GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on June 08, 2017, 03:35:30 PM
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Ever wondered how much of the energy in a PCP goes into accelerating the air along with the bullet?.... Lloyd just added that calculation to his Internal Ballistics Spreadsheet, and it's quite an eye-opener.... I'll just give you a few of examples....
.25 cal with a 20" barrel, running regulated on 2000 psi, 25.4 gr. pellet at 900 fps, with an efficiency of 1.5 FPE/CI.... Weight of air in barrel = 9.15 gr. (36% of pellet weight)....
Same gun, but keep the valve open until the pellet reaches the muzzle.... Velocity 1079 fps, efficiency drops to 0.66 FPE/CI.... Weight of air in barrel = 29.9 gr. (more than the pellet)....
.308 cal with a 24" barrel, running on 3000 psi, 120 gr. bullet at 900 fps, with an efficiency of 1.0 FPE/CI.... Weight of air in barrel = 64.7 gr. (54%)....
If you wonder why Helium makes such a big difference in velocity.... take the last example, and replace the air with Helium (no other changes)....
.30 cal with a 24" barrel, 3000 psi of Helium.... That 120 gr. bullet will now hit 1033 fps, at an efficiency of 1.22 FPE/CI.... because the mass of He is only 9.62 gr. (8%)...
Bob
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Great, now to get the most from my gun, I'll need to buy a scale that will measure air molecules ???
Interesting stuff, Bob!................but now my head hurts :o
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An air scale, I want one.
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That is far beyond what I would have ever guessed Bob.
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Now I feel stupid for laughing at Betty Lou when she asked me if my SCUBA tank was heavy because it had just been filled ....
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I think this helps me understand how efficiency is greater in our guns at higher pressure and lower dwell times. didn't think to blame the air! I just thought it was only because you use less thus, you are more efficient but now I see that it's using less and having to put less energy into the air itself to get it all moving. who would have guessed I would get smarter from air gunning? one of these years I'll understand how to hit what I aim at but one step at a time. this does help at home though. when the wife complains I'm reading the forum too much I can say hush woman, I'm learning physics! then I'll get to shoot more as I'll be kicked outside. win win
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From observation, most of us already figured out that the law of "diminishing returns" was in force.
Now we have a numeric view of "why".
Thanks bob.
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Now I feel stupid for laughing at Betty Lou when she asked me if my SCUBA tank was heavy because it had just been filled ....
Ha ha, I know what you mean. I never really thought about this until several years ago when the owner of the dive shop was filling one of my tanks and he mentioned that the tanks were heavier after they were filled. I said "no way," and he said "Yup, about 5 or 6 pounds heavier." So I had to go home and do the math (couldn't help myself, LOL.). Since scuba tanks are sized in standard cubic feet of air, and one std-cuft of air weighs .0807 pounds, an 80 cubic foot tank, filled to capacity, has 6.45 ponds of air in it. Who'd a thunk it?
Lloyd
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I've seen evidence of this a while now. As Lloyd said, a full scuba tank is usually heavier than an empty only. If even seen th effect of this on guns with a removable reservoir. A full 255 cc reservoir will weigh about 4 ounces more than an empty one.
From what I can tell, the difference in weight can varry according to both the pressure of the compressed air in the cylinder, as well as the amount of moisture in the air going into the tank when it's compressed. Pressure becomes a little less of an influence the drier the air is, but only to a point. Then the weight starts to go up the higher the pressure goes for the given size of vessel. Could be because there's no way to be absolutely certain that 100%of the moisture in the compressed air is removed while filling. I don't know this for sure, but that's my train of thought. Smarter than I can verify/disprove this as necessary.
Keep up the good work Lloyd. Your efforts and insights really are appreciated by many.
You too Bob.
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Interesting info about sailboat fuel. Nice job Lloyd and Bob! I like what one member has on his avatar. "It's free until you compress it"
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Great topic!! When you try to understand the dynamics in the order to be more efficient, those things matter...In the same lines, if the same reservoir is filled at the same pressure and brought to a higher altitude, can it shoot faster due to LESS AIR DENSITY in front of the pellet??
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The difference in muzzle velocity at higher altitudes, in a PCP, is miniscule.... However, once you shoot the pellet, the lower air density causes less drag, so it maintains that velocity better....
