Posted by allen hill on December 27, 2001 at 11:19:08:
Please clear up
some (mis)understandings about co2.
i have been told that co2 is sort of
self regulating, that until the tank is almost completely empty that the gas
pressure is practically constant- as long as the ambient temperature is above
some(???) degrees f., below this temp. (Whatever it is) the liquid won't
vaporize.
I have also been told that co2 pressure is directly proportional
to ambient temp.
Huh????
I’m wondering because i wanted to buy a
pneumatic pistol but found a used co2 gun i might be interested in.
Posted by TCooper on December 27, 2001 at 14:40:37:
I currently own a BRNO
TAU-7 CO2 pistol and am somewhat familiar with CO2 properties.
Yes, CO2 is
self regulating. When contained in a sealed reservoir the CO2 will comprise of a
liquid and a gas. In an airgun the gas is expelled when a shot is taken. The
pressure then drops until some liquid converts to gas and brings the pressure
back up. When the liquid is eventually all converted to gas (after many
shots)the pressure will drop with each shot until the gas is pretty much all
gone. The conversion of liquid to gas is the self-regulating effect. When CO2
liquid converts to gas there is a chilling effect which causes a slight decrease
in pressure. Some CO2 guns will have a velocity decrease if fired rapidly with
little rest between shots. The "rest time" allows the CO2 pressure to stabilize
inside the reservoir. Most 10 meter style guns have valve designs which are not
as drastically affected by quick shooting as low priced guns are. I did some
testing with a TAU-7 and Crosman 2240. I was able to shoot the TAU with 10-15
seconds between shots and had no velocity variation. When I shot the 2240 at
this rate the velocity continually decreased by 1-3fps with each shot until
depleted. The 2240 required 30-40 seconds between shots to produce consistent
velocities. I would think the highend CO2 pistols would have a more efficient
valve design and not be affected by rapid shooting. This is not a huge factor
with 10m shooting since shooters pace themselves and always take more than 10
seconds between shots.
I have read that at room temperature (70F) the
contained CO2 is at 850PSI. At higher temps the pressure increases(liquid to gas
conversion) .
A full CO2 tank is apparently 70% liquid at room
temperature.
As ambient temperature drops, CO2 converts from gas to liquid
within the tank and internal reservoir pressure drops. Shooting CO2 guns below
60F degrees gives poor results. At shooting temperatures (approx 67-80F degrees)
CO2 is a wonderful propellant.
The properties of CO2 are what makes it such a
good propellant for airguns. The self regulation makes velocity consistent. The
decreased pressure when chilled makes the CO2 easy to get into pressurized
tanks. By chilling the CO2 it will flow into a tank and eventually increase to a
useful pressure when it reaches room temp again. No fancy pumps are needed to
fill a CO2 reservoir in a gun. Filling a small transfer tank also does not
require a special pump. A simple chill in a freezer works great to allow the
small tank to be filled.
CO2 tanks are filled by weight. Obviously a 9oz CO2
tank will hold 9oz of CO2 when full. Just weigh the tank before and after
filling. This system works for guns with attached tanks by weighing the gun
before and after (example: TAU-7). I have seen CO2 is three forms. The gas form
is seen when CO2 is expelled from a pressurized tank. The liquid form can be
heard when you shake a CO2 tank and hear it slop around inside. If you turn a
tank valve on for several seconds you will see CO2 in a solid state. Hold a rag
in front of the valve and solid CO2 will stick to the rag. In a few minutes the
solid CO2 will evaporate into gas and the cold white solid disappears. Solid CO2
can also be seen coming out of certain fire extinguishers.
If a CO2 repeater
pistol is fired continuously with rapid shots you will eventually see the white
frost blasting out of the barrel. At a normal shooting pace you will not see
this frost.
Sorry for the long answer.
Later,
Todd
Posted by TCooper on December 27, 2001 at 15:21:18:
Below is some testing
info that I did with my TAU-7 CO2 pistol. This info will show how consistent the
velocity and accuracy can be with a mid priced CO2 gun.
I have done some
extensive testing with my .177 caliber TAU-7 over the last year. So far it likes
the Czech made JSB pellets the best. At 10 meters it averages .230"ctc for 5 JSB
Match pellets. Velocity on the TAU-7 is adjustable from about 325fps to over
600fps.
I have tried RWS Hobby, RWS R-10 light (7.7gr), H&N Match,
H&N Finale Match pistol, H&N Finale Match CO2,RWS Meisterkugeln lights
(7.7gr),and JSB Match in my TAU-7. These are all WC pellets. The JSBs were
always the most accurate. The JSB Match are available in 7.3gr (.475 gram) and
8.0gr (.520 gram). Price is $5USD per tin.
For 10 meter accuracy testing I
clamp the bulk filled gun in the Black&Decker workmate for the entire
session. Here is a string of 5 shot groups at 10 meters (485fps with 7.3gr JSB
Match pellet): .232", .259", .180", .250", .251", .259", .213", .294", .230",
.132" Average accuracy for 10 groups is .230" ctc.
