Using the LI-8150 Multiplexer
10-37
Chambers sold by LI-COR provide an electronic signal when they are closed; this
signal is used during various operations (e.g. during calibration) to indicate when
the chamber is fully closed. Thus, when a LI-COR chamber is chosen from the
menu, the “This chamber provides a “Closed” signal” check box is automatically
checked. If a custom chamber is attached, this check box should be checked only
if the chamber provides this electronic signal. The Chamber Volume (cm
3
) is
automatically entered for LI-COR chambers; this value will need to manually
entered when using custom chambers.
The Temperature Source determines where the air temperature measurement used
for the flux calculations is obtained. When using a LI-COR soil chamber, this field
should always be set to ‘Chamber’, so that the temperature measurement is made
using the thermistor located inside the chamber. For other experimental protocols
(e.g. making flask measurements), however, you can choose to map the
temperature measurement to any of the analog voltage input channels (V1-V4 with
the Auxiliary Sensor Interface, or V2-V4 with the 8100-104/C and the LI-8150
Multiplexer), and then use the appropriate V2-V4 tabbed page to enter the
coefficients for the user-supplied temperature thermistor. The temperature derived
from that input channel will then be used for the calculations.
The Volume Corrections allow you to enter the exposed Soil Area (cm
2
) within the
soil collar, as well as the Chamber Offset and extension tube volume (if present).
The Soil Area is the area encompassed by the chamber collar. The Chamber
Offset is the distance (in cm) between the soil surface and the top of the soil collar,
and is dependent upon the depth that the collar is inserted into the ground
(discussed in Section 2, Initial Setup). The soil CO
2
flux measurement requires an
accurate estimate of the Total System Volume. The LI-8100A software uses the
Chamber Offset and the Soil Area values to calculate the volume of air inside of
the soil collar (Collar Volume). This value is, in turn, added to the Chamber
Volume, IRGA Volume, Multiplexer (Mux) Volume, and Extension Tube
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