Here is another visualization I just did.... It's only approximate, and only in 2 dimensions, but it will give you an idea what happens when you compress air....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Air%20Molecule%20Spacing_zps4xvo9y3v.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Air%20Molecule%20Spacing_zps4xvo9y3v.jpg.html)
The black dots represent an air molecule (oxygen or nitrogen).... They average about 4 Angstroms across (0.0000004 mm, and yeah, they are not spheres).... In a liquid (lower left), they are pretty much touching, but vibrating and changing positions, not locked into a crystal structure like in a solid.... As a gas, at 1 bar pressure, they average about 40 Angstroms apart (upper part of the diagram).... As you increase the pressure, they get closer together.... At 125 bar (1800 psi, lower right), they are about 8 Angstroms apart, about 1/5th the spacing at 1 bar (roughly enough room for one more molecule between each pair).... Since we are working in 3 dimensions, the volume occupied by a given number of molecules is the cube of the spacing.... so we have (1/5 x 1/5 x 1/5) = 1/125th the volume.... because the pressure is 125 times as great....
You may have heard me state that at very high pressures the pressure goes up faster than Boyle's Law for ideal gasses predicts.... What is happening is that the space between the molecules gets so small that they start developing additional pressure from hitting or interfering with each other, and not just the walls of the container.... It is called the VanDerWaals effect, and this diagram shows you how that might be occurring.... Now imagine each and every one of those molecules travelling in random directions at an average speed of 1650 fps.... like a crowded pool table gone mad.... in 3 dimensions.... :o
Bob
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Shows how much of a variable the pellet weight to air weight ratio is in terms of efficiency. Exactly the same point I made in another thread. Very clear here that increasing pellet weight in the same caliber increases efficiency due to the air column still weighing the same but the ratio of pellet weight to air weight changes...
I wish there were heavier pellets than 34 gr in .25 cal that were actually worth their weight ;)
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Totally agree with you Matt!!! I refer to Bob`s drawing to get the picture of Helium compressed and is it right to say that in compressed Helium ,the molecules would be TIGHTER bounded together??? Is the lack of weight in Helium causes the acceleration to be FREE of dragging the weight of air..
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Interesting that the weight of the air slows itself down.
Somehow I would invision the momentum of the heavier air bringing the efficiency difference between it and helium closer with heavier projectiles.
Of course I have no formula to quantify this theory.
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Totally agree with you Matt!!! I refer to Bob`s drawing to get the picture of Helium compressed and is it right to say that in compressed Helium ,the molecules would be TIGHTER bounded together??? Is the lack of weight in Helium causes the acceleration to be FREE of dragging the weight of air..
I would imagine there are more helium molecules in terms of ppm in that diagram compared to nitrogen/oxygen molecules which is what most of the compressed air we're dealing with is compromised of of course...
Nitrogen's structure is hexagonal where helium is hexagonal close-packed
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Back to basics, guys!.... The number of atoms or molecules in one MOLE is the same, regardless of the element or compound.... Avogadro's Number, 6 x 1023.... Also, 1 mole occupies 22.4 liters at STP.... What IS different is the mass of those atoms/molecules.... At any given pressure, they aren't packed any tighter.... although big atoms/molecules will have less space between....
Helium produces more velocity and energy simply because less energy is expending accelerating itself, because it has less mass.... It's Atomic mass is 4, whereas Nitrogen it 28 and Oxygen is 32.... Air, being a mixture, is about 29.... Therefore, air is 29/4 = just over 7 times as heavy as Helium.... Gasses have no "structure", of course, that is only found in solids....
Bob
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Bob is correct the structure I referred to was crystal structure, and I wasn't certain if it applied or had any significance when in gas form. I certainly can say its quite interesting that there's such difference in between the two forms :)
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Hot air? Well my ex-gf weighed 130 pounds and she was full of it. You figure it out.
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so the next step in attaining higher efficiency is to figure out how to evacuate the air out of the barrel, after loading the pellet, so it would be shot in a vacuum.
kj
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KJ!! Due to the enlightment of the new discovery by Bob, I might say that the better efficiency will probably comes with bigger caliber and valve opening in those calibers.... The theory behind the AIR WEIGHT is meaning at the same pressure, it will move the at the same speed down the barel independent of the caliber used..
