# Kestrel absolute pressure question?

Discussion in 'Equipment Discussions' started by bowhunthard, Feb 8, 2013.

1. ### bowhunthardWell-Known Member

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996
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Sep 5, 2008
Ok, I've just been getting my head wrapped around the idea of absolute pressure, which I need to set my "BARO" setting to "0", correct? So, I did.

I then set my "ALTITUDE" setting to 29.92 inHg, right? (It calculates the altitude comprable to standard baro pressure, I guess.)

So, if my Kestrel 4500 NV is set up in the previous manner, is it taking absolute pressure? Also, will the "DENS ALT" (DENSITY ALTITUDE) be the correct value? I need the density altitude number to plug into "Shooter" for my Droid 4.

Thanks for the enlightenment .

2. ### otan3lvlNew Member

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Aug 13, 2013

To measure the absolute pressure aka station pressure with you Kestrel 4500NV (nice choice btw) you do as you have done: you go to the Baro-screen and set Reference altitude to 0feet above sea level. When setting the reference altitude to zero you tell the Kestrel that you are at sealevel and thus prevent it from converting the absolute pressure to sea level pressure. These weather stations are mainly used for weather-stuff, and the weather geeks use pressure converted to sea level. When shooting we are interested in the absolute pressure (the actual pressure at your location). Now of course it is equally important that you zeroed in using absolute pressure.

Ok, so you were spot in with your first bit, but....the altitude menu. if you use the standard atmospheric pressure of 29.92inhg you would not get an accurate altitude reading unless the absolute pressure at sealevel actually IS 29.92inhg at that exact time. The standard atmospheric pressure is just an average reading, so the actual pressure at sealevel fluctuates with the weather. If it is sunny and warm you are in a highpressure area most likely above 29.92 and if it rains itnwould be opposite and below 29.92.

The thing is, you had just told the kestrel that you are at sealevel when setting reference altitude to zero. when you go to the altimeter menu, you are suppose to tell it what the sealevel pressure is that day, so it can calculate the difference between sealevel and your location and thus figure out your altitude. So for a correct read of the altitude you would HAVE to get the ACTUAL pressure at sea level at THAT time. You can find that online from a weather station. Of course you could get this pressure yourself from the Kestrel by going to the Baro and put in your actual altitude (but then you would already know your altitude)

So here is what I suggest:
1. Use a topographical map or your gps to find your current altitude - write down
2. Put current altitude as reference altitude in Baro screen to get mean sea level pressure - write down.
3. Set reference altitude in Baro screen back to zero to get back to absolute pressure and getting ready to shoot.
4. Go to Altimeter screen and set reference value to mean sea level pressure that you wrote down in step 2.
This way you have the right barometric pressure for shooting, and as long as there are no big changes in weather the altimeter screen will also show the correct altitude throughout your day.

step zero would be to check that Sync baro and Sync alti are set to off, you find these settings where you punch in the reference values.

I hope you understood what I meant as I feel my explanation is not one of the best. But I DO have a proper understanding of this myself so feel free to ask any followup questions

3. ### bowhunthardWell-Known Member

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996
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Sep 5, 2008
I really appreciate the answer, very informative! I'm glad I was doing things correctly haha! I did finally figure it out, but that was the best explanation I have had to date, including contacting Kestrel directly. I did figure out the dens/alt factor, as I completely eliminated the need for it by setting the ballistic app to calculate with absolute pressure (just needed to reset rifle/load zero @ absolute pressure also). As you basically stated, ACTUAL PRESSURE (at shooting location) IS IMPORTANT, IF YOU HAVE THAT, ALTITUDE DOESN'T REALLY MATTER.