Big power difference on air vs. CO2

[nervoustrigger]

It has been a long time since I have played with both CO2 and air on the same platform.  And when I think back on it, I don't think I had ever done a side-by-side comparison with the same state of tune but I recently had the occasion to do that.  The results weren't what I expected so I figured I'd share the numbers.

Anyway, the platform is the ubiquitous QB78 in .177 cal.  It has all the usual porting restrictions opened up (valve, barrel, bolt probe, etc.).  I started with CO2, loading a couple of cartridges into the tube.  The temperature was 90F which puts the CO2 at about 1100psi after I left it outside for a couple of hours to dwell in the heat.  I then grabbed the chronograph and dialed the hammer spring preload to the velocity knee which ended up at 623fps / 6.8fpe

I plinked with it a while until the cartridges were finally depleted, then I swapped the end cap with one I had adapted to a Foster fill.  I pumped it up to 1300psi which is about 200psi over the CO2 pressure to see if I could wring out a useful bell curve from this tiny volume (about 3ci).  Without touching the hammer spring preload, I ran a string of 21 shots.  Average power for those shots was 11.5fpe with a high of 12.7fpe.

So the average was about 70% more than CO2 and the peak was 87% more.  I wasn't expecting such a big difference.

What would you guys generally expect?

[Bentong]

WoW...21 shots off QB78 tube, Sure is plenty of shots for dispatching nutters @ 12fpe. What pellet did you use? 👍👍

[moorepower]

Did not expect as much difference, even with CO2 being much denser.

[nervoustrigger]

I like the way you think

There were only about a dozen shots inside of a 4% ES, but yeah probably 18 that would have been good for my usual squirrel range of 30 yards or so.  That’s more than I’ve ever used on a single outing so I might just take it out when the season rolls around. 

Despite the fact I was using a lightweight pellet (7.6gr), efficiency calculated out to 1.70fpe/ci with nothing more than an MDS nylon hammer to help it out.

[PG in San Diego]

I plinked with it a while until the cartridges were finally depleted, then I swapped the end cap with one I had adapted to a Foster fill.  I pumped it up to 1300psi which is about 200psi over the CO2 pressure to see if I could wring out a useful bell curve from this tiny volume (about 3ci).  Without touching the hammer spring preload, I ran a string of 21 shots.  Average power for those shots was 11.5fpe with a high of 12.7fpe.

So the average was about 70% more than CO2 and the peak was 87% more.  I wasn't expecting such a big difference.

What would you guys generally expect?

Hey Jason,

Funny you bring this up. I bought a Co2 Adapter for my AF Talon and after shooting it with the Co2 yesterday, I swapped back to air using a 13ci bottle at 850 (Attached it to my co2 adapter) and the pellets went though my target, and the thin plyboard using the same pellets and power wheel number. I want to chrony it later this week to check out the FPS, but there is no question, the air was much more powerful.

.

[nervoustrigger]

Moorepower, thanks for reminding me.  The density of air is about 34% less than CO2.  The resulting velocity difference was right at 30% which is pretty similar (whereas the energy difference of 70% involves a squaring term).  However I have no idea if this correlation is generally true.  For example, I can see where other factors are changing as well like valve dwell and how far down the barrel is the pellet when the valve closes.

For some reason I had it in my head that the difference in energy is usually on the order of 20 – 30% but maybe it’s actually the velocity.

[Ribbonstone]

Have run the same rifle on both several times....not very far off your results,but it does matter how the rifle is adjusted/tuned.   

More going on than just pressure.  Can word it several ways,but basically CO2 is a denser/thicker gas than air.  Even at the same pressure,the "fat gas" shoots slower....not unlike the folks that experiment with He. in PCP's.

[rsterne]

At equal pressure, CO2 will shoot slower because the gas is heavier (more dense).... For the same valve dwell and pellet weight, the weight of CO2 released will be greater, and hence a higher percentage of the total energy goes into accelerating the CO2 and less ends up in the pellet.... There may be a slight loss of velocity due to the increased "viscosity" of the CO2 as well (less ability to flow through a port or barrel).... In addition, since CO2 cools as you shoot, the pressure, and hence the velocity, will drop on successive shots, which you would be hard pressed to see with HPA, particularly if the gun is regulated....

Having said all that, most of the difference between HPA and CO2 is often due to using increased pressure with the HPA.... The pressure increase alone will results in an FPE increase roughly proportional to the ratio of pressures....

Bob

[Gippeto]

... most of the difference between HPA and CO2 is often due to using increased pressure with the HPA...

Bob

This meshes with my results. Only did one test using co2 and air at the same measured pressure, and got 750fps on co2, 830fps on air....an extra 10-11% due to the gas alone. Then with the old .30 cal rifle I worked on, it was 775fps on 1000psi air, 698fps on co2...pretty much spot on for the percentage...pressure makes the difference.