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Sorry, kj, a vacuum in the barrel will only improve the performance of a PCP by a very small amount.... A perfect vacuum would only increase the force across the pellet by 1% if you are using 100 bar HPA.... and only by 1/2% if you are using 200 bar air.... etc.etc.... Likewise, the air in front of the pellet does not weigh very much, because it is only at 1 bar pressure.... In the first example I gave (25 cal, 20 barrel), the air in front of the bullet only weighs about 0.3 grains.... adding just over 1% to the weight to be accelerated.... Applying a vacuum to the barrel would at most add 1% to the velocity and 2% to the FPE....
You are likely thinking of the impressive performance of ping-pong ball vacuum powered cannons.... They get that performance from very large calibers with long barrels, and a very light projectile.... where the weight of the air in the barrel is many times more than the ping-pong ball, and the driving pressures are quite low....
Bob
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well hot air molecules do matter... say the rifles reservoir is 150 cc of 3000 psi in a -10*f Montana winter Snow Shoe hunt...duh molecules is much
more densely packed than the same rifle on a 100*f summer Nevada Jackalope hunt... has a charge that weighs less...
Energy state/velocity of course matters.. ;)
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well hot air molecules do matter... say the rifles reservoir is 150 cc of 3000 psi in a -10*f Montana winter Snow Shoe hunt...duh molecules is much
more densely packed than the same rifle on a 100*f summer Nevada Jackalope hunt... has a charge that weighs less...
Energy state/velocity of course matters.. ;)
Well, -10F and +100F are both outside my comfort zone, LOL, but you do make a good point about how temperature affects the energy available in a compressed gas. For CO2, we know that would make a big difference. But for a PCP running on air, I am not positive, but I think it relates to degrees Kelvin.
-10 F is 250K and 100F is 311K, or 24 percent higher. Does that mean 24% more energy is available? Maybe a little off topic, but it seems relevant.
Lloyd
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From what I can tell, the difference in weight can varry according to both the pressure of the compressed air in the cylinder, as well as the amount of moisture in the air going into the tank when it's compressed. Pressure becomes a little less of an influence the drier the air is, but only to a point. Then the weight starts to go up the higher the pressure goes for the given size of vessel. Could be because there's no way to be absolutely certain that 100%of the moisture in the compressed air is removed while filling. I don't know this for sure, but that's my train of thought. ................................
Ricky, moisture in the air can certainly be a nuisance for people who pump-their-own in places with high temps and high humidity. (Is that you? ;) ) This sounds like a good topic for another thread. Just for yucks, air pumped at 95 degrees F and 95 percent relative humidity will have 10 times as much water in it :( as air pumped at 60 degrees F and 30 percent RH. Think of that ice tea glass in the humid summer and how the water just pours off of it.
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I am curious on the subject matter of temperature effecting available energy.
I understand that pressure rises/lowers as temperature rises/lowers...with that aside
Does temperature have another effect on compressed air and available energy? IE:
3000 psi @ -10f vs 3000 psi @ 100f, or 250k vs 311k?
I was under the impression that temperature only effects pressure by either raising or lowering it. I am too uneducated on thermodynamics to state the above concretely though D:
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If you fill two identical tanks at standard temp and pressure (STP) and then chill one to -10F and heat the other to 100F, the pressures will be different, but the density and weight of the gas will remain the same.
Pv=nrT {pressure x volume = number of gas molecules(kinda, ha ha) x constant x temperature}
So, if the container volume and the number of gas molecules stay the same, when the temp goes up, the pressure has to go up too. If you have a digital gauge on a tank and watch it throughout the day, you might think the gauge is goofy with the readings always changing, but really, the pressure in the tank is changing with the temperature.
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After reading all the post from the beginning, is it right to say that the weight of air is costing air gunners a price in performance??? In one post it says humidity in the air causes the tank to weight more.. But to push that air charged with humidity cost in the ACCELERATION DEPARTEMENT.
