INSTRUCTION MANUAL Bowen Ratio Instrumentation Revision: 9/05 C o p y r i g h t ( c ) 1 9 8 7 - 2 0 0 5 C a m p b e l l S c i e n t i f i c , I n c .
Warranty and Assistance The BOWEN RATIO INSTRUMENTATION is warranted by CAMPBELL SCIENTIFIC, INC. to be free from defects in materials and workmanship under normal use and service for twelve (12) months from date of shipment unless specified otherwise. Batteries have no warranty. CAMPBELL SCIENTIFIC, INC.'s obligation under this warranty is limited to repairing or replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective products.
Bowen Ratio Table of Contents PDF viewers note: These page numbers refer to the printed version of this document. Use the Adobe Acrobat® bookmarks tab for links to specific sections. 1. System Overview ......................................................1-1 1.1 Review of Theory ................................................................................. 1-1 1.2 System Description............................................................................... 1-3 1.2.1 Water Vapor Measurement ........
Bowen Ratio Table of Contents Figures 1.2-1. Vapor Measurement System............................................................. 1-3 1.2-2. Thermocouple Configuration............................................................ 1-4 2-1. CSI Bowen Ratio System .................................................................... 2-2 2.2-1. Placement of Thermocouples and Heat Flux Plates ......................... 2-3 2.2-2. TCAV Spatial Averaging Thermocouple Probe.............................. 2-4 2.3.
Section 1. System Overview 1.1 Review of Theory By analogy with molecular diffusion, the flux-gradient approach to vertical transport of an entity from or to a surface assumes steady diffusion of the entity along its mean vertical concentration gradient.
Section 1. System Overview β= H PC p ( T1 − T2 ) = Le λε (e1 − e 2 ) (6) where PCp λε is the psychrometric constant. The surface energy budget is given by, Rn − G − H − L e = 0 , (7) where Rn is net radiation for the surface and G is the total soil heat flux. The sign convention used is Rn positive into the surface and G, H, and Le positive away from the surface. Substituting βLe for H in Eq. (7) and solving for Le yields: Le = Rn − G 1+ β .
Section 1. System Overview FIGURE 1.2-1. Vapor Measurement System 1.2 System Description 1.2.1 Water Vapor Measurement It is common practice in Bowen ratio measurements to measure wet bulb depression to develop the water vapor gradient. The position of the two psychrometers is periodically reversed to cancel systematic sensor errors (Suomi, 1957; Fuchs and Tanner, 1970).
Section 1. System Overview A datalogger is used to measure all sensors and control the valve that switches the air stream through the cooled mirror. The resolution of the dewpoint temperature measurement is ±0.003°C over a ±35°C range. The limitation is the stability of the Dew-10, approximately 0.05°C, yielding better than ±0.01 kPa vapor pressure resolution over most of the environmental range.
Section 1. System Overview 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.
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Section 2. Station Installation Figure 2-1 shows the typical Bowen ratio installation on the CM10 tripod. The 023A enclosure, mounting arms, and SP20R solar panel all mount to the tripod mast (1 1/4 in. pipe, inside diameter) with U-bolts. The size of the tripod allows the heights of the arms to be adjusted from 0.5 to 3 meters. The mounting arms should be oriented due south to avoid partial shading of the thermocouples.
Section 2. Station Installation Q7-BR (system) FIGURE 2-1. CSI Bowen Ratio System 2.2 Soil Thermocouples and Heat Flux Plates The soil thermocouples and heat flux plates are typically installed as shown in Figure 2.2-1. The TCAV parallels four thermocouples together to provide the average temperature, as shown in Figure 2.2-2). It is constructed so two thermocouples can be used to obtain the average temperature of the soil layer above one heat flux plate and the other two above the second plate.
Section 2. Station Installation To minimize thermal conduction down the sensor lead wires, they should be buried for a short distance back from the sensor. In particular, do not run the leads directly to the surface, but wrap them around the edge of the hole, keeping the leads at the same level as the sensor for as long as possible. Once the sensors are installed, backfill the hole. Install the CS616 as shown in Figure 2.2-1. See the CS616 manual (Section 5) for detailed installation instructions.
Section 2. Station Installation 24 GAUGE CHROMEL CONSTANTAN HI (PURPLE) LO (RED) 40 GAUGE CHROMEL CONSTANTAN STAINLESS STEEL TUBE FIGURE 2.2-2. TCAV Spatial Averaging Thermocouple Probe 2.3 Wiring Table 2.3-1 lists the connections to the CR23X for the standard Bowen ratio sensors measured by the example program.
Section 2.
Section 2. Station Installation CR23X FIGURE 2.3. A Block Diagram for the Connections between the Datalogger, the BR Relay Driver and Components, and the External Battery.
