hammerweight and different hammersprings of course.
I want to make this PR900W as efficient as possible.... thats my goal, to begin I need to understand how it all works, even if I think I know its always better asking twice so I dont make misstake, I also dont like to step on security, I dont want to risk any failures when making wall thinner and so on and risking getting hurt if a wall burts..
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I didn't do a before and after.
I did just run a string with 7.9 grain. 170 bar to 125 bar (2500psi to 1800psi). 27 shots. 980fps up to 1041fps and back to 982fps. Big spread but 1.64 efficiency at 18fpe is ok in my book.
If your looking for efficiency, you also need to know what power level you want.
I can tell you for sure, porting this gun without regulation is not going to get you alot of shots within in a 3% spread with it's tiny air reservoir.
My gun is set up for backyard shooting under 30 yards. When I first got this gun I was thinking power mods but, the small air reservoir and the overall mechanical strength of the gun is beginning to force me to turn mine into a 12/15fpe regulated gun.
Unfortunately, it's very hard for me to invest in such a cheap gun. Yes it's fun but so many things need attention. My main focus now and has been accuracy and i feel as if I am as close as I can get out of it.
Besides the valve and gauge block, I use a short stiff spring behind the hammer. The short stiffer spring is what gives me the efficiency. I am sure an SSG system would be better but I typically use short stiff springs.
Good luck and be safe. There's alot of information here on GTA for power and efficiency modifications. All our valves are basically the same except for the balanced ones.
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For more efficiency make a SSG (anti bounce hammer) for more power put longer pin. The pin exits the valve only 2.3 mm...
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2.3 mm should be plenty of travel for the valve stem.... If you are using that much valve travel, your gun will be VERY inefficient.... Typical PCPs, properly tuned have a valve lift of only about 1/4 of the throat diameter or less.... Anything approaching 1/2 the throat diameter and you are shooting an air hog....
Bob
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If you want to reduce power make HST (hammer spring tension) or make shorter springs. The weight of the hammer is 33 grams. Orginal spring dimensions OD 9.8 mm x 89 mm x 1 mm, 23 active coils 25 total...
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looking at those loooong valvepins, geek, :o
Im sure you bob has some answer to this: if a typical unregulated pcp like the 900W 5.5mm has everything same size hole(lets say 3.5mm), and its only regulated by the transfer port hole, how does the curve for power/efficency look when going from small (1mm) to big (3.5mm) TP hole, is it linear?
What I know is 1mm TP hole=10 joule, and from reading here it seems 3.5mm TP hole=40 joule, then what is to expect from 1.5, 2, 2.5 and 3mm TP hole in Power? and how does the efficency go with that, does it gets better with bigger TP hole?
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2.3 mm should be plenty of travel for the valve stem.... If you are using that much valve travel, your gun will be VERY inefficient.... Typical PCPs, properly tuned have a valve lift of only about 1/4 of the throat diameter or less.... Anything approaching 1/2 the throat diameter and you are shooting an air hog....
Bob
Mr.Bob how much pin should max. exit the valve in mm?
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Roughly speaking, with everything else remaining the same, the maximum FPE is proportional to the transfer port (smallest port) diameter.... Note I said MAXIMUM, which means a shot where the valve is open until the pellet exits the muzzle.... If the shot is limited by valve dwell, that no longer applies....
The efficiency at constant FPE and pressure will favour the larger transfer port, because that allows you to use less valve dwell.... You are flowing more air in a shorter time.... If on the other hand you are comparing the efficiency at maximum power (dwell long enough that pellet exits the muzzle), you will likely find the opposite, and the efficiency will increase as the power decreases with smaller ports....
The valve stem only needs to protrude from the back of the valve about 1/2 the valve throat diameter, because in proper tunes you will only need about half that in valve lift.... However, if you are trying for maximum power for bragging rights, you may want to allow a bit more than that, so that the hammer never hits the back of the valve.... About 3/4 of the throat diameter is plenty.... Just remember you won't actually use that much for normal tunes, because if you do the efficiency will be terrible, and the gun will be an air hog....
Bob
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Roughly speaking, with everything else remaining the same, the maximum FPE is proportional to the transfer port (smallest port) diameter.... Note I said MAXIMUM, which means a shot where the valve is open until the pellet exits the muzzle.... If the shot is limited by valve dwell, that no longer applies....
The efficiency at constant FPE and pressure will favour the larger transfer port, because that allows you to use less valve dwell.... You are flowing more air in a shorter time.... If on the other hand you are comparing the efficiency at maximum power (dwell long enough that pellet exits the muzzle), you will likely find the opposite, and the efficiency will increase as the power decreases with smaller ports....
The valve stem only needs to protrude from the back of the valve about 1/2 the valve throat diameter, because in proper tunes you will only need about half that in valve lift.... However, if you are trying for maximum power for bragging rights, you may want to allow a bit more than that, so that the hammer never hits the back of the valve.... About 3/4 of the throat diameter is plenty.... Just remember you won't actually use that much for normal tunes, because if you do the efficiency will be terrible, and the gun will be an air hog....
Bob
Valve throat is 6 mm, means 3mm max. thank you
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It means that a proper, operating tune will require only about 1.5 mm of valve lift or less, so that 2.3 mm is fine for the way you will end up tuning the gun.... If you are looking for maximum power (which you will then never actually use), then you may use about 3 mm of lift.... To insure that the hammer never hits the back of the valve, you may need more than that so you don't see strange things in testing, like increased hammer strike causing decreased velocity when you are trying for maximum power....
If I were designing a valve with a 6mm throat, I would allow for 4 mm of stem travel, knowing that I will then have plenty.... However, I would expect that the 2.3 mm you now have would produce exactly the same results, once the gun is properly tuned.... Don't forget that lengthening the valve stem decreases the hammer stroke.... If the gun is already hard to cock you will make that even more of an issue, because you will need a stiffer hammer spring because of the reduced hammer travel.... If that is not an issue, then a bit less hammer stroke won't matter, of course....
I presented this argument about valve stem length because it is a common error that people think that making it longer will allow the valve to open further and give move power.... It is a rare gun indeed where it actually doesn't have enough valve stem travel.... Most of the time, fitting a longer stem results in a decrease in power, because of the reduced hammer strike.... I was trying to save you the headache of finding that out and wondering what happened....
Bob
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Bob Sterne,my deep respect for your lifework in the airguns world...
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very well explained, still learning :)
But is it possible for a "tune" with all holes wide open let say 3.5mm all the way, and a very weak tension OR a weak spring behind the hammer to get more efficiency compared to a restriction in TP hole of 1mm and a harder hammer tension.... reason asking is cause here all airguns have 1mm TP hole, big hammerweights and strong hammersprings, so you need to strike that valvepin hard to get that 10 joule, I think that the other way around may increase the efficency? bigger TP hole instead of the small 1mm and less tension or weaker hammerspring to still be max 10 joule but hopefully increase efficency?