Gas Ram Velocity re Temperature Variations, Hard Air Magazine

[TonyL]

So, in the last couple of days we see an article on Hard Air Magazine regarding velocity variation vs temperature change.

Using the new Sig, reported velocities at about 17 degrees C are significantly higher than at about -6.

So, higher velocity down to higher ram pressure (obviously a point would be reached where, if the internal pressure at lower temps was higher then the corresponding higher pressure at higher temps would then become a negative, making it too harsh).

But shall we also deduce that the piston seal to bore fit must be an "easy sliding fit", otherwise greater friction with the seal expanding with higher temperatures would cancel out the faster piston acceleration due to higher ram force?

Couple of things would be nice to know:-

Firstly, I wonder if any grease is used on the piston? Increased viscosity giving greater drag at lower temperatures.

Secondly, do we know the piston seal design? Ie parachute type? Or more like the TX / Gamo design?

[TonyL]

Also, just thinking a little more on this and as well as the relative expansion / contraction rates at differing temperatures of the synthetic seal vs the cylinder, I wonder whether at such low temperatures the seal (depending on material properties) becomes less "pliable" and can't conform to the internal diameter of the cylinder so well?

Many different factors at play here, methinks. And I remember Pete's (flyingfish) HW98 not wanting to play last Winter at such low temperatures, but that performance was restored once ambient air temperatures increased once again.

I wonder what sort of temperature the assembly line would be at and if any "fine tuning" is done at point of assembly re piston seal fit, or if it's more a case of "pot luck"? Also if there may be variation in the start pressure of the rams?

[Hsing-ee]

Ah, the gas-ram. The answer to a question no-one asked.

In a world full of gas-rams, if the simple steel spring was invented then everyone would get a conversion from gas-ram to spring.

[look no hands]

Hi Tone,

I asked the same question about temp/velocity change with Nitrogen rams the other day to Nelson of Quickshot in Brazil about the gas ram in my Indy, here's some replies he sent me (I've just copied and pasted the messages straight across, so they may read a bit strange)

"Well, I guess that the Theoben rams that were affected with variations in the temperature were those equipped with air, which you could adjust the pressure.
The main reason for manufacturers using nitrogen in their gas ram is just because it is an inert gas and does not change its density due to increase or decrease in temperature.
Anyway, when the variation in temperature is big, the air in the compression chamber of the airgun may cause the pellets to hit higher or lower, due to variation in its velocity".

I then asked why there was two different length rams listed?

"The 270 mm long Gas Ram is intended to fit in different air rifles with different piston stroke. Actually as we install a 270 mm GR where 260 mm long one would fit well, the extra 10 mm keeps the GR rod more compressed and this results in more power at the end of the GR decompression, which is not always necessary.
Gas Ram manufacturers use to identify their rams by the power applied with the rod being pushed inside the cylinder at 1 cm. As the rod is pushed deeper the pressure inside the cylinder increases an so does the compression force.
So, depending on the design of the parts inside the gas ram and its compression rate, we may have a higher compression force at the beginning (10 mm stroke, for example) but with a lower pressure curve at the end. This results in a smoother gas ram that is softer to compress and causes less felt recoil, without sacrificing the airgun energy at the muzzle".

Not sure if that sheds any light on the situation or if he's telling a few porkies to try and get sales, I'm going to be doing some tests with the chrono and different temps with the Indy soon, to see if these comments (I won't say rumours) are true or not.

Pete

[look no hands]

Ah, the gas-ram. The answer to a question no-one asked.

In a world full of gas-rams, if the simple steel spring was invented then everyone would get a conversion from gas-ram to spring.

Except if it's a bloody Theoben that needs a sodding rocket up it's arse to get power once the air's been let out or if it has a central piston rod (like what's in a Prosport or Airsporter) that would puncture a hole in a ram.

Pete

[Geezer]

A quick chairgun on their figures suggests that with a primary zero of anything from 30-50 yards, a decent pellet will be within 0.5-1” of POA across those distances at the high (UK FAC) velocities recorded, without aiming off, irrespective of a nominal 55 FPS variation in velocity.

Add in aiming off, and the claim that “you could miss your target” appears true only if your target is very small - like under half an inch in diameter. Assuming the target is the brain of typical pest species, and the range is kept sensible (sub-35M), with a sensible primary zero range (27M, for example), the variation in velocity should not matter in the field.

But, yes, the claim that gas-rams are, unlike every other form of projectile-launching device, immune to temperature changes, is obviously wrong.

The real question is are gas rams less affected by temp than springers? Which doubtless varies according to ram type, design, and springer type, design, lube etc. A meaningful test would be to pit the SIG rammer against an HW80 or Diana 52 running at similar m/e levels. I suspect the difference would be inconsequential, but would like to know for sure.

