Air pressure inside a soil CO2 flux chamber must be equal to the ambient air pressure during a measurement. Any difference will affect the flux rate.
A vent to the atmosphere works; however, this approach is only effective under calm conditions. Under windy conditions, chamber pressure drops as wind blows over the vent’s open end due to the Venturi effect. This causes a mass flow of CO2-rich air from the soil into the chamber, leading to a significant overestimation of soil CO2 flux.
LI-8100A chambers feature a patented pressure vent to maintain equal pressure between the chamber and the atmosphere (Fig. 1). As air is forced into the vent by wind, the average flow rate drops. As the flow slows, most the dynamic pressure is converted to static pressure, virtually eliminating the Venturi effect. The vent is radially symmetric to eliminate sensitivity to wind direction.
Fig. 1. Cross section of pressure vent. For more information about the patented vent design, see Xu L., et al. 2006. On maintaining pressure equilibrium between a soil CO2 flux chamber and the ambient air. Journal of Geophysical Research. 111, D08S10, doi:10.1029/2005JD006435.
Data from field experiments on differential pressure measurements between air inside the chamber and the outside ambient air show that chambers equipped with this vent maintain the pressure of outside air under both calm and windy conditions.