If the laws of physics are valid, the more an object weight, more energy is required to move it.. That being said,an object in motion weighting a certain amount retain it`s momentum more that a lighter object.. I think in both cases, the weight factor cancel out because the lighter DRY AIR accelerate faster but contains less momentum..
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Per Bob's original post, the lighter the gas (generally) the higher the achievable velocity. Helium guns are a lot faster than air or nitrogen guns.
For the internal ballistics of an airgun, momentum is rather irrelevant. You are accelerating a specific mass, along with the mass of the air (a=F/m) and the heavier the mass, the slower the acceleration. But after the pellet leaves the barrel, momentum is a good thing. Heavier pellets carry more momentum (weight x velocity) and more energy (weight x velocity squared), there fore, they can generally hold their velocity better, resist cross winds better, and hit harder.
As for the additional weight of the water in the air, at worst case (filling a tank a 95 degrees F and 95% RH), the weight of the air in the tank will increase by about 3%, but only if the water stays in the compressed air and does not condense out inside tank.
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Doesn't the Available Energy increase as pressure increases? Meaning 1 psi @ 1 cc has around .905 FPE available when at 1,000 psi, where as 1 psi @ 1 cc has around 1.023 available FPE when pressure is increased to 4,000 PSI (with adiabatic expansion only for simplistic reasons)...which is only a 13% increase in energy over the 300% increase in pressure but, does the Mass of air increase that same 13% which would inevitably negate the increased energy available at the higher psi since it has to accelerate its own heavier mass?
Physics hurts my brain :(
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Ultimately, I think it is the pressure that is the critical factor in PCP performance, not the temperature.... As you raise the temperature, the pressure in the reservoir increases, but in an unregulated PCP that can actually reduce the velocity.... If you fill your tank cold, and then warm it up, the pressure will increase, and with it, the shot count, THAT is where the increase in the heat energy shows up.... as increased number of shots.... for most PCPs....
There are a couple of exceptions where hotter means faster.... One would be a gun tuned for a dump shot (maximum power on the first shot), where the increase in pressure caused by the higher temperature would produce more velocity.... The other is in something like Lloyd's "Hypergun", that hit 2162 fps.... The faster moving molecules in hotter air may help the performance of such a beast, as least in theory....
For the most part, changes in temperature or humidity will have little effect on PCP performance.... However, the density of the gas, which affects the amount of work that expanding gas must do in accelerating itself.... has a very real, and profound affect.... That is why CO2 or Argon are slower, and Helium is faster.... compared to air or Nitrogen.... Nitrogen is marginally more powerful than air, by (29/28) = 3.5% in its potential effect on FPE....
Bob
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Matt, to answer your question, the density of the air at 1000 psi is 83.2 kg/m^3, but at 4000 psi is 302.4 kg/m^3.... Although the pressure is 4x higher, the density has only increased by 3.63X.... That is the VanDerWaals effect in action....
Bob
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Heres some good reading if anyone is interested and my air weighs less than your air nanana lol. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch4/deviation5.html (http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch4/deviation5.html)
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Matt, to answer your question, the density of the air at 1000 psi is 83.2 kg/m^3, but at 4000 psi is 302.4 kg/m^3.... Although the pressure is 4x higher, the density has only increased by 3.63X.... That is the VanDerWaals effect in action....
Bob
Thanks for the answer Bob. Based on what I understand of that, higher pressure operates more efficiently due to having less mass thus providing you with slightly more FPE per psi/volume when compared to lower psi. So it does not negate itself, the #'s you helped provide me with for fpe available at 1 psi per cc at specific pressure is due to the above effect?
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Wow, Getting Technical, this sounds a lot like a Refrigeration Class, since air is ruffly 78% Nitrogen and Nitrogen is basically inert (within normal environmental temps), only 22% of the air's content which is other gases are affected by temperature, so it's these other gases which constitutes the difference in pressures when exposed to a heat source.
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Wow, Getting Technical, this sounds a lot like a Refrigeration Class, since air is ruffly 78% Nitrogen and Nitrogen is basically inert (within normal environmental temps), only 22% of the air's content which is other gases are affected by temperature, so it's these other gases which constitutes the difference in pressures when exposed to a heat source.
