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air pressure - relative or absolute?

 
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Old 12-06-2011, 11:40 AM
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air pressure - relative or absolute?

I recently got to taking and logging air pressure when I do a range session, because I have acquired some hunting in the mountains and will be taking shots with 6000 feet more elevation than where I normally zero and shoot.

So I figure that if I can track the differences in the cold bore POI with pressure changes I can use that info for my high altitude shots.

So do I need to pay more attention to relative or absolute pressure?
My Skywatch Xplorer4 gives both, and it was only recently that I discovered the difference.
Now, if your logging work is good, I suppose it shouldn't make any difference, because it calculates the relative air pressure by taking the absolute then factoring in the altitude, but that is also pressure based, and here is where I start to have doubts in the accuracy of using relative.


With absolute you should have an actual environmental value, and I should be able to set up an air pressure to POI log, which should also translate into altitude, no?
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Old 12-06-2011, 04:14 PM
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Re: air pressure - relative or absolute?

Its indavidual preference AS long as you do it consistently.

My preference is to document raw pressure (pressure at location) and skip the altitude. I document the pressure, temp, humidity etc...

IMO, using raw pressure only leaves one less step/value out of the picture to minimize mistakes.

Just my .02
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Old 12-06-2011, 04:44 PM
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Re: air pressure - relative or absolute?

That's what I was thinking. Bow do you measure humidity? Have you ever known it do make any real difference in your trajectory?
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Old 12-06-2011, 05:23 PM
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Re: air pressure - relative or absolute?

Quote:
Originally Posted by EXPRESS View Post
That's what I was thinking. Bow do you measure humidity? Have you ever known it do make any real difference in your trajectory?
Some of the Kestral stations provide humidity measurments. So do some of the Brunton units. I use the Brunton. Some models have it and some dont. Honestly, the difference between 0% and 100% humidity has a very small effect on trajectories. At least using average loads up to 1K. For my 308 the difference is somewhere around 6" at a full 1000 yards. This can definately be a deciding factor between a hit and a miss. At least it is a managable variable in the case you dont have a pocket station that measures humidity whereas pressure and temps are not quite as managable on a long shot. You REALLY need to know the pressure and temp.

If you cant afford the top end models that measure humidity, you can document it from the nearest airport etc....during you range sessions. When you are out hunting it should be fairly obvious whether or not it is average, dryer or more humid. A minor addition or subtraction is typically adequete. I cant see where anyone would actually sight in their rifle in 0% or 100% and then shoot the exact opposite. There are extremes such as sighting in at Phoenix and hunting in Ketchikan. Even then you could expect 20% versus 80% which is not too bad. On average, the difference will be between 50% and 80% which is minimal as far as we are concerned.

Beyond 1K and you had better not 'wing' any variable.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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