Section 2. Station Installation 2.4 Battery Connections Two terminal strip adapters for the battery posts (P/N 4386) are provided with the 023A (Figure 2.4-1). These terminal strips will mount to the wing nut battery posts on most deep cycle lead acid batteries. FIGURE 2.4-1. Terminal Strip Adapters for Connections to Battery The SP20R solar panel, BR relay driver, and CR23X each have a separate power cable.
Section 2. Station Installation thermocouples. A camel-hair brush and tweezers can be used to clean the thermocouples. To turn the hygrometer and pump on, set flag 6 high. The thermocouples can also be dipped in a mild acid to dissolve spider webs. For example, muratic acid (hydrochloric acid) is available in most hardware stores. Rinse the thermocouples thoroughly with distilled water after dipping. 2.
Section 2. Station Installation If the LED is not already on, turn the potentiometer clockwise until it turns on and then counter clockwise until it goes off. Now, slowly turn the potentiometer clockwise until the LED comes on again. 7. Return the switch to its normal operating position. The LED will turn off several seconds after the switch is moved to the normal operating position. 8. Set flag 4 low to write the time that site maintenance ended.
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Section 3. Sample CR23X Program The example program is available on the Campbell Scientific FTP site, ftp://ftp.campbellsci.com/pub/outgoing/files/br_023a.exe. The example program measures the standard Bowen ratio inputs: vapor pressure and air temperature gradients, net radiation, and soil heat flux (flux at 8 cm and change in temperature of the soil layer above). If additional measurements are to be made or if a different installation is to be used, the program will have to be altered.
Section 3. Sample CR23X Program TABLE 3-1. Sample CR23X Bowen Ratio Program Flow Chart Table 1 1 Second Execution Interval Measure Panel Temperature Measure Lower Thermocouple (Single Ended) Measure Upper Thermocouple (Differential) Measure RTD on Cooled Mirror Subtract Upper TC Temp. from the Lower TC Temp.
Section 3. Sample CR23X Program Table 2 10 Second Excitation Interval 40 Second Interval ? Yes No Reset Flag 1 Flag 5 Set ? Yes No Flag 4 Reset ? Yes Call Subroutine 1 No 2 Minute Interval ? Yes No Set Flag 1 4 Minute Interval ? Yes No Set Port 2 High Set Port 1 High Set Flag 2 Reset Flag 2 Delay 0.
Section 3. Sample CR23X Program Subroutine 1 Output the time processing is re-enabled Reset Flag 5 (Re-enable Output) [output process] Day, Hour:Minute Subroutine 2 Turn the cooled mirror and pump on/off in response to a user flag or battery voltage Flag 6 Set ? Yes No Set Port 3 High (Turn on Pump and Mirror) Reset Flag 6 Flag 7 Set ? Yes No Set Port 4 High (Turn off Pump and Mirror) Reset Flag 7 Battery Volts < 11.
Section 3. Sample CR23X Program TABLE 3-2.
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Section 4. Calculating Fluxes Using SPLIT SPLIT (LoggerNet software) can be used to calculate the fluxes from the Bowen ratio measurements. This section describes these calculations on the data output from the example datalogger program. It requires two passes with SPLIT to compute the fluxes. The first pass operates on the raw data files generated by the datalogger. The definitions of points in this data is given in Table 3-2 which is the Output from the sample program.
Section 4. Calculating Fluxes Using SPLIT heat of the soil and the change in soil temperature, ∆Ts, over the output interval, t, are required to calculate the stored energy. The heat capacity of the soil is calculated by adding the specific heat of the dry soil to that of the soil water. The values used for specific heat of dry soil and water are on a mass basis.
Section 4. Calculating Fluxes Using SPLIT TABLE 4.2-1. Input Values for Flux Calculations VARIB. VALUE UNITS DESCRIPTION CP CW CS* EW** P* 1.01 4190.0 840.0 2450.0 87.18 kJ/(kg K) J/(kg K) J/(kg K) kJ/kg kPa T** D** BD* 1200 0.
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Appendix A. References Bowen, I. S., 1926: The ratio of heat losses by conduction and by evaporation from any water surface. Phys. Rev., 27, 779-787. Brutsaert, W., 1982: Evaporation into the Atmosphere. D. Reidel Publishing Co., 300 pp. Dyer, A. J., and W. O. Pruitt, 1962: Eddy flux measurements over a small irrigated area. J. Appl. Meteor., 1, 471-473. Fuchs, M. and C. B. Tanner, 1970: Error analysis of bowen ratios measured by differential psychrometer. Ag. Meteor., 7, 329-334. Gash, J. H. C.
Appendix A. References Wallace, J. M., and P. V. Hobbes, 1977: Atmospheric Science: An Introductory Survey. Academic Press, 350 pp.
Appendix B. 023 Bowen Ratio (Pre July 1993) FIGURE B-1.
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