In the case of passive sources, a device is required to perform a direct measurement of the source, from either a solid or liquid surface. Coverings or ‘hoods’ are usually used to achieve this and one such device is the static flux hood. This hood involves covering an area of the source and injecting neutral gas into the hood at a fixed rate of 4 to 5 lps, to create mixing via minor turbulence. A sample is then obtained. The input and output of neutral gas and sample mixture respectively may be of equal flow rate or have a greater input then output.