Connecting to non-LI-COR gas analyzers

While the LI-8250 Multiplexer is optimized for use with LI-COR gas analyzers, it can be configured with non-LI-COR gas analyzers.

The LI-8250 Multiplexer logs timestamp, soil temperature and moisture, location, and other data alongside the analyzer. The datasets can then be merged using SoilFluxPro Software to calculate fluxes.

Making the connection

Configuring the LI-8250 Multiplexer with a non-LI-COR gas analyzer requires two things.

  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 strongly recommended.

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 non-LI-COR gas analyzer, an optional 2 m 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 m (part number 392-19109) and 25 m (part number 392-19110) sealed to standard Ethernet cables are also available. It is strongly recommended that you use these cables with the LI-8250 Multiplexer, and not cables from a different manufacturer.

Timestamp synchronization

The LI-8250 Multiplexer includes a GPS receiver for high-precision GPS time data. For LI-8250 Multiplexer data files to be merged with non-LI-COR 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 non-LI-COR 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 non-LI-COR 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,


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


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


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


Thus, the total volume used in equation C‑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 non-LI-COR 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:



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 C‑5 into C‑6 and rearranging to solve for Nadded yields:






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


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 C‑10 to be simplified, eliminating the need to know temperature or pressure:


You will need to follow this protocol to calculate the effective volume of a non-LI-COR gas analyzer. LI-COR Technical Support is happy to assist you with this configuration.