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Old 16th June 2010, 05:16 PM
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RobF RobF is offline
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Join Date: Mar 2010
Member of: Southampton Buccaneers, Parkstone, South Dorset
Location: Poole, Dorset
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Default Heat, light and gun and scope...

Naff all happening this afternoon at work, but clear blue skies, so decided to sneak out and see if I could unravel the shift seen on the March. Slapped some temp strips on the scope, and grabbed a digital temp probe that an engineer had left behind one day.

First test was to see if both read the same, and indoors, in the shade they both had everything at 19 degrees. Interestingly the strips moved pretty fast dropping from the 25 degrees of the car to the 19 of the scope within a minute or so of being attached to the scope body. The temp probe moves very fast, and often picked up just the effect of a cool breeze.

The thing to remember in this is that heat transfers. It does this by 3 methods, conduction, convection and radiation. Whilst heat is linked to the radiation that makes you taste your fillings and hair loss, we're not concerned with that aspect.

Using the illustration of a fire, radiation is what you can feel on your face as you get near it, convection is what you can see as the hot air and smoke rises and draws in air, and conduction is what happens to a metal bar placed in a fire, with some exposed out... when you grab it it's hot. Metal is a very good conductor, bad conductors are known as insulators... a wooden stick in the same scenario is not likely to burn your hand, as the heat hasn't transfered all the way through it and finds it difficult to do so. Organic substances tend to be good insulators.

Ok, so that's the basics out of the way.

The primary source of heat outdoors, is the sun... this is radiation. Radiated heat has the ability to pass through things, and different sunstances absorb different parts of the heat spectrum. That's the reason why an object left in the sun may be hotter than the air around it. You can burn your feet walking barefoot on the beach, yet you don't burn all over in the air you're walking through. Different parts of the spectrum pass through different things. Infra red for instance passes through air well, but not you, which is why it's used for heat lamps. Microwaves pass through a lot of stuff, but not water, so they heat food rather than the air...

Black things are black because they absorb a lot of the colours/wavelengths of light (which is on the same scale as heat) and silver or white things reflect most of it. Things have a colour because they absorb most wavelengths, but reflect one particular band of wavelength of light.

Anyway, onto the scope.

Taking the rig down to South Dorset allowed me to put it in light and shade with ease. First test was to see what she measured in the car after an hour of airconned journey. Temp strips read 19, and a probe into the bag read 19, but the air in the back of the car was 25. That suggests the zipped up foam lined bag was doing a very good job of insulating the rig against the air in the car heated by the sun. Something to remember if your akin to getting a shifting rig out and trying to shoot with it in different conditions from home.

Taking her out of the bag and onto the heavily shaded range, which was also at 19, i did a quick rangefind test, and she measured 55 bang on the money. Letting her settle for 10 mins, no change. The plinking range has now got a heavy canopy and is much darker than when I first set the ranges up, but they all came in true.

Next test was to take her out in the sun, but I wanted to see what insulating her from the sun would do. So I left the scope on the ground, covered by my coat, which has a silvery grey inside, which i turned inside out and covered just the scope with. I left it for 10 mins. Being an outdoors walking coat, it's a good insulator as well.

After 10 mins, measuring the air with the probe here had the temp near the scope at 23 degrees. The scope was reading 19 under the coat, but the exposed black muzzle brake inside was reading 25.

After 20 mins, the air was 22, the scope still 21, the muzzle was 30. The temp strips read 24.

Testing the rangefinding again in the shade and she was still bang on 55.

Next test was leaving her naked in the sun, no protection.

After 10 mins, the air was 24, the scope was 28, the muzzle was 30. Under the objective scope cap, which had been left closed was 33, the strips read 32. The reg, which is silver coloured, read 25 on the probe.

After 20 mins, the air was 26, the scope was 32, the muzzle was 30. Under the cap was reading 35. The strips were reading 37. The reg had risen to 30.

After 30 mins, the air was 26, the scope was 32, the muzzle was 30. Under the cap was reading 37. The strips were reading 40. The reg held at 30.

Taking her straight away onto the shaded plinking range, and the scope was under ranging by 4 yds at 55.

I had set up a camera to meter a the target, and this showed little change between the start and finish, and the difference was only probably attributable to the sun moving around... even though it was in shade, it was probably hitting now at more of a steeper angle.

Within 10 minutes of being in the shade, the scope had cooled to under 25 degrees. It was difficult to determine how long it took to get where, but within 15 minutes it was all at 20 degrees, save the reg, which was just under 30 still. As quick as I could, I looked at the shift, which diminished by the second, until it was gone.

So, conclusions...

1) The scope reacts quickly to radiation from the sun, and radiates and cools quickly when left in the shade. 20 mins seems to be around the time to see it at almost it's maximum. This goes for the rest of the gun as well, apart from the reg.

2) The silver of the reg, or perhaps it's different material, means it doesn't heat up as fast, however it holds it longer. Whether this is because it's silver, and traps the heat in, or because it could be a different material, i don't know. What I do know is that the reg heating up, and the scope, did not impact POI at all.

3) Shielding the scope from the heat with something that reflects signficantly slowed the scope from absorbing radiation from the sun. I suspect that it would also slow the radiation of the scope, thus making it cool slowly as well. Things that insulate tend to do so both ways, resisting the heat transfer both in and out.

4) The stock was 3-4 degrees different on one side than the other, however the carbon shrouded barrel was only about 1-2 degree different. A more severe exposure of the sun may yeild different results, and i guess it also depends on what's underneath the gun.

Pondering further... coating with white or silver may slow down absorbtion of the sun's heat. However, as it is in direct contact with the surface of the object, if it conducts, then any heat that gets through will be conducted into the object quickly, especially if that is metal. Shiny aluminium foil is supposed to reflect in the order of 90% of radiated heat, but it is also a good conductor, meaning the 10% could be transfered easily to whatever it touches,again especially if that's metal. That might suggest that coating alone could be improved upon with an insulating layer between it and the object you want to protect, or that coating alone might still have it's weaknesses.

Also to ponder, is that eventually, the scope will absorb heat by conduction from the ambient air temperature... but considering that was only 20 degrees, (the air was measured near the ground where the air was, so may well be picking up some reflected heat off the ground, or radiated heat from it) today objects got up to 35-40 just from absorbing radiation... so reducing this right down could mean temp shift could be heavily mitigated, if it's slowed to the point to where it stays within a more comfortable bracket.

I guess the only way of testing that up is to work out what you want to shield and really bake and monitor it.

Also, removing whatever you use to shield from the sun even for a few minutes, may over time, quite reduce it's effect as it receives hit after hit of heat, and if you then shield it, you may trap the heat in that it's absorbed while exposed.

Interesting to know if the reg is alloy, because if it is, it might suggest that silver paint may have it's tipping point, and that you may need to spend more time acclimatising a scope then... but once there it should hold for longer if it finds somewhere cool.

So really it's up to you if you want a steep or shallow temperature curve against time, and whether you can work out how to keep that within a more acceptable bracket.

You might not even have a scope that shifts... or maybe it does it by a small amount. Or may be your gun does. But at least I have now much more of an idea of what's going on on mine
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