Campbell CS106 Manual de instrucciones descargar pdf (Pagina 20)

CS106 Barometric Pressure Sensor

8.3 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

(VoltDiff()) to measure the CS106.

8.4 Output Resolution

When storing the values from the CS106 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. If you use this option with barometric data scaled in

millibars (hPa), a reading above 799.9 mb 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 threshold, or output the

barometric reading in high resolution format. This can be done by using the

IEEE4 format. The default data output format for CR200(X) series datalogger

is IEEE4.

8.5 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 3 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 3 was derived (Wallace, J. M. and P. V. Hobbes,

1977: Atmospheric Science: An Introductory Survey, Academic Press,

pp. 59-61).

= − −

































1013 25

1 1

44307 69231

5.25328

(3)

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 (4) to convert feet to meters.

E ft

ft m

( )

3 281

(4)

The corrections involved can be significant: e.g. at 1000 mb and 20 °C,

barometric pressure will decrease by 1.1 mb for every 10 meter increase in

altitude.

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Campbell CS106 Manual de instrucciones Pagina 20

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