Flatland96 Sensors Previous Chapter

3.0 Sensors


3.1 General


During FLATLAND96 three surface flux measurement sites were maintained by SSSF, Station 1, Station 2 and Station3. The sensors operated at each of the Stations are shown schematically in Figure 3.

Figure 3. Sensors at Stations 1, 2 and 3

The sensors operated at the Stations and are listed in Tables 7 and 8. Data from the sensors at Station 1 were acquired via the Flux-PAM system, EVE, and the ASTER system, ADAM. Data from the sensors at Stations 2 and 3 were acquired via the Flux-PAM system.

TABLE 7 Sensors operated at Station 1

------------------------------------------------------------------------------------------------------
           Sensors            Rate             Make                 Location              Height         
------------------------------------------------------------------------------------------------------
   3-Dimensional sonic ane     10    Applied Technologies         Sonic boom        3m above canopy *   
           mometer                        Inc model K                                                   
   Fast-response humidity      10   Campbell Scientific Kr         Sonic boom        3m above canopy *   
                                          hygrometer                                                    
    Band pass hygrometer       10             NCAR                Sonic boom        3m above canopy *   
  Wind speed and direction      1      NCAR / R.M. Young          PAM mast                10 m          
                                            propvane                                                    
  Platinum resistance ther      1      NCAR Hygro thermom       PAM crosspiece             2 m          
    mometer and humitter                      eter                                                      
      Pressure sensor           1       Vaisala PTB 220         Electronic box             1 m          
Surface infrared radiometer      1    Everest Interscience         PAM mast           10 m, 45 o down    
                                                                                         looking        
      Solar radiation           1       Licor LI 200 SA      Four leg radiation             3 m          
                                                                    stand                               
       Net radiometer           1           REBS Q7          Four leg radiation             3 m          
                                                                    stand                               
         Rain gauge             1             ETI                  Surface                1 m           
      Soil temperature          1            REBS                    Soil         surface - 5 cm slant   
    Soil heat flux plate        1            REBS                    Soil               5cm deep        
    Soil moisture probe         1       Campbell CS 615              Soil              2.5cm deep       
            Ozone               1    ThermoElectron Model      Intake at sonic      3m above canopy*    
                                               49                                                       
         Fast ozone            10    Gas phase chemilumine     Intake at sonic      3m above canopy*    
                                         scense sensor                                                  
         Fast ozone            10    Surface effect chemilu        Sonic boom        3m above canopy*    
                                       minescense sensor                                                
    The Flux-PAM boom was periodically raised to maintain a nominal height of 3 m above the canopy.     
------------------------------------------------------------------------------------------------------

TABLE 8 Sensors operated at Station 2 and 3

---------------------------------------------------------------------------------------------------
           Sensors            Rate            Make               Location              Height         
---------------------------------------------------------------------------------------------------
   3-Dimensional sonic ane     10      Applied Technolo        Sonic boom        3m above canopy *   
           mometer                     gies Inc model K                                              
    Band pass hygrometer       10            NCAR               Sonic boom       3m above canopy*    
  Wind speed and direction      1       NCAR propvane           PAM mast               10 m          
 Platinum resistance thermom     1      NCAR Hygro ther       PAM crosspiece             2 m          
      eter and humitter                   mometerr                                                   
      Pressure sensor           1      NCAR barometer       Electronics box             1 m          
Surface infrared radiometer      1   Everest Interscience         PAM mast        10 m 45 o downlook   
      Solar radiometer          1      Licor LI 200 SA       Four leg radia             3 m          
                                                               tion stand                            
       Net radiometer           1         REBS   Q7         Station 2:: Four             3 m          
                                                             leg radiation                           
                                                                  stand                              
                                                           Station 3: Tripod             2 m          
                                                             radiation stand                         
         Rain gauge             1            ETI                Surface                1 m           
      Soil temperature          1           REBS                  Soil         surface - 5 cm slant   
    Soil heat flux plate        1           REBS                  Soil               5cm deep        
    Soil moisture probe         1      Campbell CS 615            Soil              2.5cm deep       
  The Flux-PAM boom was periodically raised to maintain a nominal height of 3 m above the canopy.    
---------------------------------------------------------------------------------------------------

3.2 Sonic anemometer


Applied Technology Incorporated, ATI, model K sonic anemometers were used at all three station. The anemometers were mounted at the end of the sonic anemometer boom projecting 2 meter to the south-west.The height of the sonic boom was adjustable and several times during the program the booms were raised to maintain the sonic anemometers approximately three meters above the crop. The absolute azimuthal direction of each sonic anemometer was determined by theodolite both at the beginning of the program and after any change in the height of the boom was made. The sonic anemometers required considerable maintenance during the deployment. However there was practically complete operational status for the duration of the program. (For a complete record of the adjustments and maintenance see LOGBOOK: SONICS).

3.3 Fast Krypton hygrometer


A Campbell Scientific, model KH 20, Krypton hygrometer was mounted at station 1 on the sonic anemometer boom 30 cm behind the anemometer. The windows of the hygrometer were cleaned several times to improve the strength of the signal.

