Question about scuba tank fullness....

[MikeMargolis]

background:

Steyr LP-10
30 cubic foot scuba tank, bestfittings adaptors for my gun

So my tank was down to about 2/3 level, and was only filling my PCP air tanks to 150bar vs the 200bar when the tank was full.

I took it to the scuba store, they said, "Well, it's not even half empty, so we filled it for free, a little past 200bar."

Filled my gun's tanks, and my point of impact was way off compared to how nicely it was zeroed with the tank 2/3 full.

So the question is:

Can having a 200 bar tank on the gun affect the sights vs having a 150 bar tank on the gun?

Thanks in advance...

[brent375hh]

No. The regulator, if working correctly, offers the pellet the same ride from 200 down to 65-70 bar.

Do you have a chronograph available?

I know my point of impact changes sometimes, but it's me, not the gun.

[nmondal]

So the question is:
Can having a 200 bar tank on the gun affect the sights vs having a 150 bar tank on the gun?
Thanks in advance...

Yes, it can. But most surely not due to the reason you think.
Most possible reason is increase in the weight of the cylinder. So when you are making the shot - it is staying low.
Check - if the point of impact is higher or lower?
If lower, it is definitely due to weight.
If higher - then it is possible still due to weight - where your hand is trying to over compensate for the weight.
Solution.
Practice with two different type of cylinders.
Unfortunately - I do have the same combo. Old Walther Cylinder with more weight and they are larger.
New Walther Cylinder with pretty less weight and they are smaller.
I have noticed similar issue in my case. Because it is a THING - you need to be careful with the extra weight you incur.
It is not a gradual change - it is sudden and thus I will definitely not recommended to change cylinder during match.
It can literally be 10.9 vs 8.9.

[brent375hh]

You suggest there is a big weight difference due to the additional air volume compressed to 200 bar?

[spektr]

Absolutely....

[David M]

Firstly, what pistol are you talking about?
Secondly, how far did the group move?
Most PCP pistols work from 200 bar down to about 90 bar, with less than 2-3 m/s variation.
The weight difference bewteen a half full air tank and a full tank is negligable and will not make a difference.
The Morini 162 after lockout (manually overiden) can be shot down to 72-75 bar (about 20
shots) with 2 clicks up after 10 shots and then 3 clicks more.
Useful if the lockout cut in and locked during a final (better to re-fill before final) but gave me a out.

[Rover]

Unless the gun is defective, it's YOU!

Weight of the air is nonsense, but MIGHT affect you with C02 (yes, I laugh).

I DID test a Morini where the accuracy deteriotated near lockout, but that's not the same thing.

I refill my SCUBA tank when the pressure gets below 1800 psi, but that only affects the number of shots, not velocity.

[MikeMargolis]

Thanks for the replies.

Steyr LP-10, two tanks from 2013 and 2014.

The gun was shooting like 6 ring 3 o'clock, very odd.

I haven't done much testing since...

[Green_Canoe]

Just for fun I tried to calculate the weight of air in the air cylinder of my Steyr. I used dimensions and weights from the internet so there are lots of opportunities for errors. My result? 20 grams going from empty to 200 BAR. Of that you might use 13 grams going from full to 70 BAR. I get about 110-120 shots from a full cylinder so I'll have just over a gram weight change for each 10 shot string. Dang it!... Now I've got one more thing to worry about... ;^)

[GoodEnuf]

Just for fun I tried to calculate the weight of air in the air cylinder of my Steyr. I used dimensions and weights from the internet so there are lots of opportunities for errors. My result? 20 grams going from empty to 200 BAR. Of that you might use 13 grams going from full to 70 BAR. I get about 110-120 shots from a full cylinder so I'll have just over a gram weight change for each 10 shot string.

This weight change may affect some very accomplished shooters, but 50 bars (in my cylinder) is about 5 grams, with the CG of the weight change not too far away from the system CG. These small values would have to upset the cart in an awful way, to put the POI as far off as reported.

Weight of the air is nonsense, but MIGHT affect you with C02 (yes, I laugh).

PS to Rover: Wouldn't it be He that causes you to laugh, not CO2? But in that case, with He being displaced with NO2, O2, would not the cylinder gain weight with firing successive shots? Oh no, I don't factor the 150bar He being displaced with an equivalent 1 bar air...it gives me a headache.

My FWB cylinders are each imprinted with the tare weight and the net weight; the net weight is 22 grams @200 bar, validating Mr. Canoe's (accurate) estimates. It would seem that if declining air weight with pressure drop was a significant problem, it would exist with every cylinder use, no matter what the initial pressure would be, i.e. shoot a few shots, poi changes with weight change. As others have noted, this may be a weight change to which an exceptional few shooters would react, but I don't think I'm in that gang.

Taking the original poster's observations at face value, he reports what he see's, I speculate to cause: The cylinders are not new (6-7 years old). Is it possible the valve has become internally corroded, or internal corrosion/rust flakes are preventing consistent operation against the cylinder pressure? Those effects might vary with declining pressure, and prevent the regulator from seeing consistent >80-90 bar pressure, with subsequent calibrated volume and pressure delivery errors. The effects would be expected to be erratic. Again, all I know is FWB mechanics. The valve is easy to push in and out with no cylinder pressure, but when under operational pressure, cannot be moved by hand. The reaction to forces at the cylinder valve may change with pressure if it is eroded or damaged.

