The effect of hot weather on velocity in springers

[dvd]

Its not only expasion/contraction, it is seal hardness. Harder (colder) seals often have less friction.

Yes I agree that very cold seals tend to behave like metallic piston rings. Your idea to investigate velocity changes induced by variable air temperature using the FWB 300 is very good. Should give a very good indication of the effect of low ambient air temperature in front of the piston just before the gun is shot, without having to account for the plastic nature of the seal.

[andrewM]

All very interesting. It is clear, from my experience, that different air rifles respond differently. It was very apparent on my 1970s Airsporter and, now, on my otherwise excellent Omega, that the effects are marked and, moreover, there seems to be a tipping point, at which point the power falls away significantly, causing the sights to have to be raised by nearly 2 inches at 25-30yds.

I have not done sufficient tests to find out what happens on other air rifles. I had rather considered the Airsporter to be a 'one-off' but this is not the case. I think the Omega has a synthetic seal whereas the Airsporter uses a leather seal.

In some export markets, the impact of hot weather must be significant.

I wonder what tests the manufacturers have carried out. It seems to me there is a big hole in the knowledge/research over this topic. It would be helpful if research pointed in a particular direction to say that factor 'x' was the biggest cause of the problem, for example, but we do not know this and can only speculate, without 100% certainty that a particular cause is for the most part responsible.

[evert]

IF we get low temperatures this winter, I'll try and get the experiment done. keep a fwb300 in the car trunk over night, bring it into the cold garage for quick chronographing.
Then, keep it in the heating room in the basement for some hours, and shoot it again. I might be able to get close to a 50 degree temperature difference if we get any severe cold periods.

Or perhaps anyone has access to a cold storage facility?

[TonyL]

b) The cylinder (or washer) itself expands slightly. This might lead to an expansion within the chamber, causing the washer to pull against the chamber walls.

It is peculiar that no one appears to have conducted any testing to see what happens and how the influence of temperature can be addressed.

The impact of hot weather has an influence on power and, therefore, on accuracy, so it cannot be underestimated.

A

with springers, 99% of temperature related velocity change is down to seal fit / friction.

Sorry to jump in so late; I've only just seen this thread.

As Jon says, yes, most of this phenomenon is caused by seal expansion and the correspondingly greater friction. Jim Tyler has indeed addressed this issue on more than one occasion in his excellent articles in Airgun World and how temperature shift affects both PCP and springer velocity changes.

One of our esteemed members, NickG, has poured much effort and experimentation into this very subject in recent times and has been enjoying great success with his own design of synthetic piston ring as regards this very phenomenon.

[TonyL]

IF we get low temperatures this winter, I'll try and get the experiment done. keep a fwb300 in the car trunk over night, bring it into the cold garage for quick chronographing.
Then, keep it in the heating room in the basement for some hours, and shoot it again. I might be able to get close to a 50 degree temperature difference if we get any severe cold periods.

Or perhaps anyone has access to a cold storage facility?

My guess would be that the 300, with its metallic rings of small cross-section, will be affected far less than a rifle with a synthetic seal. Especially a synthetic seal of the parachute type which uses a greater amount of material and is more susceptible to expansion.

[trajectory]

My guess would be that the 300, with its metallic rings of small cross-section, will be affected far less than a rifle with a synthetic seal. Especially a synthetic seal of the parachute type which uses a greater amount of material and is more susceptible to expansion.

Sounds logical to me.

Anyone got a Webley Senior & a premier they fancy popping in the freezer & then on top of a radiator to look at velocity changes with phosphor bronze v PTFE to save waiting a few months? Maybe not...

[andrewM]

Sorry to jump in so late; I've only just seen this thread.

As Jon says, yes, most of this phenomenon is caused by seal expansion and the correspondingly greater friction. Jim Tyler has indeed addressed this issue on more than one occasion in his excellent articles in Airgun World and how temperature shift affects both PCP and springer velocity changes.

One of our esteemed members, NickG, has poured much effort and experimentation into this very subject in recent times and has been enjoying great success with his own design of synthetic piston ring as regards this very phenomenon.

Very helpful comment.

Is there any chance you can supply us with the article by Jim Tyler?

I was also unaware of the research carried out by NickG. Can you tell us something about his piston rings? Does he suggest that these should replace all existing washers so as to overcome the problem? In addition, do leather washers have the same difficulty or are they somewhat less affected on the whole by high temperatures?

It would be most helpful to see research on this phenomenon.

Evert's experiment will be interesting to see but, as you say, the 300 might be less affected. It would be helpful to see the results in another make of air rifle by way of comparison.

Rgds
A

[TonyL]

I'll try and dig the issues with the articles out and advise, Andrew. Maybe they'll be available as back issues or electronically? I might not be able to oblige in lightning quick time, so if anyone else is looking in and can help, please feel free to jump in.

Pretty sure Nick's seal is still performing well in endurance testing. As regards the details re composition, Nick did tell me verbally at a Bash. Not wanting to be awkward, but I'm not sure if it was meant to be general knowledge. If he looks in we can let him decide how much detail he wants to share. I can say that it is a very expensive and hard to come by synthetic mix, though.

Jim confirmed my suspicions in a previous thread that "rings" with less material used will suffer less than types using more material.

I wonder if, for UK use, those that like to size seals might use an ambient temperature of, say, 15 degrees Centigrade, giving a little "wriggle room" for use in different conditions?

Although most will give higher power at lower temperatures, this will be dependant on seal fit. So a nice, "easy, sliding" fit at 20 degrees may prove too slack a fit at very low temps. A member who I know well once experienced drastic power loss at close to freezing temperature with a HW98. I had a wild, lucky guess and advised him to retest once warmer rather than strip the gun down which he duly did and, hey presto, all back to normal. Which will be exactly why people like Nick started to research different materials and designs. Prior to this, Nick also had much success with 'O' and 'X' rings.

Not sure how leather fares.........guessing not brilliantly?

[AC99]

I also have just noticed this thread and agree that the 'reduced velocity / increased temperature' phenomena is most likely attributable to seal composition/friction and maybe other factors also.

In their 1976 publication "Air gun from trigger to muzzle", GV & GM Cardew (brothers I think) were aware of this cold weather phenomena and carried out experiments in an attempt to determine why. They firstly wrapped a heating band around the cylinder of their experimental gun and warmed it up, monitoring the temperature. Before each shot was fired the gun was left in the cocked position to allow time for the temperature to stabalise. They were surprised to find that the velocities did not change over a range of temperature expected on a summer's day.

They then cooled the cylinder by injecting refrigerant into a jacked surrounding the cylinder again allowing time for all parts to reach the same temperature. The resulting velocities were very much the same as for the gun at normal temperatures and they concluded that temperature, within reasonable margins, had little or no effect on the internal ballistics of air weapons.

The explanation therefore must lie elsewhere as has been suggested in previous posts on this thread.

[andrewM]

Tony and AC99, great posts, thank you. We appear to be narrowing it down, based on the Cardew brothers' tests which, it seems, have removed the questions over the cylinder, at least. Presumably, the heat and cold applied to the cylinder did not penetrate sufficiently to the washers. It is unfortunate, however, that they only used one rifle for the test (presumably). Perhaps a wider range of rifles might have produced a different outcome. I wonder if they simply tested a rifle that was largely immune; it is possible.

I should be very interested to see the articles and research by Jim Tyler and, in addition, any research and findings by NickG.

One would think that the big manufacturers such as HW were aware of the impact of weather but perhaps they never bothered to research it adequately. If so, that is a glaring omission on their part.

It seems that this is still work in progress and, perhaps, one of the last and perhaps the last technical question that remains unsolved in the sphere of air-gunning.

Rgds
A

[TonyL]

I guess that the manufacturers' main aim will be to specify their guns with a seal that is robust and displays longevity above all other factors. Many, many rifles will have been bought over time by non-tinkering / less "technical" types and the main thing is that these rifles work and keep working. There are probably many thousands of rifles delivering sterling service out there at a ripe old age that are still sporting their original piston seals? If the manufacturers are aware of this characteristic, maybe they think it's down to the end user to get to know his/her gun's points of impact at different ranges / temperatures?
Although point of impact variation can have much more severe consequences for hunters, it's most likely the competitive arena of FT and HTF that have driven the science more in recent years as regards accuracy, so the more technically minded shooters in those disciplines with the most inquisitive minds are the ones most likely to explore?