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Thread: Altimeters

  1. #1 Altimeters 
    Forum Freshman Mister URL's Avatar
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    How can a hand-held altimeter based on barometric pressure ever be accurate? Since the "normal" atmospheric pressure constantly varies locally based on high and low pressure air masses, it would automatically throw a pressure based altimeter off.

    For example if you are at sea level in a hurricane and looking at your device, you will appear to be somewhere above 0 feet. If you are there during a balmy high pressure day, you will seem to be below sea level. What's the Deal? How can they sell these things and expect people to use them to determine their altitude without taking into account the local pressure variations?

    Thanks.


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  3. #2  
    Forum Junior c186282's Avatar
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    Good question.

    The variation in pressure resulting from weather is small compared to the variation in pressure with height. I have driven over mountain passes have have had my potato chip bags pop because of the pressure but never have I seen a potato chip bag pop because of changing weather.

    See: Atmospheric pressure


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  4. #3  
    Forum Freshman Mister URL's Avatar
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    OK, that would account for these being sold if the error in feet is really small. I ought to be able to calculate that. It would not be linear since air is so compressible.

    The difference in pressure due to altitude is substantial: at sea level it is about 29.9 inches and at 2000 feet is it only 27.8 inches.

    But some hurricanes get to that 27+ reading, so I am still confused as to why you could rely on the altimeter reading in anything other than fine "normal" weather.

    I think, if I want to use a barometric altimeter, that what I need to do is determine what the typical pressure deviations are, discounting especially low or high readings, which must be rare.
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  5. #4  
    Forum Freshman Mister URL's Avatar
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    OK, that would account for these being sold if the error in feet is really small. I ought to be able to calculate that. It would not be linear since air is so compressible.

    The difference in pressure due to altitude is substantial: at sea level it is about 29.9 inches and at 2000 feet is it only 27.8 inches.

    But some hurricanes get to that 27+ reading, so I am still confused as to why you could rely on the altimeter reading in anything other than fine "normal" weather.

    I think, if I want to use a barometric altimeter, that what I need to do is determine what the typical pressure deviations are, discounting especially low or high readings, which must be rare.
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  6. #5 Re: Altimeters 
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by Mister URL
    How can a hand-held altimeter based on barometric pressure ever be accurate? Since the "normal" atmospheric pressure constantly varies locally based on high and low pressure air masses, it would automatically throw a pressure based altimeter off.

    For example if you are at sea level in a hurricane and looking at your device, you will appear to be somewhere above 0 feet. If you are there during a balmy high pressure day, you will seem to be below sea level. What's the Deal? How can they sell these things and expect people to use them to determine their altitude without taking into account the local pressure variations?

    Thanks.
    This is why the altimeters on small aircraft have a small knob and a window reading barometric pressure in inches of mercury. The knob is used to adjust the altimeter to compensate forthe barometer reading.

    Larger airports have an ATIS frequency which broadcasts terminal information on a loop. This info includes the present barometer reading. You enter this into your altimeter, and then you know its accurate.

    Smaller airports(with no tower) have the airport elevation posted on a sign at each end of the runway. You just adjust your altimeter until it reads the airport altitude before you take off. This is only really accurate for short flights when you know the barometric pressure is not going to change that much between take off and landing.
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  7. #6  
    Forum Freshman Mister URL's Avatar
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    Since you don't know the local pressure relative to "zero", and you don't know your altitude, these hand-held "altimeters" are about worthless. They have one parameter (atmospheric pressure) that supplies what it calls an altitude reading, but in fact it is only showing pressure. Do GPS satellites provide altitude, maybe by bouncing radar off the earths surface?
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