Posted: 12/10/2009 9:35:01 AM EDT
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For starters... barometric pressure is essentially how an altimeter works, correct?
Ok... this is the "Up in an airplane" experiment I'm trying to put together. As I understand it you get a container, put a semi-inflated balloon in it. If you did this at sea level and sent it up in a plane the balloon should "inflate" or increase in size inside the container, correct? Is there any other way to do it like this, but not using liquid? I saw a way to make a homemade glass barometer, but that won't do me any good. Thanks in advance |
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Yes, altimeters are barometers. Aneroid barometers, specifically. The problem with using a sealed quantity of air is thermal effects have a great bearing on the reading. Aneroid barometers use an evacuated vessel for the reference pressure, a vacuum of fairly high quality. To expand the sensitivity, the vacuum container is a bellows, allowing it to move greater with changing atmospheric pressure. So now you need a bellows, preferably metal. And a way to achieve a fairly good vacuum inside it, like a refrigeration pump. Then seal the vacuum inside. Atmospheric pressure is the inverse of the length of this assembly. |
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Quoted:
Yes, altimeters are barometers. Aneroid barometers, specifically. The problem with using a sealed quantity of air is thermal effects have a great bearing on the reading. Aneroid barometers use an evacuated vessel for the reference pressure, a vacuum of fairly high quality. To expand the sensitivity, the vacuum container is a bellows, allowing it to move greater with changing atmospheric pressure. So now you need a bellows, preferably metal. And a way to achieve a fairly good vacuum inside it, like a refrigeration pump. Then seal the vacuum inside. Atmospheric pressure is the inverse of the length of this assembly. How about one of those FoodSaver vaccuum containers? I'm not sure what I could use for a metal bellows though. I'm looking to set iit up so that, in theory, once it reaches a certain altitude it it set up with an alarm to notify you that the desired height has been reached. With the balloon experiment this is a little simpler I think... but as you said, may be a little less reliable. |
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Balloons suck because they are permeable to air and thermal effects will constantly haunt you. Remember, the lapse rate is 3º F per 1000 feet, this will compensate for lower pressure, making your altimeter much less sensitive. For example, the balloon, if inflated at sea level and 75 ºF will be the same size if taken to 3500 feet and 0 ºF. But with a true aneroid cell, the pressure change is absolute and altitude will be within 50 feet if properly compensated for normal atmospheric variation. |
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a simple experiment is to rubber band a sheet of latex ( think balloon cut open) across a plastic bowl. Set the bowl in a base, and have a wall at 90 degrees to the base.
Tape a stick in the middle of the balloon, and have the mid point resting on the edge of the bowl. You can then mark the postition of the stick in relation to altitude on the wall you have attached to the base. Something like this 
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Quoted: a simple experiment is to rubber band a sheet of latex ( think balloon cut open) across a plastic bowl. Set the bowl in a base, and have a wall at 90 degrees to the base. Tape a stick in the middle of the balloon, and have the mid point resting on the edge of the bowl. You can then mark the postition of the stick in relation to altitude on the wall you have attached to the base. Something like this http://i.ehow.com/images/GlobalPhoto/Articles/5321784/Step-9-balloon-main_Full.jpg Still, that isn't temperature compensated. |
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Quoted:
Quoted:
a simple experiment is to rubber band a sheet of latex ( think balloon cut open) across a plastic bowl. Set the bowl in a base, and have a wall at 90 degrees to the base. Tape a stick in the middle of the balloon, and have the mid point resting on the edge of the bowl. You can then mark the postition of the stick in relation to altitude on the wall you have attached to the base. Something like this http://i.ehow.com/images/GlobalPhoto/Articles/5321784/Step-9-balloon-main_Full.jpg Still, that isn't temperature compensated. If he is doing something very precise, then that might be important. My understanding is this is a high school project, or something simular, The device will be in a heated cab as I understand it. OP: Take a look at the bottom of page 243, this is what you are asking for, just not the same use. http://zugzwanged.org/np/improvised_munitions_handbook_v3.pdf |
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Quoted:
OP: Take a look at the bottom of page 243, this is what you are asking for, just not the same use. http://zugzwanged.org/np/improvised_munitions_handbook_v3.pdf Always a comfort knowing this is readily available on the net. That's pretty much what I was looking to make. Thanks dude. |
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It's not precise at all, but we do this with empty plastic 2-liter soda bottles all the time. We live in Colorado and have out-of-state visitors. Screw the cap on tight at the top of a mountain, drive home, enjoy the collapsed bottle. (We're easily entertained.) |