Barometric Altimetry
Using the Vernier LabPro
Purpose of Report
• Improved altitude determination for Balloon Fest
•
and other activities
Determine characteristics of Vernier Barometer
sensor
– Resolution
– Uncertainty
– Accuracy
• Establish reliable procedures for testing and use.
Barometer Sensor Specs
• Sensitivity: 13.08 V/atm
• Digital resolution: 0.003 mBar (~0.8m)
Atmospheric Pressure Profile
• Standard Atmosphere
Model
h 

Ps ( h)  P0  1 

 44329m 
5.255876
Ps ( 1000ft)  0
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
Slope
Intercept
Volts
-659.3 m/V
+1635 m
kPa
-84.50 m/kPa
+8560 m
Atm
-8562 m/Atm
+8560 m
In Hg
-24.96 m/inHg
+8560 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 101.3 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 v2 
• Error in altitude determination due to
dynamic pressure: H = P84 m/kPa
• At 10 MPH: H = 1 m
• At 60 MPH: H = 36 m
Field Tests
• USGS topo maps: typically 20 ft contours
• Google Earth: appears good but has
unknown accuracy
• 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:
•
•
•
•
•
– 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)
Read about 1 kPa (9m) higher than official sources.
Have a digital resolution of about 0.8 m.
Random variations of about ± 2 steps
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:
– “warm-up” drift
– Pressure drift (weather changes)
– Dynamic pressure effects (due to wind or
motion)