On the performance of SF6 permeation tubes used in determining methane emission from grazing livestock

In a technique for measuring methane emitted by grazing ruminant livestock, a calibrated source of inert tracer sulfur hexafluoride (SF₆) is inserted into the rumen of each participating animal prior to collection of “breath” samples for gas analysis. Each source comprises a “permeation tube” from which an SF₆ charge slowly escapes through a permeable membrane, to be sampled with the breath. This paper reports analyses of the permeation characteristics of such tubes and provides evidence that the permeation rate slowly changes rather than stays constant as the technique supposes. This feature has been observed routinely over several generations of tube fills in our laboratory. Failure to take account of a changing permeation rate can lead to a systematic error in the inferred methane emission rate of up to about 15%. A quality control strategy is proposed that enables permeation rates to be extrapolated with confidence, based on the monitored performances of control tubes as proxies for inserted tubes.