All groups were measured
with a vernier caliper and calculated to center-to-center measurement. Groups #2
and #8 had a slight flyer but both 5 shot groups were measured and used for the
test. All groups are 5 shots.
As we all know, all guns are individual and
what works for my gun may not work for other TAU-7 guns.
I was always a bit
curious about the velocity consistency of a CO2 pistol like the TAU-7. As
always, I had to find out for myself by doing a test. I shot numerous series of
pellets with 10-15 seconds between shots and recorded velocities. All shooting
was with a bulk filled gun.
Here is a 20 shot string that I recorded with 7.3
grain JSB pellets:
484, 486, 487, 486, 487, 485, 490, 487, 491, 492, 491,
493, 492, 495, 494, 495, 495, 498, 494, 495.
Here is a 10 shot string with
RWS R-10 pellets: 422, 424, 422, 422, 424, 426, 423, 426, 425, 426.
RWS
MeisterK Lights: 434, 423, 430, 430, 430, 428, 432, 428, 431, 433, 433, 429.
I then did a test using 3 different JSB pellets. I alternated between the
pellets so I could see what the velocity consistency of each pellet would be
throughout a bulk fill charge. I shot 5 pellets with one type, then switched to
another pellet and shot 5 , then to a third pellet and shot 5, then to the first
pellet type again, etc.
The 3 pellets are: A - JSB 7.3 grain Match
Diabolo 4.49mm
B - JSB 7.3 grain Match Diabolo 4.50mm
C - JSB 8.0 grain
Match Diabolo 4.50mm (heavier)
I just kept shooting one pellet after another
with maybe 10 seconds between shots at the most. Here are the velocity
results:
#1 - A - 486, 492, 491, 492, 493
#2 - B - 489, 490, 484, 489,
491
#3 - C - 474, 476, 477, 471, 473
#4 - A - 492, 494, 492, 495, 495
#5 - B - 496, 487, 493, 492, 489
#6 - C - 475, 476, 476, 473, 475
#7
- A - 495, 496, 492, 492, 491
#8 - B - 495, 490, 493, 495, 490
#9 - C -
476, 476, 474, 473, 472
#10-A - 494, 498, 495, 492, 493
#11-B - 494,
490, 492, 492, 493
#12-C - 475, 477, 478, 475, 476
#13-A - 498, 499,
494, 494, 492
#14-B - 489, 484, 484...out of gas
Bulk filled again
#1 - B - 487, 485, 487, 490, 493
#2 - C - 471, 471, 474, 471, 476
#3
- A - 493, 491, 493, 498, 494
#4 - B -491, 495, 490, 490, 492
#5 - C -
476, 476, 477, 475, 475
#6 - A - 496, 497, 497, 494, 497
#7 - B - 490,
494, 494, 491, 495
#8 - C - 471, 477, 477, 478, 477
Took a break for a
while
#9 - A - 485, 483, 486, 482, 488
#10- B - 482, 485, 485, 492, 491
#11- C - 474, 473, 474, 475, 478
#12- A - 495, 499, 496, 492, 495
#13- B - 492, 497, 491, 490, 490
#14- C - 477, 475, 474, 475, 476
#15- A - 491, 487, 481...out of gas
All the above shooting was done in
my basement at approx 68 degrees F. The TAU-7 seems to be very
consistent.
Later,
Todd
Posted by Stan Pace on December 27, 2001 at 22:28:16:
I tested a FWB Mdl 2
fairly carefully with a friend of mine who is a technician at NASA Langley. We
were interested in finding the best groups for this pistol.
Without going
into all the details, we attached a thermocouple to the CO2 cylinder while we
did batch testing using a Ransom Rest. What the thermocouple showed us was that
if we fired pellets as frequently as every 30 seconds, the cylinder cooled
fairly dramatically. As might be expected, FPS started to show variations and
the resulting groups were larger. When we increased the time between shots to 60
seconds, this cooling did not occur, and group sizes became smaller.
Posted by TCooper on December 28, 2001 at 01:07:18:
Stan,
I wish I had
the same equipment to do a test as precise you do. Nice work.
The TAU-7 CO2
reservoir is located under the grip so I cannot even feel the cooling but the
barrel sure gets cold from shooting.
When I did my testing I would shoot 5
pellets with about 10-15 seconds between and then change the target. My
chrongraph was positioned in front of the B&D Workmate so I could shoot
groups at the same time as record velocities. There would have been a longer
break between 5 shot groups as I changed targets but each group was shot with
little rest. On highend CO2 guns I still believe the valve is designed better
than on lower priced pistols. The 2240 I tested was awful until I rested 3-4
times as long between shots as with the TAU-7 (between every shot). I have no
idea why this was a fact. I can only record the results that I experienced and
give no explanation.
