Barometric Altimetry Using the Vernier LabPro

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Barometric AltimetryUsing the Vernier LabPro
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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.

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Barometer Sensor Specs

• Sensitivity 13.08 V/atm
• Digital resolution 0.003 mBar (0.8m)

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Atmospheric Pressure Profile

• 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)
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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.
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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

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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

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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 230pm 6/9/07
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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.

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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)