Variables, data, and diagnostics
Normally, the LI-720 reports flux results every 30 minutes with averages of biological and meteorological variables. It can be configured to provide the results in as short as five minutes or as long as one hour in five minute increments. In addition, high-speed raw data can be recorded at 10 Hz.
Flux results are reported to LI-COR Cloud via an IoE Module, stored in the IoE Module SD card, or logged to an external data logger. When enabled, raw data (10 Hz) are continually output over the RS-232 serial wires, and can be recorded to the IoE Module SD card or an external data logger.
Time stamp
When used with an IoE Module, the LI-720 does not use its internal clock. Instead, it enters an external trigger mode upon recieving the first trigger from the IoE Module via SDI-12. In this mode, the IoE Module controls the timing of flux intervals. The IoE Module clock is automatically aligned with time stamps from GPS satellites.
When not connected to an IoE Module or data logger that can trigger a measurement period, the LI-720 disciplines its internal clock to the GPS time stamp from its own GPS receiver. If the LI-720 recieves one trigger but no follow up trigger, it will self-trigger a new measurement period after 36,000 samples have been recorded (1 hour of raw data). This is a fallback; a properly configured data logger will issue the trigger command on a schedule of 5 to 60 minutes (30 minutes recommended).
All time stamps are logged in UTC with no offset. You can apply an offset for the local time zone manually on the IoE module or you can choose a time zone when you download data from LI-COR Cloud. The offset applied to the time stamp is indicated in the data file.
Note: When setting a time zone on the IoE Module, use Standard Time. Do not apply an offset for Daylight Saving time to prevent artificial gaps and overlaps in the time series.
When data from a sensor, such as LI-710 or LI-720, are requested by the IoE Module, data are delivered with time stamp from the IoE Module. Those results are delivered to LI-COR Cloud with the time data intact. LI-COR Cloud will read the time that is included with sensor data, and it uses that information to construct the time series for displayed parameters. When you download data from LI-COR Cloud, you can choose a time zone to be applied to the data. The time stamp will be adjusted by adding or subtracting hours. The offset is indicated in the time field (+0000 is UTC, -0006 is US Central Time, and so on).
Raw data
Raw data are provided at 10 Hz. You can view the data in the CarbonWare app or record it on the IoE Module or an external data logger.
Table 6‑1. Variables, definitions, and units for raw data.
| IDX
|
Index - a count at 10 Hz |
# |
| U
|
Horizontal wind speed |
m s-1 |
| V
|
Horizontal wind speed |
m s-1 |
| W
|
Vertical wind speed |
m s-1 |
| T_SONIC
|
Air temperature computed from the vertical anemometer. |
°C |
| T_SONICH
|
Air temperature computed from the horizontal anemometer. |
°C |
| SOS
|
Speed of sound computed from the vertical anemometer. |
m s-1 |
| TA
|
Air temperature measured by the air temperature sensor. |
°C |
| RH
|
Relative humidity measured by the RH sensor |
% |
| PA
|
Air pressure |
kPa |
| V_r0
|
|
|
| V_w0
|
Detector voltage for water reference band |
Volts |
| V_c0
|
Detector voltage for CO2 reference band |
Volts |
| V_ref
|
|
Volts |
| V_h2o
|
Detector voltage for H2O absorption band |
Volts |
| V_co2
|
Detector voltage for CO2 absorption band |
Volts |
| V_par
|
Quantum sensor output voltage |
Volts |
| V_det
|
Thermistor output voltage |
Volts |
| T_det
|
Optical detector temperature. |
°C |
| T_src
|
Optical source temperature. |
°C |
| RH_src
|
Relative humidity inside the upper gas analyzer housing. |
% |
| VTX
|
|
|
| PITCH
|
Offset from vertical front-to-back |
° |
| ROLL
|
Offset from vertical side-to-side |
° |
| HEADING
|
Offset from magnetic north |
° |
| Status
|
Performance indicator |
# |
| SEQ
|
|
# |
| VIN
|
Power supply voltage |
# |
| RSSI
|
Residual signal strength indicator |
# |
| PPFD
|
Photosynthetic photon flux density |
µmol m-2 s-1 |
| H2O_ABS
|
Optical absorption by H2O |
# |
| CO2_ABS
|
Optical absorption by CO2 |
# |
| H2O
|
Water vapor mol fraction |
mmol mol-1 |
| CO2
|
Carbon dioxide mol fraction |
mmol mol-1 |
| CO2_MOLAR_DENSITY
|
Carbon dioxide molar density |
mmol m-3 |
| H2O_MOLAR_DENSITY
|
Water vapor molar density |
mmol m-3 |
Computed results
Computed results are fluxes over the thirty-minute period and averages of biological and meteorological parameters over the same period. Computed results can be logged to an external data logger (SDI-12), LI-COR Cloud via an IoE Module, or an IoE Module that is not connected to LI-COR Cloud. The parameters delivered are the same.
