Applications of the LI-7820

In this chapter we provide information about using the instrument in field or laboratory applications. We cover connecting plumbing to the air inlet, retrieving data, and other applications.

Flow schematic

During normal operation, air is drawn into the analyzer through the air inlet (Figure 3‑1). Air flows through the optical bench and phase adjuster and is exhausted through the air outlet. In the optical bench, the pressure is drawn down to about 40 kPa. When the instrument is powering down, air is circulated in a closed loop through the desiccant to ensure that air remaining in the system is free of moisture, preventing condensation when the instrument is powered off.

Figure 3‑1. Simplified flow schematic of the gas analyzer. During a measurement, (left) air flows through the optical bench. During shutdown (right), the instrument purges the optical path of water vapor.

Soil gas flux measurements

The LI-7820 works well for soil gas flux measurements when coupled with the LI-8250 Multiplexer and the Smart Chamber. All hardware, cables, and tubing required for the connection is available for purchase. The digital communication connection is established through the LI-8250 or Smart Chamber web interface. Full details are provided in the LI-8250 and Smart Chamber instruction manuals.

Connecting the air inlet and outlet

Air is drawn into the sample cell through the air inlet. Connect tubing to the inlet with the nut, bushing, and ferrule from the accessories kit. Optional stainless steel inserts (300-18126) are included for use with soft tubing. Tubing can be connected to the inlet and the outlet in the same way.

Caution: Be careful when working near standing water. If water is drawn into the air inlet, the instrument will have to be repaired at the factory.

Figure 3‑2. When connecting a tube to the air inlet, insert the tube through the nut, bushing, and ferrule. If the tubing is soft, place a stainless steel insert (300-18126) in the tubing to make it more rigid.

To connect a ¼" outside-diameter metal or plastic tube to the compression fitting, insert the nut, bushing, and ferrule over the tube. Then tighten the nut over the ferrule until it is finger tight. Tighten it an additional 1-¼ revolutions if you are connecting the tube for the first time. For highly pliable plastic tubing, place a stainless steel tube insert (300-18126) inside the tubing to make it rigid enough.

When reconnecting a plastic or metal tube that has been connected previously, simply tighten it ¼ turn beyond finger tight.

Important: Abrupt pressure transients up to 35 kPa above or below ambient pressure may cause momentary status warnings, inlet plugged warnings, and measurement inaccuracies. Longer lasting pressure excursions or larger pressure transients may cause the instrument to reinitialize measurement control loops.

Plumbing a subsample

Note: We are currently refining numerous sampling protocols for the LI-7820. Contact LI-COR technical support (envsupport@licor.com) if you have questions about your application.

A typical sampling application will use the LI-7820 to subsample a gas from a main sample line. Air passes a single time through the gas analyzer before it is discharged. Air supplied to the sample inlet should be between ambient and 35 kPa (5 PSI) above ambient. If the pressure is higher than that, install a 30 cm vent in the sample line to ensure that air from the external pump does not pressurize the analyzer. The vent must be sufficiently long to prevent diffusion of ambient air into the sample air supply.

Figure 3‑3. A basic open system, where air is supplied to the gas analyzer by an external pump. If the pressure exceeds 35 kPa above ambient, install a 30 cm vent in the air supply line. Attach the inlet to the 'leg' of the T-fitting to avoid isotopic fractioning.

Configuration options

The instrument supports two operating configurations that can be set in the interface: standard and high altitude.

Standard

The standard configuration is the default - it will be loaded automatically unless the configuration has been changed under Options > Settings > Software > Configuration.

High altitude

The high altitude configuration does not require any hardware modifications. The reduced flow rate hardware modifications cannot be used while the instrument is in the high altitude configuration; if the kit has been installed, remove it before using the high altitude configuration. To use the high altitude configuration, select it under Options > Settings > Software > Configuration and restart the instrument. Be aware of slight differences in the performance specifications of the instrument while in the high altitude configuration.

See Support: LI-7820 N2O/H2O Trace Gas Analyzer for details.

Installing the reduced flow rate kit

Two reduced flow rate kits have been produced for LI-COR Trace Gas Analyzers. Both kits enable the analyzer to operate at a nominal flow rate near 70 sccm, rather than the standard nominal flow rate near 250 sccm.

Part number 7800-112 is older, consisting of a blue Balston filter, vent, barbed unions, and two orifices. Part number 7800-113 is current. It consists of two quick-connect unions and a single orifice. The older kit is not compatible with closed-loop applications because the inlet and outlet flow rate are different. The current kit is compatible with closed-loop applications.

These instructions describe how to install 7800-113.