I once took my TAU-7 outdoors to shoot longer distances
with higher velocities. I shot at 600fps at distances of 30 and 40 yards with
the TAU clamped into a Workmate. My shooting was very fast with maybe 5-10
seconds between shots and sometimes faster. I was trying to beat the air current
changes that affect the light pellets at longer distances. I would watch the
long grass and shoot 5 quick shots when things were calm. With preferred pellets
my accuracy at 30 yards averaged slightly under 3/4"ctc and at 40 yards averaged
slightly under 1"ctc.
Here are the groups at the longer distances:
30
YARDS
CR Premier Lt - .95, 1.00, .80, .95, .90 Ave= 0.92"
JSB Exact - .70,
.90, .70, .40, .90 Ave= 0.72"
RWS Super Pt - .75, .80, 1.00, 1.00, 1.10 Ave=
0.93"
40 YARDS
CR Premier Lt - .95, 1.20, 1.15 Ave= 1.10"
JSB Exact -
1.05, .75, 1.05 Ave= 0.95"
RWS Super Pt - 1.65, 1.75, 1.40 Ave=1.60"
Even
with the fast shooting I did not experience any noticeable vertical stringing of
groups. You can see the 30 yard groups at http://www.ctaz.com/~stevent/40y.jpg
and the 40 yards groups can be viewed at http://www.ctaz.com/~stevent/40y4.jpg
.
I fully believe the CO2 temperature and reservoir temperature will cool
with every shot. This is a physical property of CO2. I would sure like to know
why the TAU doesn't seem to be affected much by the temp change and quick
shooting. Maybe the short rest between groups was enough.
If anyone can add
to this conversation I would like to read some explanations. I'm stumped.
All
my shooting distances are carefully measured and all groups are measured with a
venire caliper and calculated to ctc.
I realize the Black And Decker Workmate
is not as stable as a heavy metal bench vise but it is all I have. A heavy vise
might produce better groups but I cannot guaranty anything. I can only report
the facts as I see them.
Later,
Todd
Posted by Stan Pace on December 28, 2001 at 15:47:43:
I forgot to mention
that we were, in fact, chronogaphing each shot at the time. I guess the FPS
thing was a clue, huh? ;-)
In terms of the cylinder cooling affecting group
sizes, I didn't see any vertical stringing. What I saw was essentially circular
groups, but the diameter of the groups were slightly larger when the cool down
was going on.
Posted by Ed Hall on December 29, 2001 at 11:46:49:
With rifled
projectiles, the path (trajectory) is not a pure parabola as might be initially
expected. In addition to the accelerated dropping toward earth (rough parabolic
path looking from the side), there is a helical motion about the parabola. IOW,
the path is more of a bent corkscrew. This is caused by the spinning of the
projectile. Due to this motion, small variations in velocity manifest themselves
in a circular fashion instead of a direct vertical manner.
Take Care,
Ed
Hall
Posted by TCooper on December 28, 2001 at 01:32:57:
Stan,
I forgot to
mention an important point. When I was shooting at 30 and 40 yards I left the
partially filled 150 gram bulk bottle attached to the TAU-7. This system would
result in less cooling of the larger tank. I think the remaining CO2 in the 150
gram tanks would be more than in a FWB Model 2 reservoir.
All the other
shooting (at 10 meters) was done with a bulk filled gun. The internal reservoir
on the TAU-7 holds about the same as a 12 gram powerlet.
If I was to pace the
40 yard shooting to 15 seconds per shot with a bulk filled gun I would likely
have obtained the same accuracy results as with the hanging tank. The 10m
velocity stats showed no noticeable velocity drop with a 10-15 second shot
pacing. I find this topic very interesting because it was something that greatly
worried me about buying the TAU. I thought my velocities would be all over and
accuracy poor if I didn't wait at least 30 seconds between every shot.
I am
not trying to disprove anyone’s testing, nor am I trying to be argumentative. I
am just interested in the facts and explanations. I am a friendly kind of
guy.
Does anyone have any further info on shooting CO2 target pistols
quickly? Accuracy fluctuations? Velocity fluctuations?
Thanks,
Todd
Posted by Matt on December 27, 2001 at 18:30:33:
If you do decide to go
with CO2, and you buy a bottle for CO2 storage, then make sure the bottle has a
valve with a dip tube. It will eliminate the need for you to invert the bottle
when filling your cylinders.
Matt
Posted by allen h. on December 27, 2001 at 20:36:23:
: please clear up
some (mis)understandings about co2.
: I have been told that co2 is sort of
self regulating, that until the tank is almost completely empty that the gas
pressure is practically constant- as long as the ambient temperature is above
some(???) degrees f., below this temp. (whatever it is) the liquid won't
vaporize.
: I have also been told that co2 pressure is directly proportional
to ambient temp.
: huh????
: I’m wondering because i wanted to buy a
pneumatic pistol but found a used co2 gun i might be interested in.
Posted by Jeonggyu Seo on February 15, 2002 at 12:23:44:
Dear Ms. or Mr.
I am a graduate student of Petroleum Engineering Department at Texas A&M
University.
I am doing experimental work with my advisor. I would like to
know the compressibility of CO2, not compressibility factor.
Thanks a lot
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