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CS115 Barometric Pressure Sensor
13
*Table 3 Subroutines
End Program
-Input Locations-
1 P_mb 1 1 1
* Proper entries will vary with program and datalogger channel, and input location assignments.
5.3 Output Resolution
When storing the values from the CS115 to a datalogger’s final storage
location, or to a data table, care must be taken to choose suitable scaling of the
reading or to store the value with adequate resolution to avoid losing useful
resolution of the pressure measurement. The default resolution (Low
Resolution) for the Campbell Scientific dataloggers is limited to a maximum of
four digits. Even then the maximum digit value that can be displayed is 6999
(7999 for the CR5000/9000 dataloggers). If you use this option with
barometric data scaled in millibars (hPa), a reading above 699.9 mb (799.9 mb
for CR5000/9000) will lose one digit of resolution, e.g. at 900mb, the
resolution is limited to 1 mb.
To retain 0.1 mb resolution, you either need to deduct a fixed offset from the
reading before it is stored to avoid exceeding the 699.9 (or 799.9) threshold, or
output the barometric reading in high resolution format. This can be done by
using the Resolution (P78) instruction in the CR500, CR510, CR10(X),
CR23X, CR7 and 21X dataloggers, or the IEEE4 format for the CR5000 and
CR9000.
6. Correcting Pressure to Sea Level
The weather service, most airports, radio stations, and television stations reduce
the atmospheric pressure to a common reference (sea level). Equation 1 can be
used to find the difference in pressure between the sea level and the site. That
value (dP) is then added to the offset (0.0 mb) in the measurement instruction.
U. S. Standard Atmosphere and dry air were assumed when Equation 1 was
derived (Wallace, J. M. and P. V. Hobbes, 1977: Atmospheric Science: An
Introductory Survey, Academic Press, pp. 59-61).
dP
E
=−−
1013 25 1 1
44307 69231
5.25328
.
.
(1)
The value dP is in millibars and the site elevation, E, is in meters. Add dP
value to the offset in the measurement instruction.
Use Equation (2) to convert feet to meters.
Em
Eft
ft m
()
()
.
=
3 281
(2)