[Geezer]

Hi Tone,

The main reason for manufacturers using nitrogen in their gas ram is just because it is an inert gas and does not change its density due to increase or decrease in temperature.....

Not sure if that sheds any light on the situation or if he's telling a few porkies to try and get sales

In the words of Scottie off Star Trek “ye cannie change the laws of physics!” Of course nitrogen is affected by temperature. Everything is.

https://www.engineeringtoolbox.com/n...re-d_2039.html

[look no hands]

A quick chairgun on their figures suggests that with a primary zero of anything from 30-50 yards, a decent pellet will be within 0.5-1” of POA across those distances at the high (UK FAC) velocities recorded, without aiming off, irrespective of a nominal 55 FPS variation in velocity.

Add in aiming off, and the claim that “you could miss your target” appears true only if your target is very small - like under half an inch in diameter. Assuming the target is the brain of typical pest species, and the range is kept sensible (sub-35M), with a sensible primary zero range (27M, for example), the variation in velocity should not matter in the field.

But, yes, the claim that gas-rams are, unlike every other form of projectile-launching device, immune to temperature changes, is obviously wrong.

The real question is are gas rams less affected by temp than springers? Which doubtless varies according to ram type, design, and springer type, design, lube etc. A meaningful test would be to pit the SIG rammer against an HW80 or Diana 52 running at similar m/e levels. I suspect the difference would be inconsequential, but would like to know for sure.

In theory it would be an ideal test for my Indy as I have both spring and ram to power the same mechanism, I'll try and do a few different temp chrono tests to see if there's any difference, it's got the ram installed at the moment but my spring set up is being copied at the moment so I won't have that back for a while but once it is, I'll put that back in and do some tests with that set up as well.

Pete

[look no hands]

In the words of Scottie off Star Trek “ye cannie change the laws of physics!” Of course nitrogen is affected by temperature. Everything is.

https://www.engineeringtoolbox.com/n...re-d_2039.html

I do wonder sometimes if these engineers who make these rams in countries like Brazil just don't really worry because the temp doesn't really change that much, unlike here, so won't effect the rams performance as much.

Pete

[zephyr]

The following is just my mind pondering. Much of it not based on specific knowledge, just simple engineering extrapolation. Or in other words, the extravagances of a deluded mind!

My first consideration - is the fps drop in the spring and gas ram guns similar and its not just a gas ram thing?

Apart from the way the energy is stored - compressed spring vs compressed gas - is there any other fundamental difference:
- piston? friction, movement inside the cylinder, speed/rate/duration of applied force during the power stroke
- transfer port? correctly (whatever that is) sized for ambient temperature
- temperature? of the air during the compression of the spring/ram and while being driven through the transfer port during firing cycle
- temperature? when you compress a spring there is not much of a temperature increase in the metal of the spring - when you compress a gas (as in a gas ram, presumably) there is a noticeable increase in the gas temperature, is this significant? A memory from my A-level physics of yesteryear (yester 'century'!) about the General Gas Law has me frustrated as I don't know the effect in a gas ram air rifle... anyone?
- temperature? differential expansion
- temperature? rapid reduction in pressure can cause very low localised temperature (or is that caused by change of liquid state to gaseous state?) - air humidity and freezing water vapour?
- Piston seal? the same in spring and gas rams?
- Lube? is there a difference in how springs and gas rams are lubed?
- Do gas rams have top hats, piston sleeves, guides? are the different in some way? or something else that could be the causal factor?
- Cylinder/piston? diameter, length, weight, stroke
- cocking mechanism?
- Maybe the velocity drop has incorrectly been ascribed to the gas ram... but it is attributable to something else?

Given more time than will be healthy, I'm sure I could think of more obfuscating rubbish... like seals inside the gas ram, or phase change of the gas in the ram, or water vapour in the gas ram gas, or or or... sod it, i'm going to bed to read Guy Martin's latest book!

[Geezer]

In theory it would be an ideal test for my Indy as I have both spring and ram to power the same mechanism, I'll try and do a few different temp chrono tests to see if there's any difference, it's got the ram installed at the moment but my spring set up is being copied at the moment so I won't have that back for a while but once it is, I'll put that back in and do some tests with that set up as well.

Pete

That would be really interesting. Please do so.

I do wonder sometimes if these engineers who make these rams in countries like Brazil just don't really worry because the temp doesn't really change that much, unlike here, so won't effect the rams performance as much.

Pete

Even here, the weather (apart from one week of “snow crisis” in Jan/Feb and one of “heatwave crisis” in Jul/Aug) is typically between “a bit chilly” and “a bit warm” and most of the year is between I guess 10 and 20 degrees C.

The Americans can help here. When I lived near DC, we would every year get from sub-zero (C) snow and ice to over 30 degrees (with horrible humdity) for weeks in the summer.

