Driver_-_Li710

SDI-12 interface driver for the LiCor Li-710 Evapotranspiration sensor

Error Response

Device has two potential error responses (in addition to general SDI-12 and sensor errors)

LI710_ERROR -> 0xA0010000

LI710_WARNING -> 0xF01A0000

The sensor errors (described in detail in manual) are split between warnings are errors and the bit position of the given diagnostic code is reported as the error code sub-type

Example:

Error Code -> 0xA0010712

Indicates an error code of 'Poor sonic signal' on sensor connected to Talon 1, Port 2

Below is a listing of fault descriptions, type, and corresponding codes

Bit Position	Description	Type	Error Code
0	Flow out of range	Error	0xA00100tp
3	Low RH sensor voltage	Error	0xA00103tp
4	Cell temperature out of range	Error	0xA00104tp
6	No sonic	Error	0xA00106tp
7	Poor sonic signal	Error	0xA00107tp
8	Rain detected	Warning	0xF01A08tp
9	High humidity shutdown	Warning	0xF01A09tp
10	Cold temperature shutdown	Warning	0xF01A0Atp
11	High cell pressure relative to ambient	Error	0xA0010Btp
12	Low cell pressure relative to ambient	Error	0xA0010Ctp
14	Low pump voltage	Error	0xA0010Etp
15	High pump voltage	Error	0xA0010Ftp

Note 1: t and p indicate insertion of Talon value and Port value respectively

Note 2: Bit positions not listed are considered 'reserved' by the system and no description is given - they will however be passed on as Error type if they are reported by sensor

In general a 'Warning' is a state which will be self corrected, or at least has no ability for intervention (too cold, too humid, etc). Conversely 'Error' type codes generally require intervention -- See Li-710 Manual (Section 9, 10) for more details.

Interface Notes

Interface Errata

Sensor Invalid Response

Depending on the specific sensor reporting, an out of range value can be reported at -9999 or 9999999 - this does not appear in the documentation but is observed in testing

Sensor Not Ready Response

When the sensor first starts up and a trigger (aXT!) command has not been issued, the sensor responds with ill defined values in the Diagnostic (DIAG) field.

Example response:

Power to Sensor

aR0!->a-9999-9999-9999-9999-9999-9999-9999-9999-99

The system uses a -9999 value to indicate a null value, while this is not specified in the sensor manual or in the SDI-12 specification, it is generally accepted in the community.

However, the sensor also responds with a -99 for the Diagnostic Value, which is not specified in the list of potential diagnostic codes - take note of this for parsing and decoding responses!

Send Data Idiosyncrasy

Generally one would get sensor data using Start Measurement (aM! or aM0! thru aM9!) followed by Send Data (aD0! thru aD9!), however for the Li710 (as of sensor firmware version 1.0) this does not work as expected.

Instead of sending the entire group results with each Send Data call, the sensor instead only sends 3 at a time in the following way.

After sending an aXT! command to update data, the sensor responds in the following way to Start Measurement and Send Data commands

Start Measurement Commands - aMx!

aXT!->0READY

aM0!->00119

delay 11 seconds

aD0!->a+ET+LE+H

aD1!->a+VPD+PA+TA

aD2!->a+RH+SEQ+DIAG

While this does not violate SDI-12 specifications¹, it is inconsistent when compared with using the Continuous Measurement commands, despite indication from the SDI-12 specification² that they would be formatted in the same manor, and as a result could lead to confusion.

Continuous Measurement Commands - aRx!

aXT!->0READY

aR0!->a+ET+LE+H+VPD+PA+TA+RH+SEQ+DIAG

Note: At this point the Send Data command can be used to retrieve this information as well

aD0!->a+ET+LE+H+VPD+PA+TA+RH+SEQ+DIAG

This idiosyncrasy seems to arise from a maximum length discrepancy in the SDI-12 specification³ regarding length of data returns from various sources. The total length of a single group exceeds this 35 character limit Given this, it seems that either using Continuous Measurement (aR0! thru aR9!) commands or using Concurrent Measurement (aC0! thru aC9!) commands.

Concurrent Measurement Commands - aCx!

aXT!->0READY

aC0!->001109

delay 11 seconds

aD0!->a+ET+LE+H+VPD+PA+TA+RH+SEQ+DIAG

Additional Documentation

Sensor Responses to Metadata Calls (not in manual)

aIM_001->a,ET,mm,Actual Evapotranspiration (ET);

aIM_002->a,LE,W+1m-2,Latent Energy flux (LE);

aIM_003->a,H,W+1m-2,Heat Flux (H);

aIM_004->a,VPD,kPa,Vapor Pressure Deficit (VPD);

aIM_005->a,PA,kPa,Atmospheric Pressure (P);

aIM_006->a,TA,C,Air Temperature (T);

aIM_007->a,RH,percent,Relative Humidity ambient (RH);

aIM_008->a,SEQ,#,Sequence Number;

aIM_009->a,DIAG,#,Diagnostic Value;

...

aIM1_001->a,ET,mm,Actual Evapotranspiration (ET);

aIM1_002->a,LE,W+1m-2,Latent Energy flux (LE);

aIM1_003->a,H,W+1m-2,Heat Flux (H);

aIM1_004->a,PA,kPa,Atmospheric Pressure (P);

aIM1_005->a,TA,C,Air Temperature (T);

aIM1_006->a,RH,percent,Relative Humidity ambient (RH);

aIM1_007->a,SEQ,#,Sequence Number;

aIM1_008->a,SAMP_CNT,#,Sample Count;

aIM1_009->a,DIAG,#,Diagnostic Value;

...

aIM2_001->a,AH,g+1m-3,Absolute Humidity ambient;

aIM2_002->a,RH,percent,Relative Humidity ambient (RH);

aIM2_003->a,SVP,kPa,Saturated Vapor Pressure ambient;

aIM2_004->a,VPD,kPa,Vapor Pressure Deficit (VPD);

aIM2_005->a,PA,kPa,Atmospheric Pressure (P);

aIM2_006->a,TA,C,Air Temperature (T);

aIM2_007->a,TD,C,Dewpoint Temperature;

aIM2_008->a,TILT,degrees,Inclinometer Tilt;

...

aIM3_001->a,PUMP_V,V,Pump Voltage;

aIM3_002->a,PA_CELL,kPa,Cell Pressure;

aIM3_003->a,RH_CELL,percent,Cell Relative Humidity;

aIM3_004->a,TA_CELL,C,Cell Temperature;

aIM3_005->a,RH_ENCL,percent,Enclosure RH;

aIM3_006->a,FLOW,sccm,Flow;

aIM3_007->a,INPUT_V,V,Input Voltage;

aIM3_008->a,DATA_QC,#,Data QC;

Footnotes

1. "If the expected number of measurements is not returned in response to a D0! command, the data recorder issues D1!, D2!, etc. until all measurement values are received (The expected number of measurements is given in the response to an M, C or V command.)" - SDI-12 Interface Standard v1.4, Page 15 ↩

2. "Continuous Measurements ... (formatted like the D commands)" - SDI-12 Interface Standard v1.4, Page 8 ↩

3. "The maximum number of characters that can be returned in the <values> part of the response to a D command is either 35 or 75. If the D command is issues to retrieve data in response to a concurrent command, or a high volume ASCII command, the maximum is 75. Otherwise, the maximum is 35." - SDI-12 Interface Standard v1.4, Page 16 ↩