The reduced flow rate kit is supported by instrument firmware version 2.0.25 and newer. Firmware version 2.4.47 eliminated the need to select an option in the interface by automatically detecting the kit on startup. If you are running firmware older than 2.4.47, the low flow configuration must to be selected in the interface and the instrument rebooted. For firmware, go to licor.com/78xx-support, select an instrument, and then click Software Downloads. One updater can be used to update all Trace Gas Analyzer models. To install the kit:

  1. Power off the instrument. 
  2. Identify the tube that connects the pump to the optical bench.
  4. Observe the orientation of the T-fitting. In early models, this tube comes off of the stem of the T. In later models, it comes off of the cross of the T. In either case, the tube connects the T-fitting to the pump.
  5. Cut the tube where indicated.
  7. Observe the flow direction indicator on the assembly and install it between the two cut ends with the arrow pointing from the optical bench to the pump.
  8. For each quick connect, insert the tube and push firmly until it stops.

  11. Options > Settings > Software

    Reduced Flow Rate

To restore the normal flow rate, remove the reduced flow rate kit, connect the cut tubes with quick-connect fitting from the kit, and restart the instrument.

Retrieving data from the instrument

The instrument records all of its data after it has warmed up. There is no way to turn data logging on or off because data are always logged. Typically, the instrument can store a total of 5,356,800 records, which is about 62 days of continuous data. Old records are overwritten when the memory is full. Therefore, we recommend that you routinely download important data from the instrument. When used with the Smart Chamber or LI-8250 Multiplexer, measured values are transferred automatically to the Smart Chamber or LI-8250.

The instrument also supports two protocols for transferring data:

Downloading a data file

To retrieve data from the instrument, click Options > Export. Specify a date range and time period. Dates are displayed as YYYY-MM-DD. Time options are given in a 24-hour clock (00:00 through 24:00). Click Export. The web browser will prompt you to save or open the file, and then provide a text file with the requested data. The file has a .data extension. Measurements are recorded as tab-delimited text that can be opened in a text editor or spreadsheet application.

Click to export data.Data can be dowloaded for date range.
Figure 3‑4. To download data, click Export from the settings menu, select a date and time range, and click Export.

Components of the data file

The text file will include a file header, data header, and data.

File header

The file header provides information about the instrument that measured the data.

Header Label Description
Model The model of the instrument
SN The serial number of the instrument
Software Version The software version on the instrument
Timestamp The date and time of the beginning of the requested data (according to the instrument clock; yyyy-mm-dd hh:mm:ss).
Timezone The timezone setting on the instrument when the data is requested.

Data header

The data header identifies the columns of values that are in the file. You'll see two rows: one called DATAH, which gives the variable names for the corresponding columns, and one called DATAU, which gives the units for the corresponding columns.

DATAH DATAU Description
SECONDS secs Seconds past the universal epoch (Unix time).
NANOSECONDS nsecs Nanoseconds of the seconds
NDX index A count of scans. At four scans per second, the value increases by four counts per second.
DIAG diag Diagnostic code (see Status codes)
REMARK - The remark entered in the Remark field
DATE date Date of the record in yyyy-mm-dd
TIME time Time of the record in HH:MM:SS (according to the instrument clock)
H2O ppm Water vapor concentration
N2O ppb N2O mole fraction in dry air
CAVITY_P kPa Optical cavity pressure (typically near 39)
CAVITY_T °C Optical cavity temperature (typically near 55)
LASER_PHASE_P kPa Laser phase pressure
LASER_T °C Laser temperature
RESIDUAL n/a Difference between raw and best fit spectra
RING_DOWN_TIME µsecs Indicator of cavity resonance
THERMAL_ENCLOSURE_T °C Optical enclosure temperature
PHASE_ERROR counts Dimensionless indicator of mode lock state
LASER_T_SHIFT °C Shift in laser center wavelength from factory calibration
INPUT_VOLTAGE V Power supply voltage
CHK CHK Checksum; to ensure that the receiving software received the data without error, and to reject corrupted data lines

Time, data , and diagnostics

The time, data, diagnostics, and remark are given under the header.

The relationship between Unix Epoch time and the time stamp

The instrument measures time based upon the number of seconds past the Unix epoch (GMT: Thursday, January 1, 1970 12:00:00 AM). This value is represented in the Seconds column of the data set. You can easily convert the Unix epoch to date and time using online resources (e.g., https://www.epochconverter.com). If you have selected a time zone, the Date and Time columns will represent the Unix epoch time adjusted by an offset for the time zone.

Installing the instrument in a rack mount

The LI-7820 can be mounted in an instrument rack with a stationary or sliding shelf with dimensions of at least 33 cm by 61 cm (13" by 24") with 18 cm (7") of vertical clearance. The instrument dimensions are given in Figure C‑1. Be sure that the vents on the sides are not obstructed. Allow 2 to 3 cm (~1 inch) of clearance on either side of the vents. Ensure that air can flow freely around the case.