Campbell Bowen Ratio Instrumentation, Instruction Manual

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 

µ

m and 

one 76 

µ

m diameter.  It was hypothesized that the 25 

µ

m diameter would 

suffer less from radiation loading and the 76 

µ

m would be less prone to 

breakage.  The current design uses a single set of TCs (76 

µ

m 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.