Home Introduction Specification Photographs Screenshots Hardware Software Utilities PHYSICS History Traces Links

Altimeter physics Units Easiest to work out in metric, and then convert to imperial where necessary. This way all the units work out for you: 1000 mbar = 100kPa = 14.5Psi = 29.5 inches of mercury 1 foot = 0.3048m 1 m = 3 feet 3.37 inches Other conversions can be found here. Calculating altitude from pressure There is an international altitude formula which defines a standard pressure atmosphere relative to a mean sea level of 1013 mbar. We can then derive an agreed altitude from the current pressure reading: where the units are as follows: Pressure at sea level 1013.2 mbar Temperature at sea level 288 Kelvin Lapse rate 0.0065 Kelvin/m This is known as QNE and is used by airliners flying in the higher flight levels. Two other altitude readings are also obtained using this formula. Subsituting the pressure at an airfield for sea level yields QFE (field elevation) which is zero when you have landed. QNH can be derived from QFE and is the actual height above mean sea level. You know an airfield is (for example) 200 feet above mean sea level no matter what the pressure reads! It is worth noting that this formula can be approximated and simplified. This is particularly useful for simpler instruments and works out as: altitude/m = -8590 ln (pressure / 1013) For this altimeter I've implemented the full blown formula for QNE expressing it in feet. I then derive QFE and QNH by simply adding offsets to the height rather than pressure. This isn't totally accurate as should adjust the pressure instead. It's close enough for our use unless you make a habit of getting close to airspace! Variometer calculations The variometer (vario) is the rate of change of altitude. Paraglider pilots use metres for altitude and therefore m/s as rate of climb (or sink). Hang glider pilots use feet and so note their ascent in 100s of feet per minute (fpm, or simply "up" or "down"). Sailplane pilots use knots! There is approximately a 1 mbar drop in pressure for every 34 feet. For this vario I was after an accuracy of 0.2up/down, ie 20 feet per minute. In pressure terms this is then 0.01 mbars/second. We have an ADC resolution of 65535 /(1050mbar-330mbar) = 91/mbar; ie we can just about detect a 0.2up/down change once a second! It is for this reason that the vario updates once a second. This is a little poor for audio so we accept some error and update the sounder twice a second with a loss of some accuracy. Pitot tube and impact pressure Crudely speaking if you stick a tube into the airflow you will get a higher "impact" pressure in the tube relative to measured pressure in still air. This is the method used by nearly all aircraft to measure airspeed. It's simple and there aren't any moving parts to worry about. Powered aircraft have to concern themselves with icing up but this is not something we should have to worry about! The formula for airspeed derived from pitot tube readings works out as follows: Where airspeed is in metres per second and temperature is in Kelvin. If you drop this into a spreadsheet (or just load up asi.xls from the ffc.zip download in the software section) you can see that we can get usable readings above 17mph with the PIC's onboad 8bit ADC. This will give a reading of Indicated Air Speed (IAS) and not True Air Speed (TAS). To obtain true air speed from indicated air speed a good rule of thumb is to add 2% for every 1000 foot of elevation. So if we were at 3000 feet we would multiply IAS by 1.06 to get TAS. The altimeter software is setup to output TAS but is easy enough to revert to IAS if that is your preference. Back to top ...