I admit, I did not measure the co2 pressure on the old .30, but the highest I've ever measured co2 is 1000psi on a hot day....despite what the vapor charts say....

Al

[Ribbonstone]

The revese worked out well too.   If you only need/want co2 perfomance on air, can use a lower pressure HPA.

[nervoustrigger]

Hi Robert, you said you’ve gotten results that aren’t far off from mine.  Did you happen to save any side-by-side figures?
 
In most cases, I’ve followed the path I presume Bob was referring to…changing from CO2 to air at some elevated pressure.  That requires a re-tune which means I don’t have occasion to make a direct comparison between CO2 and air.

[nervoustrigger]

Okay thanks Bob.  We were chatting about the density difference but I wasn't thinking about it in terms of viscosity as well.
 
FWIW, the self-cooling nature of CO2 wasn't a factor in my comparison because the chronograph figures that yielded the 623fps / 6.8fpe were taken after the AG had been dwelling in the 90F heat.
 
Regarding the pressure difference, I ended the HPA test at 650psi and it was still producing 10.9fpe.  Like Gippeto, I don't know what the CO2 pressure was.  Perhaps it was indeed lower than the 1100psi predicted by the vapor charts but I'd have to think it was comfortably higher than the HPA end pressure of 650psi.  Yet air at such a low pressure was still holding a 60% energy advantage.
 
So at this point I'm getting mixed messages as to whether a difference of this magnitude is expected.  When I swap back to CO2, I will double check that the hammer strike is adjusted optimally (~5% under peak velocity).  The HPA bell curve is centered on 1000psi so I don't think it could be far off of where it should be for CO2 on a hot day, but I want to be sure the CO2 energy figure is not unfairly represented.

[Ribbonstone]

Neither of thesewere hot-rodded on air...kind of calmrifles,but running on low pressure (non-regulated) air.
Simple rifle (HiPAc/.177 rebarrled 2260).Air pressure not regulated,but about the same as co2 pressure that day in the middle of the 10 shot string...so the average vel.of the airstring is atabout the same pressure.


The results are very very close to what you posted.





Another simple rifle. FDPCP.  Again,air prssure was about the same as co2 pressure at about the midle of the shot string.






When I HPA'ed a couple of QB's, only wanted 12foot pounds, so used 850PSI out put tanks.  Isn't really a more efficient use of air, but given a wider range of pressure per fill (2150psi worth of regulated air), can get 200-205 good shots per fill with minimal adjustments/mods.

[nervoustrigger]

Hi Robert.  Wow, the 2260 figures are not just similar proportionally but almost identical at 7fpe / 12fpe.
 
I see the difference for the XS60C wasn’t as big but like you said, the way it’s tuned may explain it.
 
Thanks for validating what I’m seeing.  I feel better about it now.

[Ribbonstone]

Those two (and a couple of 2K fill PCP's)were about as close as I could come to adjusting them (as PCP's) to down-tune to run well at co2 type pressures.

Not that it really made a whole lot of difference in accuracy... co2 was a little bit LESS accurate.

Lots of "could be's"...could be a different pellet might have helped,but to keep it even used the same pellets from the same tin...could be that the tuning was better for air than co2...could be vibration...could be one "oops"shot.....could be a whole lot of "snit".

.177/2260  (from above)...take offHammerli 850 barrel fitted to the 2260:





FDPCP (from above post):





 HAve a couple of hours of bordom to fill.....have adjusted the 5mm QB conversion for really good 12gr. use.....have a spare HPA bottle....have bulk fill QB end cap.

Different in that this one starts out as tuned for good co2 perfmance rahter that starting out as an "air"gun like the two previously posted.

Lets see what happens if I feed it 850PSi air rather than 86F co2.

EDIT:for current tempertures of 86F (it rained)

[AndyKunz]

Those groupings are "the same," it just looks like the scope wasn't reset for the greater drop.  I wouldn't call that "less accurate."

Andy

[nervoustrigger]

Robert, I was reading your comments in the other recent CO2 thread about a slight accuracy edge going to air even at modest distances.  My experience tends to agree, although I’d be quick to acknowledge I probably run CO2 only 15% of the time so that may have something to do with it.  On a good day (for me) and calm conditions, the differences seem to be pretty hard to distinguish and there’s no way I could account for all the variables over the course of a session or two, but over the long haul the trend seems to favor air.
 
Based on the fact the results really begin to diverge at 40 yards and beyond, I usually reserve CO2 for inside of 30 yards or so for precision stuff.  That leads me to think the biggest factor is wind.  Since domes generally have the best average BC somewhere between 800 – 900fps, that stands to reason since none of my CO2 setups run much over 700fps.  Frankly I don’t want them any higher though because it annoys me to be constantly swapping cartridges.  Instead I prefer to just enjoy using it for the task at which I think it’s best suited.
 
Really the only other objective difference I can think of is the lock time.  With a lower muzzle velocity, the pellet spends longer in the barrel so there is more opportunity to pull off target a bit between the time the sear breaks and the pellet leaves the muzzle.  Also since the amount of hammer strike needed is proportional to pressure, and CO2 at 850psi will produce a lower velocity than HPA at 850psi, whatever amount of cyclic vibration is introduced by the hammer whacking the valve stem will disproportionately affect the CO2 setup.

[Ribbonstone]

ABOUT AS inelegant a test as I have ever posted...wanted answers more than elegance.

(OK...honest...have done things just as ugly in serach of data when it "made no nver mind" how it looked.)

Nothing neat about this,just wanted the air vs. co2 data.

DSCN2849 by Robert Dean, on Flickr

Two weeks back,tuned it up for the 12gr. co2 perfomance I wanted. Adjustedthat QB 5mm for really good co2 use by way of a lite striker,SAME striker spring, smaller transfer port,standard QB valve (with piercing pin/fiber filter removed).

Same side ways mounted (and tested against other chronographs) cheap ($20-$30) chronograph used with the co2 tests.....same rifle other than the HILLBILLY tank hanging off the  bulk fill adpator.

(Some how....I feel like I should say "HEY ya'll,lookie here"...thinking I really am a bit of a snob....results are results).

DSCN2812 by Robert Dean, on Flickr

Totally empty of co2,just put in the bulk fill valve and attached an HPA tank directly to an ASA fitting I used for bulk filling co2 from paintball tanks....and just let it dangle (didn't leak).

Just a summation of numbers from the two pellets that my hands happened to fall on when testing:

14.3gr.Accupells(think unsorted 5mmCP's:

 12gr. CO2 : 562fpa/10FPE
850Air: 594fps / 11.2FPE


23gr.Eun Jins:

12gr. CO2: 478fps / 11.6 FPE
850Air: 507fps / 13.1 FPE


DO REALIZE that bulk fill co2always did increase velocity a bit...just less "clutter"in the waywhen the piercing pin was taken out....and it may have been even more equal(if I still had any tanks oc co2 to bulk fill with).

AM thinking about this.


Optimizing for co2 use kind of negates low pressure air use for power... would need to UP the pressure.  850PSI air is a little more powerful, a LOT more temp. insensitive (so more a year-round power source)...but does NOT have to be more powerful.

[nervoustrigger]

Robert,
 
Ah, okay thanks for the extra data points.  This arrangement shows a much smaller difference between CO2 and air.  I’m curious what the relevant factors are. 
 
I’m looking at the 10fpe figure you got on CO2 with a 14.3gr pellet…I hadn’t mentioned it yet but I had temporarily fitted a .22 cal barrel on the setup described in my original post and it yielded 11fpe on CO2 so that agrees pretty closely with your result.  I say that in spite of the fact there are probably several differences in how our two setups are configured but it’s an interesting similarity nonetheless.
 
I see you described running a lightweight striker and a reduced transfer port for “really good CO2 use”.  Looking at the fairly modest energy increase (~15%) that came from using a much heavier 23gr pellet, I take that to mean the hammer strike is adjusted a little lower on the velocity curve than mine, so I’m guessing that accounts for at least part of the reason the air vs. CO2 results are closer together.
 
And whereas the porting in my setup is pretty generous, you fitted a reduced transfer port so that could be another important difference.  However it seems to me a restriction would favor the less dense/viscous gas (air) because it can squirm its way through more easily than CO2, so that’s no explanation for why CO2 and air are producing similar energy.
 
Anyway, I’m just thinking out loud.  Does that make sense?   Do you have other thoughts on why the gap between air and CO2 is so small?

[Ribbonstone]

The old bulk fillfitting only fits two of the current QB gas tubes....tried it on another.  .177 Carbine (restocked) version:

Same "hillbilly"tank attachement....but if it makes a rat'sarse of differcne,I'dlike to know how)....same 850PSi HPA tank vs. Co2.

DSCN2850 by Robert Dean, on Flickr

Also adjusted for co2 use (transfer port/striker weight/striker spring)...it's a pest-bird shooter(which explains the stright wrist stock made for shooting fast and "up" into trees).

A bit different...never did replace the valve or put back in a pirecing pin (so I could put back in the HiPac unit if I wanted go go back up in powerby readjusting).  So it's a 1 x12gr. shooter, but without the valve clutter.


co2 (86F by the thermoneter): 10.5gr.@665fps/ 10.3 FPE
850-PSI air:  10.5 @  720fps / 12.1 FPE


First time I ran this backwards (optimized for co2 vs whatever I get from 850PSI air).  Uasually it's the other way around.

Not 100% sure what to make of it

Evidently set up the striker  to snap the valve open/closed quikcly on co2....so it snaps open/closed just about as quickly on air of about the same pressure...opening foce about equal to closing force.....the difference in energy produced most likely from the same volume of gas,air is just more energetic/less "thick"?