3.4 Band pass hygrometer


The BandPass hygrothermometer, BPH, is a temperature/humidity instrument providing data used in the calculation of humidity fluxes. The sensing transducer is a 50Y Vaisala humitter. The sensor is mounted in a force ventilated radiation shield. The instrument is mounted on the sonic anemometer boom with the inlet tube pointing toward the sonic transducer array. The raw voltages are collected by the A/D system of the sonic anemometer where they are digitized and appended to the sonic serial message. From the initial start of operations until June 20th, the calibration coefficients for the BPH sensors were incorrect. The correct coefficients were loaded on the 21st. From June 21st to July 1st the temperature response at station 3 was questionable. Over this time period a number of different tests were done (refer to logbook entries). On July 1st the fan was replaced and the temperature response looked ok. On july 19th the humidity transducer was cleaned with distilled water at site 1. On Aug 19th the humidity transducer was cleaned at site 3. At the end of the project the BPH sensors were mounted next to its corresponding hygrothermometer for a 24 hour intercomparison check. The results of those tests can be found in logbook entry 495.

3.5 Fast O3 sensor, Kokod.


The "Kokod" fast ozone sensor is a gas-phase, nitric oxide/ozone chemiluminescence detector. It is a custom-made device normally operated aboard research aircraft. This system was custom designed and fabricated by Dr. Greg Kok of the Research Aviation Facility of ATD. (For details concerning the characteristics of the system contact Dr. Greg Kok). For this application the device was housed in an air-conditioned instrument shelter near the base of the Flux-PAMIII tripod, with the associated vacuum pump and nitric oxide gas cylinders located near-bye. The 1/4" teflon intake tube for the sensor stretched up the mast and along the sonic anemometer boom to the immediate vicinity of the sonic anemometer. Dry ice was used to maintain the photomultiplier at a low temperature. Every day the system was monitored, the nitric oxide pressures noted and the dry ice reservoir resupplied. The dangerously toxic nature of nitric oxide was impressed upon the site operators and a "safety first" procedure was observed. The system was installed during late July and became operational on 24 July. For the first week the system was only operated during the daytime but after 29 July it was decided to operate the system continuously. In early August it was noted that the air-conditioner cycling was introducing a spurious periodicity to the output signal. On 6 Aug, the air conditioner compressor was switched off, leaving only the fan operating. The shelter was closed each evening and opened each morning. This regimen resulted in diurnal temperature fluctuations but eliminated the more troublesome short term temperature fluctuations. The system was operated until the end of the deployment

3.6 Fast O3 sensor, Seclod


The "Seclod" fast ozone sensor is a surface effect, Cumarin on silica gel, chemiluminescence detector. The device fielded during FLATLAND96 was a prototype dropsonde sensor developed at the Deutsches forschungsanstadt fur Luft- und Raumfahrt, DLR. The prototype was modified to utilize a 16 VDC power supply, rather than use batteries and an amplifier added to enable the output signal to be cabled 15 meters to the ADAM. The sensor was housed in a 3" (7.6 cm) diameter, 8" (20 cm) long PVC pipe with the intake projecting 10 cm. This cylinder was mounted to the boom, 70 cm behind the sonic anemometer. The system was installed on 2 Aug and became operational on 3 Aug. It was attended on 9 Aug to fix a light leak. The device was removed from the boom on 12 Aug to change the high voltage setting for the photomultiplier, was reinstalled on the 14 Aug and remained operating until the termination of the deployment.

3.7 Slow O3 sensor, Tecod.


The ThermoElectron Model 49, "Tecod", is an ultraviolet photometric detector which measures ozone mixing ratio using basic Beer's law principals. It is a slow, but intrinsically accurate sensor with a response time of 10 seconds. The detector was housed in an air-conditioned instrument shelter together with the fast gas phase chemiluminescence detector, Kokod. The 10 meter length of 1/4" teflon intake tube for the sensor stretched up to draw sample air from the same location as the fast gas phase chemiluminescence detector. The detector was installed on 24 Aug, at the same time as the fast sensor and was operated until the termination of the deployment. The Tecod output twice exhibited periods of wild oscillation when the data was unusable. The second more severe episode, on 4 August, required a power cycle to cure. These episodes were believed to be due to the thermal gradient induced in the instrument by the cycling of the air conditioner. They did not recur after the air conditioner had been switched off.

3.8 Prop-vanes


At each station a Flux-PAM prototype propeller-vane anemometer, mounted on a horizontal boom, was installed on the PAM mast at 10 meters. The directional alignment of the sensor to the boom was done prior to erection of the mast and after erection a theodolite was used to determine the azimuthal direction of the boom. The readings from station 1 became suspect in late July and on 1 August the PAMIII prototype was replaced with a new RM Young design.