If he owns one of those pressure releasing devices that you screw on to the end, he might try cycling the pressure a few times, blowing the cylinders' contents out quickly. If he feels resistance or grittiness at the valve when the cylinder is empty, he might follow up with 1 drop of silicone oil on the protruding valve. (I'm aware of the 'don't use silicone oil on metal to metal' advice usually proffered, but there seems to be negligible movement or sliding forces at work at this valve; the silicone would prevent additional corrosion, and not be expected to provide lubrication against galling.)

JE

[David M]

Thanks for the replies.

Steyr LP-10, two tanks from 2013 and 2014.

The gun was shooting like 6 ring 3 o'clock, very odd.

I haven't done much testing since...

OK, something to look at is the Stabilizer function.
It actions by trigger and powered by air pressure.
It may not be setting properly on cocking or it may not be releasing properly.
Impropper function will change the impact point in the vertical plane.
It may need resetting and/or cleaning.

[David M]

Just for fun I tried to calculate the weight of air in the air cylinder of my Steyr. I used dimensions and weights from the internet so there are lots of opportunities for errors. My result? 20 grams going from empty to 200 BAR. Of that you might use 13 grams going from full to 70 BAR. I get about 110-120 shots from a full cylinder so I'll have just over a gram weight change for each 10 shot string. Dang it!... Now I've got one more thing to worry about... ;^)

Hummm.....13 grams, I don't think so

Tank volume 0.08 litre at sea level empty (atmospheric) , 15deg C (57 F), weight of dry air 0.098 grams (1.225g/l)
At firing empty 80 Bar (1160 psi), compressed volume is 6.31 litre, equals weight of 0.62 grams.
Full tank at 200 bar (2900 psi), volume is 15.78 litre, equals weight of 1.54 grams
Difference between 200 bar (full) and 80 bar (shot empty) is weight of 0.92 grams or 0.03 oz.
So 2/3 of 3/5 of sweet FA....

[spektr]

I had the exact issue happen with an FWB P44. The pin valve in the cylinder was lightly corroded and all I can figgure was that I have 2 cylinders and used only one for quite a while. The fix was to lightly oil the valve and use a bamboo skewer to exercise it. Its never happened again. I also use both tanks in a rotation.

[robjob]

Or... how bout just weigh a full and empty cylinder? My thought is if the weight of air in your cylinder is actually causing your group to move, you might as well give up now because there are a lot of other things that will mess you up a lot more.

I have at times experienced "mystery" poi shifts. After adjusting sights to compensate I usually find myself adjusting back again a few targets later... mystery solved...

[Rover]

"I have at times experienced "mystery" poi shifts. After adjusting sights to compensate I usually find myself adjusting back again a few targets later... mystery solved..."

I have had the same experience. That's why I have recommended NOT adjusting sights near the start of a match. You will be distracting yourself AND chasing your shots.

I put a sighter target behind my match targets for twenty or thirty shots. THEN I look at the large hole and determine if I want to make a sight change.

[Green_Canoe]

Just for fun I tried to calculate the weight of air in the air cylinder of my Steyr. I used dimensions and weights from the internet so there are lots of opportunities for errors. My result? 20 grams going from empty to 200 BAR. Of that you might use 13 grams going from full to 70 BAR. I get about 110-120 shots from a full cylinder so I'll have just over a gram weight change for each 10 shot string. Dang it!... Now I've got one more thing to worry about... ;^)

Hummm.....13 grams, I don't think so

Tank volume 0.08 litre at sea level empty (atmospheric) , 15deg C (57 F), weight of dry air 0.098 grams (1.225g/l)
At firing empty 80 Bar (1160 psi), compressed volume is 6.31 litre, equals weight of 0.62 grams.
Full tank at 200 bar (2900 psi), volume is 15.78 litre, equals weight of 1.54 grams
Difference between 200 bar (full) and 80 bar (shot empty) is weight of 0.92 grams or 0.03 oz.
So 2/3 of 3/5 of sweet FA....

6.31l*1.225g/l=7.72g
15.78l*1.225g/l=19.33g

I used a slightly larger volume as I used outside dimensions of the cylinder. But otherwise using your numbers I get roughly the same answer when mathed correctly.

[Rover]

The answer to your question is, "NO!"

[brent375hh]

By now, I assume that your cylinder has tapered down from 200 to 150 bar a few times.
Have you noticed a drift to the right as that happens?

[Green_Canoe]

Or... how bout just weigh a full and empty cylinder?

So I did just for a sanity check...

The pressures are estimated from the manometer on the cylinder end. As we all know these pressures are just estimates, since the cylinder manometer is not NIST traceable. My scale is a cheap Chinese electronic scale. Again not NIST traceable. Good enough to separate cast bullets into 1 grain weight groups but not accurate enough to measure gunpower when reloading.

My LP10 cylinder at 100 BAR weighs 191.1 grams. I added about 70 BAR from my SCUBA tank and brought it up to about 170 BAR. The new weight was 197.4g. A gain of 6.3g for roughly 70 BAR so the math approximates real life.