Barometric Altimetry Using the Vernier LabPro

Purpose of Report Improved altitude determination for Balloon Fest and other activities Improved altitude determination for Balloon Fest and other activities Determine characteristics of Vernier Barometer sensor Determine characteristics of Vernier Barometer sensor –Resolution –Uncertainty –Accuracy Establish reliable procedures for testing and use. Establish reliable procedures for testing and use.

Barometer Sensor Specs Sensitivity: V/atm Sensitivity: V/atm Digital resolution: mBar (~0.8m) Digital resolution: mBar (~0.8m)

Atmospheric Pressure Profile Standard Atmosphere Model Standard Atmosphere Model Near sea level, this exponential model yields almost identical results as the Adiabatic, Constant Temperature, and Meteorological models do. We can assume that over a small change of altitude (i.e. 1000ft) this exponential curve approximates a straight line. However, the slope of this linear approximation changes significantly as one moves to higher ground altitudes (e.g. 15% difference in slope between 0 ft and 6000 ft)

Converting Sensor output to Altitude Data Sensor Output SlopeIntercept Volts m/V m kPa m/kPa m Atm m/Atm m In Hg m/inHg m Slope is calculated from US Standard Atmosphere Model for elevations between 1000 and 2000 ft above sea level. The Intercept is based on sea level pressure being kPa.

Dynamic Pressure Effects Difference of pressure (  P) on the leading edge of a body moving through air with a relative velocity (v):  P= ½ v 2  Difference of pressure (  P) on the leading edge of a body moving through air with a relative velocity (v):  P= ½ v 2  Error in altitude determination due to dynamic pressure:  H =  P  84 m/kPa Error in altitude determination due to dynamic pressure:  H =  P  84 m/kPa At 10 MPH:  H = 1 m At 10 MPH:  H = 1 m At 60 MPH:  H = 36 m At 60 MPH:  H = 36 m

Field Tests USGS topo maps: typically 20 ft contours USGS topo maps: typically 20 ft contours Google Earth: appears good but has unknown accuracy Google Earth: appears good but has unknown accuracy Weather services report three kinds of pressure: Weather services report three kinds of pressure: –Station, Barometric, Altimeter

Barometric Pressure during the Accuracy and Drift Test as reported at SLO airport about 9 miles north of test location at an elevation of 212 ft MSL. This graph covers the time from 2pm 6/8/07 thru 2:30pm 6/9/07

Summary of Observations Warm up: Warm up: –Time: from 2 minutes to as much as 1 hour. –Increasing pressure: from 0.1 kPa to as much as 1.3 kPa –Decreasing error in altitude measurement: from 8 m to as much as 110 m. Sensors outputs vary by up to 0.4 kPa (~35 m) Sensors outputs vary by up to 0.4 kPa (~35 m) Read about 1 kPa (9m) higher than official sources. Read about 1 kPa (9m) higher than official sources. Have a digital resolution of about 0.8 m. Have a digital resolution of about 0.8 m. Random variations of about ± 2 steps Random variations of about ± 2 steps Uncertainty (std deviation) of ± 1.6 m. Uncertainty (std deviation) of ± 1.6 m.

Conclusion Relative altimetry is possible to an accuracy of 2 or 3 meters if painstaking effort is taken to eliminate or mathematically remove: Relative altimetry is possible to an accuracy of 2 or 3 meters if painstaking effort is taken to eliminate or mathematically remove: –“warm-up” drift –Pressure drift (weather changes) –Dynamic pressure effects (due to wind or motion)