The effect of hot weather on velocity in springers

[andrewM]

This is a topic to which I have referred in the past, with some helpful contributions from the technicians on the BBS.

I noticed, earlier this summer when the weather was especially warm, that my Webley Omega was especially impacted by the temperature, which was then over 30C. I first noticed this last year. I have fitted aperture sights to the Omega and I find it an accurate air rifle and am very happy with its performance.

However, in hot conditions, it is necessary to raise the sights because the power drops, to the extent of, say, nearly 2 inches at a range of 25-30 yds. Yesterday, when I used the rifle again in much cooler weather, it had restored itself to its usual format. It appears, although I cannot be sure, that there is a tipping point at which the velocity drops markedly. Extensive testing would have to be conducted to find out at what point this occurs, both to quantify and qualify the variations in performance. In this case, I have simply decided not to use the rifle in hot weather, which deals neatly with the problem.

I first noticed the difficulty with my BSA Airsporter in the late 1970s. I attributed it, then, to a possible expansion of the metal within the barrel. This was probably the least likely cause of the power drop but, again there was a tipping point. More likely, as others have very occasionally reported on this site, there are two other more likely explanations:

a) The air is less dense (ie it becomes lighter) in hot weather, which leads to a reduced 'punch' in the cylinder as the piston and washer drive home.

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. Alternatively, the expansion might be in the other direction - ie outwards - in which case the washer would form a looser fit and experience possible seepage.

It would further appear that different air rifles display different effects. Some rifles, it would seem, are relatively unaffected. Others, much more so.

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. One would think that the big airgun manufacturers would have conducted extensive testing. Perhaps they have but have not reported their results.

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

I should be interested to know what others think. Perhaps this should be posted on the General Forum of this site, to attract greater input. It appears that PCPs are unaffected by the phenomenon.

It might be that the problem is too complex to be answered by the industry and has therefore been ignored. On the other hand, it would be helpful to see any research that exists, either here, on the Continent or in the USA.

Rgds
A

[Jerry Cornelius]

. a) The air is less dense (ie it becomes lighter) in hot weather, which leads to a reduced 'punch' in the cylinder as the piston and washer drive home.

I am fairly sure this can’t be the explanation as what would matter would be pressure rather than density.

I wonder if the explanation is that the spring constant of your spring is decreasing slightly with increased temperature. However that would give a linear effect and not the tipping point you describe.

I can only hypothesise that some of the airgun fairies who live in your omega refuse to work above 25 degrees. I am afraid you must have a unionised gun, which will reduce its value considerably

[WebleyWombler]

Can I ask? Why have you posted this in the collectable airgun section?

[andrewM]

Can I ask? Why have you posted this in the collectable airgun section?

It is a good question and perhaps it should be posted in the general section. The answer is that the collectors' section tends to attract those of considerable experience and perhaps, therefore, they possess a greater degree of knowledge. I am quite happy to post it, also, in the general section if you think this would be helpful.

Rgds
A

[Jesim1]

It appears that PCPs are unaffected by the phenomenon.

It might be that the problem is too complex to be answered by the industry and has therefore been ignored. On the other hand, it would be helpful to see any research that exists, either here, on the Continent or in the USA.

Rgds
A

I don't know the answer, but what you suggest is probably there or there abouts, and a combination of things

As for PCPs not being affected, the opposite is true, primarily due to the increased potential energy of the pressurised air. This leads to an increase of approximately 1.3fps increase/decrease per degrees centigrade change in typical UK conditions. It will vary per gun, but it's always there, and it's a well documented fact. No idea why springers have not had the same focus though, perhaps someone can shed light on it?

James

[ptdunk]

I know my competition 97 well, and it’s slightly higher power in the winter.
I put this down to the spring being a bit stiffer in colder temperatures but I’m just guessing.

Cheers,
Matt

[Shed tuner]

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

[dvd]

I think that since cooler air is denser, so there is more of it to be utilised during compression resulting in efficiency gains.
My springers shoot a bit faster in winter. Maximum temp difference between winter and summer can be around 20/25 Deg C.here,
I would think with the hot summers in the UK, you should have an even bigger temp difference for testing velocities in different temps.
Devices that rely on compression of air perform better the more air is present in any given cylinder.

Engines that use external compression devices like turbos and superchargers, often rely on heat exchangers to lower the compressed air temp, effectively cramming more air into the cylinder. The same holds true for our springers, there will be more (denser)air in front of the piston the cooler it is.
Regarding expansion of seals and cylinders, I think it is so minimal as to be inconsequential. If the cylinder were to expand, the self compensating nature of seals would offset that.
Perhaps more influential would be the frictional changes of the seal induced by temperature change, but seals are made from fairly stable materials these days.

[Shed tuner]

Regarding expansion of seals and cylinders, I think it is so minimal as to be inconsequential. If the cylinder were to expand, the self compensating nature of seals would offset that.
Perhaps more influential would be the frictional changes of the seal induced by temperature change, but seals are made from fairly stable materials these days.

nope, sorry, it has a massive effect once outside the sweet zone. Jim did an article and some tests in AGW too IIRC.

[dvd]

nope, sorry, it has a massive effect once outside the sweet zone. Jim did an article and some tests in AGW too IIRC.

To clarify my views, I am writing about what I think shouldn't happen to pellet velocity, when the cylinder and seal expand or contract within the range of seasonal temperatures.
I presume by sweet zone you mean the gun's optimal operating temperature to achieve the desired velocity and smoothness. As in most airgun dynamics, other factors come into play such as the viscosity changes of lubricants, rate of seepage to the front of the seal, fit of individual parts, etc.

In other words, if one were to eliminate the variables I mentioned and look solely at expansion/ contraction of seals and cylinders say within a 30 degree C range, I still think that the velocity losses/gains due to temperature movement of seal and cylinder will be extremely small and the biggest velocity change factor will be the air temperature(density) inside the cylinder.
Having said that, I wouldn't mind having a read about the articles you mentioned, perhaps I am missing something.

[Robert j]

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

I agree

[evert]

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

I've seen some extreme changes in the winter temperatures we used to get here. Accidentally left a .177 HW95 in the car trunk over night some years back, with temeperature below -20 degrees C. It chronographed very high until it warmed up.

Was meaning to do some experiements this winter, but we didnt get any extreme low temperatures that lasted.
The idea was to test the FWB300s with its steel rings, as they should not be as affected by temperature as plastic/synthetic seals.

[evert]

....look solely at expansion/ contraction of seals and cylinders.....

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

[trajectory]

Although its not always possible to look up values for the coefficient of thermal expansion for all materials it's usual to find a generic value for steel as well as some for synthetic materials similar to those used to make some seals whether they be 'nylon' PTFE or the like. A quick look at a list of values will show the differences between them & support the probability that the majority of performance variation over the spread of ambient temperatures is down to expansion differences between the materials used in construction. It might not account for all the changes but I think it will be the major factor. Lubrication, shore hardness variation etc might have an influence too but ......

[cringe]

My tuppence' worth, I have found that my spring guns appear to shoot better in the winter month's. My theory is air density : air is heavy in winter & light in summer? all though the expelled pellet would have more resistance to travel through cold [heavy] air, or would it?
ponderable! regards Al.