User Manual
2 Introduction
2.1 What is Forced Diffusion?
The eosFD is a stand-alone soil CO2flux sensor, containing a single NDIR sensor, an internal data
logger and small diaphragm pump. The eosFD uses Eosense’s patented Forced Diffusion technology
to measure soil CO2flux directly. Traditionally, gas fluxes from the soil surface are measured using
closed chamber systems, with these “accumulation chambers” trapping gas during the measurement
period. CO2concentrations continue to increase while the chamber is down, and different mathematical
fits (typically linear or exponential) are applied to the data to indirectly estimate the original rate of flux.
Forced Diffusion is a membrane-based steady-state approach for measuring gas flux that establishes
an equilibrium between gas flowing into the chamber and gas flowing out of the chamber through the
membrane, without any external chamber movement. By carefully characterizing the diffusive
properties of the membrane used in the instruments, the eosFD chamber gas concentration can be
correlated directly to the gas flux rate. Essentially, the amount that this membrane limits the flow of gas
out of the chamber is known, and thus by comparing the internal concentration to that outside of the
instrument, the flux rate can be calculated. Unlike other automated chambers that lift and lower onto the
soil surface, the Forced Diffusion approach does not require external moving parts, allowing it to be
deployed in the harshest climates for extended periods without intervention.
2.2 How is Flux Calculated?
Forced diffusion flux is calculated by calibrating the diffusive transport of gas across the eosFD’s
membrane. This flux rate depends on the effective diffusivity of the interface, the concentrations on
either side and the path length between the two points. The relationships between these parameters
are linear, and everything but the soil and atmospheric concentrations are assumed to be constant,
which simplifies to a single calibration slope (G
). The flux is then calculated by multiplying the difference
in soil and atmosphere concentrations by the calibration slope:
The full differential equation showing the (volume/surface area) scaled
rate of change in flux rate equal to the flux from the soil surface (F
S
)
minus the difference in concentration, ∆C (scaled by both the path
length L
and the diffusivity of the interface, D
).
The change in the flux rate is assumed to be zero (steady-state) over
the timespan of the concentration measurements (~60 s). The
steady-state assumption reduces the equation to a linear dependence.
As the path length and interface (membrane) diffusivity are constant,
this can be represented by a single coefficient, G
.
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