Nitrogen is a gas and is still affected by changes in ambient temperature (about one psi for every 10° Fahrenheit or 3%) in a 30-35~ psi tire...it certainly experiences wild swings when 10-20F changes occur at 3000-4500 psi? My guess is the same 3% and that # coincides with my experiences. If I take my bottle out from my house (75F) into the sun (90+F) I experience around ~6% gains...can't give a proper % and temperature fluctuation though as that is very rough estimation
I've gone hunting from a heated house/car to outside with my bottle and the temperature drop there can be 60-70 F..if not more and the psi drop is HUGE.
To touch on that, someone shooting a string in the sun versus someone who is not, the person shooting in the sun could experience upwards of 6% if not more in shot count/efficiency ;)
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yes it does, not every thing can defy the laws of Physics.
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Pressure changes as a result of temperature in accordance with the ABSOLUTE temperature (Kelvin or Rankine).... Since 70*F is 530*R, a 50*F change in temperature either way results in less than a 10% change in pressure.... This applies to all gasses and over almost all the pressures we use in a PCP.... or a car tire.... It does NOT apply to a liquid boiling into a gas, like CO2, of course....
Bob
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So!!! To get back at the original question of the tread, does a bigger reservoir full of air penalize in weight compared to an empty one??? The biggest reservoir COMERCIALLY AVIABLE is 500 cc... In a Benjamin Discovery I dont think it is significant but in guns like the FX Royal 500 is it?
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A 500 cc reservoir, filled to 3000 psi, would weigh about 5 oz. more than it does empty....
Bob
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a question that will probably muddy the waters and make me understand even less of what's going on but here it goes. what effect does the cooling of the rapidly expanding air have on the effieciency of the air used when you fire a shot? does this contribute to higher pressure, smaller sips being more efficient overall? some really cool stuff to learn here, thanks to those that understand and have been explaining it.
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When you fire a PCP, the air cools to some extent, but the expansion is nowhere near Adiabatic, as some believe.... If pure Adiabatic expansion was taking place, the exhaust plume would be so cold as to freeze the moisture in the air to snow.... I use a model where the expansion while the valve is open in Isothermal, and after it close is Adiabatic, but even that is probably wrong (although it models well).... The expansion in both cases is probably only partially Adiabatic....
To answer your question, the cooling of the air, to whatever extent that happens during the shot, reduces the energy available to accelerate the pellet.... That is quite a different thing than why higher pressures tend to be more efficient.... With a higher pressure, more acceleration takes place early in the barrel, so you can close the valve sooner.... The air trapped between the valve and the pellet then has further to expand, extracting more energy, and producing a lower residual pressure at the muzzle.... That is why in an unregulated PCP, the report is less at the beginning of the shot string than at the end.... The small sips of higher pressure at the beginning of the shot string is more efficient than the larger gulps of lower pressure towards the end of the string....
HTHs....
Bob
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When you fire a PCP, the air cools to some extent, but the expansion is nowhere near Adiabatic, as some believe.... If pure Adiabatic expansion was taking place, the exhaust plume would be so cold as to freeze the moisture in the air to snow.... I use a model where the expansion while the valve is open in Isothermal, and after it close is Adiabatic, but even that is probably wrong (although it models well).... The expansion in both cases is probably only partially Adiabatic....
To answer your question, the cooling of the air, to whatever extent that happens during the shot, reduces the energy available to accelerate the pellet.... That is quite a different thing than why higher pressures tend to be more efficient.... With a higher pressure, more acceleration takes place early in the barrel, so you can close the valve sooner.... The air trapped between the valve and the pellet then has further to expand, extracting more energy, and producing a lower residual pressure at the muzzle.... That is why in an unregulated PCP, the report is less at the beginning of the shot string than at the end.... The small sips of higher pressure at the beginning of the shot string is more efficient than the larger gulps of lower pressure towards the end of the string....
HTHs....
Bob
I certainly observe a lot more shot noise as you shoot at lower psi's, I can even tell the shot report noise difference in just about 15~ fps...quite a difference!
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you drew a very clear picture with that explanation bob, thank you. now I think I'm getting it
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Bob is the Einstein of Airguns...... Quantum Pneumatics......... E=MRod2........
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HAHAHAHA! that's hilarious. I'll never be able to quote Einstein again without saying MROD