Mike, Yes, it can be done. Try a Motorola MPX4115A sensor. It has a 0-5v output depending on pressure. You will need to use an op amp to level shift and multiply the input to make it more suitable for the input to an ADC. Google for the data sheet. Equations you will need: 1000 mb = 29.53"Hg = 1 bar = 10^5 Pa = 100kPa Pressure at any altitude is: P(z) = P(0)*(1- 0.02255z)^5.256 where P(z) = pressure (mb) at z km above sea level P(0) = pressure (mb) at sea level Oh, and you will need a really good filter between the sensor and the ADC, as well as some digital filtering as well. Make sure that you verify that the granularity of the adc and your op-amp circuit so you get the required resoution. I can't tell you too much more than that until after Dec 3rd. (I'm working on a contest entry!) ken klumia@adelphia.net ----- Original Message ----- From: "Mike Hord" To: "Microcontroller discussion list - Public." Sent: Thursday, September 16, 2004 10:38 AM Subject: [OT] Pressure altimeter > I'm looking for a good, no-nonsense explanation of the variation of > pressure with altitude. > > Google reveals many, many pages, most of which are either over > simplified (altitude decreases pressure) or way to complicated; > plus, most of the ones that actually present equations are aimed > at altitude variations in the thousands or tens of thousands of feet, > while I am concerned about hundreds. > > Basically what I want to know is this: can I use a pressure > sensor with an absolute range of up to 30 PSI to measure within > 10 to 20 feet the difference in altitude between the current position > of a sensor package and the position at which the package was > initialized? At altitudes of 1000 feet or less? > > Please pardon the bass-ackwards US measurements, but > pressure is one of the few things I don't have a good feel for in > metric (distance, mass, speed, volume, and especially time > aren't so bad ;-) ). > > I have also found pages with pressure based altimeters for model > aircraft and rockets, but they tend to be sketchy on the details > and I really want to understand this, rather than just having an > answer for how to do it handed to me. I find understanding to be > far more valuable in the long run. > > Mike H. > _______________________________________________ > http://www.piclist.com > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > _______________________________________________ http://www.piclist.com View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist