CS115 Barometric Pressure Sensor

*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.

5.3Output 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).

 

5.25328

E

 

 

 

 

 

 

dP = 1013.25 1− 1−

 

 

44307.69231

 

 

 

 

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.

E(ft)

E(m) = 3.281ftm

(1)

(2)

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