Flux Chamber Measurements

Soil gas flux measurements are used since several decades. They have been applied within the framework of Greenhouse gas emissions and soil dynamics as well as in volcanic fields and other geothermal areas (e.g. CARDELLINI ET AL., 2003; BERGFELD ET AL., 2001). A number of different techniques can be employed for gas flow measurements. These include micrometeorological methods, open or flow-through and closed chamber methods, which can be further divided into active or passive sampling methods (see WELLES ET AL., 2001 for a brief review). In the summer field campaign of the coal fire project the closed chamber measurements (a passive method) have been applied by BGR and DMT.

The principle of the measurement using a closed “flux chamber” is relatively simple. A circular chamber (cylinder), made of steel or temperature resistant plexiglass (Figure 1), with in- and outlet connectors is placed leak-tight on the soil and the concentration increase (or decrease in case of a gas sink) is measured as a function of time (Figure 2). The gas concentration was determined by portable multi-gas instruments (Geotechnical Instruments GA 94 and Draeger Multiwarn II ) equipped with different IR- and chemical sensors.

Figure 1: Plexiglass emission chamber with multi-gas instrument Multiwarn II (internal pump circulating the gas) and temperature measurement at fire zone 3_2

Figure 2: Concentration increase in a flow chamber as a function of time

To ensure a constant homogenization of the gas in the flux chamber a small vane is placed in the chamber or the gas is circulated through the chamber and the measurement device by an internal pump. The frequency of data acquisition is depending on gas flow from below, ranging between 1sec intervals (for high gas flow rates) and 15-30 minutes intervals measurements of undisturbed soil.

The concentration of the gas in the chamber is increasing in two phases. During the first phase the increase is linear with time and after a certain time asymptotically approaching a maximum value (saturation of the chamber). The gas flow from the soil or vent is determined from the linear increasing concentration:

with dc/dt concentration increase over time, hC height of the chamber and VC, AC volume and base area of the chamber respectively. The unit of the gas flow Q is [Mass][Area]-2[Time]