Campbell Scientific Bowen Ratio Instrumentation - Manual (Page 9)

Section 1. System Overview
1-5
the gradient measurement is due only to the difference in the radiative heating
of the two TC junctions and their physical symmetry minimizes this.
Conversely, contamination of only one junction can cause larger errors.
Applying temperature gradients to the TC connectors was found to cause
offsets. The connector mounts were designed with radiation shields and
thermal conductors to minimize gradients.
The prototype systems used two sets of TCs on each system, one 25
µm and
one 76
µm diameter. It was hypothesized that the 25 µm diameter would
suffer less from radiation loading and the 76
µm would be less prone to
breakage. The current design uses a single set of TCs (76
µm standard) with
two parallel junctions at each height as a back up against breakage.
1.2.3 Net Radiation and Soil Heat Flux
Net radiation and soil heat flux are averaged over the same time period as the
vapor pressure and temperature differences.
To measure soil heat flux, heat flux plates are buried in the soil at a fixed depth
of between 5 to 10 cm to reduce errors due to vapor transport of heat.
Typically the plates are buried at a depth of 8 cm. The average temperature of
the soil layer above the plate is measured using 4 parallel thermocouples. The
heat flux at the surface is then calculated by adding the heat flux measured by
the plate to the energy stored in the soil layer. The storage term is calculated
by multiplying the change in soil temperature over the averaging period by the
soil heat capacity.
1.2.4 Power Supply
The current requirements of the components of the Bowen ratio system are
given in Table 1.2-1.
TABLE 1.2-1. Component Power
Requirements
Component Current at 12 VDC
Cooled Mirror 150 - 500 mA
Pump 60 mA
CR23X 5 mA
A 20 watt solar panel (SP20R) and a 70 amp-hour battery are capable of
providing a continuous current of 300 - 350 mA. The solar panel is necessary
if the system is to be used for periods longer than 2-3 days. The datalogger
can control power to the cooled mirror and pump, and can shut down the
system if the battery voltage is low or if measurements are not needed at night.