I think the original claim about gas-springs and temperature may have been made by Theoben when they introduced the Sirocco in 1982 with its mystery gas power plant (which I think was pure nitrogen, but they claimed was “HF6” or something secret and clever-sounding). If so, it wasn’t true then, and isn’t now. Not bashing Theo and Ben. I actually quite like their rifles. But do not like false claims, hype, etc.

[Geezer]

The following is just my mind pondering. Much of it not based on specific knowledge, just simple engineering extrapolation. Or in other words, the extravagances of a deluded mind!

My first consideration - is the fps drop in the spring and gas ram guns similar and its not just a gas ram thing?

Apart from the way the energy is stored - compressed spring vs compressed gas - is there any other fundamental difference:
- piston? friction, movement inside the cylinder, speed/rate/duration of applied force during the power stroke
- transfer port? correctly (whatever that is) sized for ambient temperature
- temperature? of the air during the compression of the spring/ram and while being driven through the transfer port during firing cycle
- temperature? when you compress a spring there is not much of a temperature increase in the metal of the spring - when you compress a gas (as in a gas ram, presumably) there is a noticeable increase in the gas temperature, is this significant? A memory from my A-level physics of yesteryear (yester 'century'!) about the General Gas Law has me frustrated as I don't know the effect in a gas ram air rifle... anyone?
- temperature? differential expansion
- temperature? rapid reduction in pressure can cause very low localised temperature (or is that caused by change of liquid state to gaseous state?) - air humidity and freezing water vapour?
- Piston seal? the same in spring and gas rams?
- Lube? is there a difference in how springs and gas rams are lubed?
- Do gas rams have top hats, piston sleeves, guides? are the different in some way? or something else that could be the causal factor?
- Cylinder/piston? diameter, length, weight, stroke
- cocking mechanism?
- Maybe the velocity drop has incorrectly been ascribed to the gas ram... but it is attributable to something else?

Given more time than will be healthy, I'm sure I could think of more obfuscating rubbish... like seals inside the gas ram, or phase change of the gas in the ram, or water vapour in the gas ram gas, or or or... sod it, i'm going to bed to read Guy Martin's latest book!

What zephyr said. And he has an A-level in physics, and I only have an O-level. How, experimentally, do you separate the power plant from all the other variables?

[look no hands]

The following is just my mind pondering. Much of it not based on specific knowledge, just simple engineering extrapolation. Or in other words, the extravagances of a deluded mind!

My first consideration - is the fps drop in the spring and gas ram guns similar and its not just a gas ram thing?

Apart from the way the energy is stored - compressed spring vs compressed gas - is there any other fundamental difference:
- piston? friction, movement inside the cylinder, speed/rate/duration of applied force during the power stroke
- transfer port? correctly (whatever that is) sized for ambient temperature
- temperature? of the air during the compression of the spring/ram and while being driven through the transfer port during firing cycle
- temperature? when you compress a spring there is not much of a temperature increase in the metal of the spring - when you compress a gas (as in a gas ram, presumably) there is a noticeable increase in the gas temperature, is this significant? A memory from my A-level physics of yesteryear (yester 'century'!) about the General Gas Law has me frustrated as I don't know the effect in a gas ram air rifle... anyone?
- temperature? differential expansion
- temperature? rapid reduction in pressure can cause very low localised temperature (or is that caused by change of liquid state to gaseous state?) - air humidity and freezing water vapour?
- Piston seal? the same in spring and gas rams?
- Lube? is there a difference in how springs and gas rams are lubed?
- Do gas rams have top hats, piston sleeves, guides? are the different in some way? or something else that could be the causal factor?
- Cylinder/piston? diameter, length, weight, stroke
- cocking mechanism?
- Maybe the velocity drop has incorrectly been ascribed to the gas ram... but it is attributable to something else?

Given more time than will be healthy, I'm sure I could think of more obfuscating rubbish... like seals inside the gas ram, or phase change of the gas in the ram, or water vapour in the gas ram gas, or or or... sod it, i'm going to bed to read Guy Martin's latest book!

My brain isn't as complex as yours but I often wondered if things like air density (due to the air temp) within the cylinder can cause different power and even if pulling the barrel/underlever at different rates would "suck in" different volumes of air?

Pete

[Hsing-ee]

My brain isn't as complex as yours

Pete

Therefore ideally suited to HW35 ownership. No secrets (apart from the well documented & solvable weld gaps) in the 35...

[ballisticboy]

My brain isn't as complex as yours but I often wondered if things like air density (due to the air temp) within the cylinder can cause different power and even if pulling the barrel/underlever at different rates would "suck in" different volumes of air?

Pete

As a joke once at the club I suggested that you could change the amount of air in the cylinder by the speed of pulling on the lever/barrel. One or two started trying it out but of course there was no difference.