When retrieved from LI-COR Cloud, the file will be presented as text or a MS Excel spreadsheet. The file name is the date of the dataset. The time stamp is in UTC. If a time zone offset it applied, it is indicated in the data. After extracting the compressed data, you'll see one file for each component that contributed data. You may have one file from the LI-720 and one from the IoE Module. Additional files may be present if other devices are connected to the IoE Module.
Table 6‑2. Parameters reported by the LI-720 via the SDI-12 protocol. All parameters are reported to LI-COR Cloud automatically.
| FCO2
|
CO2 Flux |
umol m-2 s-1 |
3 |
0 |
| H2O_FLUX
|
H2O Flux |
mmol m-2 s-1 |
3 |
0 |
| ET
|
Evapotranspiration |
mm |
3 |
0 |
| LE
|
Latent heat flux |
W m-2 |
3 |
0 |
| H
|
Sensible heat flux |
W m-2 |
3 |
0 |
| TAU
|
Momentum flux |
kg m-1 s-2 |
3 |
0 |
| SAMP_CNT
|
Sample count |
# |
0 |
0 |
| SEQ
|
Sequence number |
# |
0 |
0 |
| WD
|
Wind direction |
degrees |
2 |
1 |
| WS
|
Windspeed |
m s-1 |
3 |
1 |
| OFFSET
|
Offset from magnetic north |
degrees |
2 |
1 |
| U_SIGMA
|
Standard deviation of U |
m s-1 |
3 |
1 |
| V_SIGMA
|
Standard deviation of V |
m s-1 |
3 |
1 |
| W_SIGMA
|
Standard deviation of W |
m s-1 |
3 |
1 |
| T_SONIC_SIGMA
|
Standard deviation of sonic temperature |
C |
2 |
1 |
| TILT
|
Sensor tilt |
degrees |
0 |
2 |
| FLUX_SSITC_TEST
|
8 is a placeholder, then QC parameters tau, WTS (QC Flag for heat flux), WC (QC Flag for CO2 flux), and WH (QC Flag for H2O Flux) 0,1,2 or 9. 9 is missing |
# |
0 |
2 |
| USTAR
|
Friction velocity |
m s-1 |
3 |
2 |
| MO_LENGTH
|
Monin-Obukhov length |
m |
3 |
2 |
| T_SONIC
|
Sonic temperature |
C |
3 |
2 |
| TA_SONIC
|
Sonic temperature corrected for humidity |
C |
3 |
2 |
| CO2
|
CO2 mole fraction |
umol mol-1 |
3 |
2 |
| H2O
|
H2O mole fraction |
mmol mol-1 |
3 |
2 |
| AH
|
Absolute humidity |
g m-3 |
2 |
3 |
| RH
|
Relative humidity |
% |
2 |
3 |
| SVP
|
Saturated vapor pressure |
kPa |
2 |
3 |
| VPD
|
Vapor pressure deficit |
kPa |
2 |
3 |
| PA
|
Atmospheric pressure |
kPa |
2 |
3 |
| TA
|
Air temperature (not sonic) |
C |
2 |
3 |
| TD
|
Dewpoint temperature |
C |
2 |
3 |
| PPFD
|
Photosynthetically active radiation |
umol m-2 s-1 |
1 |
3 |
| INST_PWR
|
Instrument Power |
W |
2 |
4 |
| INPUT_V
|
Input Voltage |
V |
2 |
4 |
| DIAG
|
Diagnostic Value |
# |
0 |
4 |
| RSSI
|
Signal Strength |
# |
0 |
4 |
| U_RECORDS
|
Number of Valid U Records |
# |
0 |
5 |
| V_RECORDS
|
Number of Valid V Records |
# |
0 |
5 |
| W_RECORDS
|
Number of Valid W Records |
# |
0 |
5 |
| TSONIC_RECORDS
|
Number of Valid TSONIC Records |
# |
0 |
5 |
| CO2_RECORDS
|
Number of Valid CO2 Records |
# |
0 |
5 |
| H2O_RECORDS
|
Number of Valid H2O Records |
# |
0 |
5 |
| TA_RECORDS
|
Number of Valid TA Records |
# |
0 |
5 |
| PA_RECORDS
|
Number of Valid PA Records |
# |
0 |
5 |
| RH_RECORDS
|
Number of Valid RH Records |
# |
0 |
5 |
| PAR_RECORDS
|
Number of Valid PAR Records |
# |
0 |
5 |
| SITE_ALTITUDE
|
Altitude at the site |
m |
2 |
6 |
| SITE_LATITUDE
|
Latitude at the site |
degrees |
5 |
6 |
| SITE_ZGEOM
|
Sensor height above ground |
m |
2 |
6 |
| SITE_ZCANOPY
|
Canopy height |
m |
2 |
6 |
| SITE_NOFFSET
|
User or default north offset |
degrees |
1 |
6 |
