Using analyzers from other manufacturers

Although the LI-8250 Multiplexer is optimized for use with LI-COR gas analyzers, it can be used with gas analyzers from other manufacturers.

In this configuration, the LI-8250 Multiplexer logs the timestamp, soil temperature, soil moisture, location, and other data. Gas measurements and accurate time stamps will be logged elsewhere - typically by the gas analyzer. The datasets can then be merged using SoilFluxPro Software to calculate fluxes.

Connecting the analyzer to the LI-8250

Two conditions must be satisfied to use the LI-8250 Multiplexer with a gas analyzer from another manufacturer:

  1. The analyzer must be able to query the LI-8250 Multiplexer network time protocol (NTP) server for timestamp synchronization.
  2. Exhaust flow from the analyzer must not exceed 5 slpm to be compatible with the LI-8250 Multiplexer pneumatics. ≤ 2 slpm is recommended.
  3. The timestamp and file structure from the gas analyzer must be supported by SoilFluxPro Software (supported file types include .csv, .txt, .json, and .81x).

Currently, these features are supported by several models from Picarro, LGR (ABB - Los Gatos Research), Gasmet, and Aerodyne.

If you intend to use the LI-8250 Multiplexer with a gas analyzer from another manufacturer, an optional 2-meter cable assembly with two sealed RJ-45 Ethernet connectors and Bev-a-Line® tubing (part number 9982-011) is available. The sealed RJ-45 connectors will not connect to a standard Ethernet port without an adapter (part number 309-17612). This adapter includes one sealed and one unsealed RJ-45 connector.

If you choose to make your own air tubing, 5-meter (part number 392-19109) and 25-meter (part number 392-19110) sealed-to-standard Ethernet cables are also available. It is recommended that you use these cables with the LI-8250 Multiplexer, and not cables from a different manufacturer.

Synchronizing the timestamps

The LI-8250 Multiplexer includes a GPS receiver for high-precision GPS time data. For LI-8250 Multiplexer data files to be merged with gas analyzer data files in SoilFluxPro Software to calculate fluxes, the timestamps must match. For this, the LI-8250 Multiplexer contains a network time protocol (NTP) server which can be queried by other devices to sync the device's clock to the LI-8250 Multiplexer's GPS time, so that the same timestamps are being logged by both devices.

Generally, this synchronization is completed using the following steps.

  1. Connect the LI-8250 Multiplexer and gas analyzer.
  2. Identify the LI-8250 Multiplexer NTP server location through the analyzer interface.
  3. Different analyzers have different ways of identifying the location of the LI-8250 Multiplexer NTP server. Picarro analyzers, for example, have a Windows operating system embedded in the analyzer as a graphical user interface. In the file directory for the analyzer software, a "remote access" .ini configuration file allows users to identify NTP servers for time syncing. In this file, the LI-8250 Multiplexer server is identified by the LI-8250 Multiplexer serial number, and a similarly-named "remote access" executable (.exe) file performs the synchronization with the server locations indicated in the .ini configuration file.
  4. Manually query the NTP server to sync the devices, or set a task or scheduled job to automatically query the NTP server.
  5. Querying the LI-8250 Multiplexer NTP server depends on how you interface with your analyzer and locate the LI-8250 Multiplexer NTP server. Because the Picarro file structure provides an executable file to perform the query, users can manually run the .exe file through the Windows interface command prompt. Alternatively, CRON jobs, shell scripts, or scheduled tasks can also be configured to run the executable at specified times such as desired intervals, at instrument startup, etc.

Plumbing considerations

The exhaust from all connected analyzers must not exceed 5 slpm.

Determining effective volume

When using third-party gas analyzers, you must calculate the volume of the analyzer used and the length of your tubing.

First, you will need to calculate the effective volume that the third-party analyzer adds to your system. The volume of the analyzer will have at least two kinetic effects: 1) it brings additional air to the system, which dilutes trace gas entering the system from the soil surface and reduces the measured trace gas mole fraction rate of change (dC/dt); and 2) it creates a time delay in the onset of a monotonic concentration increase or decrease.

The quantitative impact of the added volume on dC/dt can be evaluated by considering the equation used to calculate flux F (mol m-2s-1). This equation is derived based on the assumption of a single fixed volume V (m3) with homogeneous air density ρ. For simplicity, in this discussion the effects of water corrections are neglected. This, however, does not change the conclusions. Thus,

D‑1

where F is the flux of trace gas (mol m-2s-1), ρ is air density (mol m-3), dC/dt is the time rate of change in mole fraction of the gas being measured (s-1), and S (m2) is the soil surface area over which the flux occurs. For a flux F, the trace gas mole fraction rate of change dC/dt is proportional to the total number of molecules in the system ρV.