3.9 Hygrothermometers (Steve S)


Temperature/humidity measurements were made with a NCAR hygrothermometer. The instrument consists of a force-ventilated radiation shield housing a Vaisala 50Y temperature/humidity probe. The 50Y probe is interfaced to a dedicated micro-controller where the analog signals are digitized and converted to engineering units. Data were transmitted at a 2 second rate to the EVE data collection platform. Each station was equipped with a hygrothermometer at the beginning of the project. On xxx, the fan at station xx failed and was replaced. On xxx the temperature response at station xx was questioned. The 50Y probe was replaced on xxx. During the operational period intercomparison tests were conducted using an Assman psychrometer. At the end of the project the BandPass hygrothermometer was mounted at the same height as the hygrothermometer for a sensor to sensor intercomparison.

3.10 Barometers


Vaisala PTB 220 barometers were deployed at each Flux-PAM, contained within the electronics box. at a nominal height of 1 meter. A tygon tube connected the sensor to a single disc pressure port projecting downward from the end of the PAM crosspiece.

3.11 Net radiometers


During the FLATLAND96 program Radiation Energy Balance Systems, model Q7 net radiometers were employed. For Stations 1 and 2, the corn sites the net radiometers were mounted at the mid point of the top beam of a four leg radiation stand. For station 3, the soybean site, the sensor was mounted on a tripod radiation stand. At all three stations the net radiometers projected to the south, At the commencement of the program all radiometers were cleaned and levelled. Periodically the levels were checked and the domes cleaned. Bird droppings and spots of mud were noted. Care was taken to avoid disturbance of the vegetation under the net radiometers. The difference in the vegetation was quite apparent from inspection of the data from the different site. It was noted that the area immediately under the net radiometer at station 3 there was a large proportion of grass compared to soybean.

3.12 Solar radiometers


Licor LI 200 SA pyranometers were deployed at all three stations to measure the incoming solar radiation. The radiometers were mounted in association with the net radiometers and utilized the same levelling system. The Licor pyranometers also required periodic cleaning as they attracted birds. The radiometer at station 1 exhibited a persistent nighttime negative offset. On 19 July the problem was rectified by a modification to the polarity of the DC supply.

3.13 Surface temperature sensors


The Everest infrared temperature sensor was mounted on the cross-arm at the top of the 10 m mast.They were angled to view to the northwest, downward at 45o. This ensured a a comparable view for each station, neither down the N-S nor the E-W furrows. With a viewing angle of 15o the footprint was approximately 2.5 m wide and 7.7 m long with an area of 2.6 m2. The radiometer at station 1 gave trouble at the beginning of the program

3.14 Rain gauges


Environmental Technology, Inc, total precipitation gauges were used at all three stations. The ETI NOAH II gauges were equipped with wind screens and at stations 1 and 2 the growing corn was kept clear of the top of the gauges. On 29 June a problem of serious under-reporting of rain rate was rectified by replacing the eproms in the Campbell dataloggers.

3.15 Soil thermometers


At each of the three sites Radiation Energy Balance Systems, Platinum Resistance Devices were used to determine soil temperature. The 6" (15 cm) probes were inserted into the soil, at a slant, to measure the average temperature of the soil from the surface to the depth of 5 cm. The first portion of the connecting cable was also buried to prevent heat conduction along its length from affecting the measurement. The alternating patterns of sun and shade moving across the surface above the probes gave rise to considerable variation in the reported temperature.

3.16 Soil heat flux plates


At each of the three sites Radiation Energy Balance Systems, heat flux plates were used to determine sub-surface soil heat fluxes. The thin (0.5 cm), square (2.5 cm x 2.5 cm) plates were buried in the soil at a depth of 5 cm. Again, as for the soil temperature sensors, the alternating patterns of sun and shade produced readily apparent effects in the reported data.

3.17 Soil moisture probes


Campbell CS615 Water Content Reflectometers were used to provide continuous values of soil moisture at the three stations. The CS615 is a time-domain measurement device which determines the dielectric constant of soil by measuring the propagation velocity of electromagnetic waves along embedded wave guides. The dielectric constant of soil is predominately dependent upon its water content. Two parallel stainless steel rods serve as the wave guides. The system is somewhat sensitive to the nature of the soil and must be calibrated by periodic gravimetric measurements of soil moisture. There is also a temperature effect which is imposed upon the measurement signal. For the FLATLAND96 deployment the probes were inserted horizontally into the soil at a depth of 2.5 cm. This configuration provided water content information for the top 5 cm of soil.

3.18 Soil samples


Periodically throughout the program soil samples were taken at each of the three sites. Triplicate sample of ~ 150 cm3 of soil, from surface to 5 cm depth, were extracted and placed in Tupperware boxes. Immediately on return to the Base each of the fresh samples was homogenized. Aliquots were weighed and placed in a drying oven at 110oC. After 24 hours of baking the aliquots were reweighed. The average of three samples is reported as the gravimetric soil moisture.

The dry bulk density of the soil was determined for each of the three site. Blocks of soil were carefully excavated from the top 8 cm. These were dried in the oven for several days and then, using the flat blade, shaped to form regular rectangular prisms. These blocks were weighed, and then, using a heat gun, were impregnated with melted Parawax rendering them water impermeable. The volume of the prisms was then determined by displacement by immersing the waterproof soil blocks into a graduated beaker partially filled with water.

Next Chapter