permeation tube
A device used for dynamic preparation of test gas mixtures by means of controlled permeation of a gaseous analyte out of a container through polymer material into a carrier gas stream. These devices containing certain condensable gases (e.g. NO2 , SO2 , etc.) when operated at closely controlled temperatures can be used as primary standards calibrated in terms of the weight loss per unit time.
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2205 (