Instruction manual

SECTION 1. SYSTEM OVERVIEW
1-3
1.3.3 NO ABSOLUTE REFERENCE
The CA27 zero offset drifts with ambient air
temperature. The zero offset drift does not
effect the flux measurements, since only the
fluctuation about some mean are of interest.
This drift does, however, preclude the
measurement of the mean vertical wind speed.
The 127 does not measure absolute
temperature, instead the 127 is referenced to
the unknown temperature of the sonic base.
Thus, the output of the 127 is the difference
between the ambient air temperature and the
temperature of the sonic base. The reference
junction (CA27 base) has a thermal time
constant of approximately 20 minutes. To
extend the thermal time constant of the base,
insulate it with hot water pipe insulation.
Referencing the 127 to the sonic base does not
limit the ability of the 127 to measure
temperature fluctuations.
The window on the source tube of the
hygrometer is prone to scaling. This scaling is
caused by disassociation of atmospheric
constituents by the ultraviolet photons
(Campbell and Tanner, 1985). The rate of
scaling is a function of the atmospheric
humidity. In high humidity environments,
scaling can occur within a few hours. This
scaling attenuates the signal and can cause
shifts in the calibration curve. However, the
scaling over a typical flux averaging period is
small. Thus, water vapor fluctuation
measurements can still be made with the
hygrometer. The effects of the scaling can be
easily reversed by wiping the windows with a
moist swab.