For a well-mixed system, when an additional volume Vadded that contains a gas of density ρadded, is inserted into the system, equation becomes

D‑2

But ρsystem = Psystem/RTsystem, where R is the universal gas constant, and similarly for the added volume. Substituting these expressions and factoring gives,

D‑3

For data processing using SoilFluxPro, an effective volume Veffective for the addition can be defined for the added analyzer and entered into the software.

D‑4

Thus, the total volume used in equation D‑1 becomes simply Vsystem + Veffective and the density is ρsystem. There are inherently small variations in Veffective due to changes in Tsystem and Psystem, but these are generally small and subsequently neglected. In many cases, the impact of an added volume on flux calculations will be modest as Veffective for many modern trace gas analyzers is small.

In cases where the volume of the addition operates at a non-uniform temperature or pressure or is not well known, Veffective can be estimated experimentally by plumbing the third-party analyzers in a closed loop and injecting a known volume Vinjection (m3) of pure CO2 into the loop. The gas concentrations in the loop pre-injection C1 (mol mol-1) and post-injection C2 (mol mol-1) are defined as:

D‑5

D‑6

where NCO2 is the number of moles of CO2 in the additional volume pre-injection, Nadded is the total number of moles in the additional volume pre-injection, and Ninjection is the number of moles of CO2 injected into the loop. Substituting equation D‑5 into D‑6 and rearranging to solve for Nadded yields:

D‑7

where

D‑8

and

D‑9

Tinjection (K) and Pinjection (Pa) are the temperature and pressure, respectively, of the gas injected into the closed loop. Substituting these into equation D‑7 and following from equation D‑4 yields:

D‑10

In practice it is difficult to know Tinjection and Pinjection with great certainty. Making the assumption that Pinjection = Psystem and Tinjection = Tsystem introduces some error in determining Veffective experimentally, but it allows equation D‑10 to be simplified, eliminating the need to know temperature or pressure:

D‑11

In many cases, the impact of added volume on flux calculations will be modest, as Veffective for many modern trace gas analyzers is small.

Another consideration is time constants. By trapping air making its way around the measurement circuit, the volume of a third-party gas analyzer may also introduce an additional time delay and have other effects that can compromise the flux measurement. The magnitudes of these effects are related to the analyzer’s volume Vadded (m3), its operating pressure and temperature, and the flow rate through it. We can qualitatively assess the kinetic consequences of adding the volume by defining a time constant for the effective volume of the added analyzer. We define a time constant tadded (s):

D‑12tadded = ρaddedVadded / Uadded

where Uadded is the molar flow rate (mol s-1) delivered to the analyzer, ρ is air density (mol m-3) in the analyzer evaluated at the analyzer’s internal temperature and pressure, and Vadded (m3) is its actual volume.

You will need to follow this protocol to calculate the effective volume and time constant accordingly if you are using a LI-COR Trace Gas Analyzer or LI-870 CO2/H2O analyzer along with a gas analyzer. LI-COR technical support is available to assist you with this configuration.

Merging files in SoilFluxPro

File merging in SoilFluxPro software is currently supported for Picarro, LGR, Gasmet, and Aerodyne analyzers, using the Column Import Routine. The example below uses data from an LGR gas analyzer, though the procedure is identical for Picarro, Aerodyne, and Gasmet datasets.

To perform the Column Import Routine in SoilFluxPro,

  1. Open a Smart Chamber data file.
  2. Click the Import tool bar button to launch the Import Data Columns dialog.
  3. Click Add Files... to launch a file explorer to add the source file(s) to the source list.

  4. Select the file format (LGR in this case).

    After selecting a source file, it will be parsed and the headers, units and flux check box are displayed.

  5. Select an H2O source.

    This is only required if you are calculating a gas flux that needs a water vapor dilution correction.

  6. Enter the Device Name.

    This can be anything you want, but it cannot be blank.

  7. Enter the Tube Length and the Analyzer Volume.

    If a General Purpose file format is selected, adjust the Date & Time settings and the Delimiter setting.

  8. If there is a known time offset between the Smart Chamber and the imported data, enter that into Adjust time of imported data.

  9. Select the variables to import, the units, and whether you want to compute a flux from the data.
  10. If you have selected an H2O source, the parameter is selected automatically.

  11. Click Import.

    Now, when you update the displayed variables, the imported columns will be available under the Meas tab.

  12. When you launch the Recompute dialog, you can now add flux computations based on the imported columns.

For calculating fluxes with imported data, it is best to import dry mole fraction/dry mixing ratios. This demonstration used LI-8100A data, which has water vapor data from the LI-8100A analyzer. Because Smart Chamber data files do not have water vapor data by default, you must provide this data. The data can be measured by a gas analyzer and imported or it may already be included in the LI-8250 or Smart Chamber data file if from a LI-COR gas analyzer that measures water vapor.