The PS-2136 PASPORT 3-Axis Acceleration/Altimeter Sensor and PS-2154A PASPORT Weather Sensor, use a barometer to determine relative altitude (height). Barometric altimeters give reasonably accurate (+/- 3%) relative altitudes without calibration; however, uncalibrated absolute altitudes may differ by as much as +/- 100 meters from published values. Fortunately, the barometric altimeter can be easily calibrated to yield more accurate altitudes.
Principle of Operation
The PASCO altimeter calculates the absolute altitude, h, from the pressure exerted by the air mass above it:
h = A ln (P0/P)
, where A = 8428 m, P is the measured local atmospheric pressure, and P0 = 101.32 kPa (the standard atmospheric pressure at sea level). The coefficient A corresponds to the scale factor for dry air (molar mass = 28.97) at a temperature T of 287.825 K and standard atmospheric pressure (P0= 101.32 kPa at sea level.):
A = kb T / m/ g
where m is the molar mass * 1.66e-27 kg and g is the gravitational acceleration near the surface of the Earth (9.8 m s-2).
From this equation, it is obvious that a change in the temperature of the air column will result in a determination of the relative altitude. For absolute altitude determinations, which involves the integrated density of air from sensor level up to the stratosphere, atmospheric density profiles are predominantly the result of a superposition of atmospheric tides.
The PS-2136 PASPORT 3-Axis Acceleration/Altimeter Sensor was designed for roller coasters for which relative changes in altitude are important, but absolute altitude is much less important.
Fortunately, the standard deviation in the relative altitude inaccuracy is only +/- 3% (corresponding to a standard deviation of +/- 3 m for the world's tallest (100 m) roller coaster) and can be reduced to +/- 1% with calibration.
Correction for Firmware Miscalculation
The datasheet of the PS-2136 PASPORT 3-Axis Acceleration/Altimeter Sensor , which is included with DataStudio 1.9.0 has value of A that is off by 1.2, leading to incorrect altitude data. If you are using a version of DataStudio prior to 1.9.1, please upgrade. If you are using a portable data logger, please connect it to a USB port of a computer running DataStudio 1.9.1 or later. Connect a sensor to the interface and collect a few data points to be sure that it is working.
The following corrections should be done within an hour of the measurements because atmospheric conditions change rapidly:
Calibration Using the Published Elevation Change of the Roller Coaster
Creation Date: 06/17/2004
Last Modified: 06/17/2004