CS106 Barometric Pressure Sensor
4.3 Long Lead Lengths
There is a 0.06 mV/foot voltage drop in the CS106 signal leads. This voltage
drop, in long lead lengths, will raise the barometric reading by approximately
1.44 mb per 100 feet.
For lead lengths greater than 20 feet, use the differential instruction
(Instruction 2) to measure the CS106.
4.4 Output Resolution
When storing the values from the CS106 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 Campbell Scientific dataloggers is limited to a maximum of four
digits. Even then, the maximum digit value that can be displayed is 7999 for
the CRBasic dataloggers and 6999 for Edlog dataloggers. If you use this
option with barometric data scaled in millibars (hPa), a reading above
799.9 mb for CRBasic dataloggers (699.9 mb for Edlog dataloggers) will lose
one digit of resolution (e.g. at 900 mb, the resolution is limited to 1 mb).
To retain 0.01 mb resolution, you either need to subtract a fixed offset from the
reading before it is stored to avoid exceeding the 799.9 for CRBasic
dataloggers (699.9 for Edlog dataloggers) threshold, or output the barometric
reading in high resolution format. This can be done by using the IEEE4 format
for CRBasic dataloggers or the Resolution (P78) instruction in the Edlog
dataloggers. The default data output format for CR200(X) series datalogger is
IEEE4.
5. Correcting Pressure to Sea Level
The weather service, most airports, radio stations, and television stations adjust
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 (500 mb in our example programs) 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
()
()
.
=
3281
(2)
10
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