Open Path Eddy Covariance System
15
Wind Vane
Wind Vector
Y
X
North
FIGURE 12. Compass Coordinate System,
Compass Wind Direction is 140 Degrees
3.2.2 Sensor Configuration
The CSAT3 and LI-7500 communicate with Campbell Scientific, Inc.
dataloggers using a proprietary digital communication protocol called
Synchronous Device for Measurement (SDM). Each sensor connected to the
SDM bus has a unique address. A maximum of 15 addresses are allowed by
the protocol and each sensor must have a unique SDM address.
Do not use SDM address F (15) because it is reserved for
use with the Group Trigger instruction.
The CSAT3 is shipped from the factory with a default SDM address of 3. The
CSAT3 address is set by a 16 position thumb switch located on the CPU circuit
board (see the CSAT3 manual for details).
The LI-7500 shipped from Campbell Scientific, Inc. is set with an SDM
address of 7. This address is set in software and can be changed using the
LI-7500 PC support software and the LI-7500 serial cable and port. In addition
to the SDM address, Campbell Scientific, Inc. sets the LI-7500 delay to 300
milliseconds (297.25 milliseconds). At a measurement rate of 10 Hz, this
programmed delay is a 3 scan delay and at 20 Hz it is a 6 scan delay. Finally,
the LI-7500 bandwidth is set to 20 Hz to limit the amount of numeric filtering
of high frequency information.
If the LI-7500 delay and bandwidth are not set correctly, the carbon dioxide
and latent heat fluxes will be underestimated. LI-7500s purchased directly
through LI-COR may or may not have the correct settings for use in a
Campbell Scientific, Inc. eddy covariance system. The only way to know for
sure is to check the settings. Contact LI-COR for information on changing
these settings or see Section 3, Outputs Page - Setting DAC and SDM Outputs
of the LI-COR published LI-7500 manual.
CAUTION