Permeation tubes are small, inert capsules containing a pure chemical compound in a two-phase equilibrium between its gas phase and its liquid or solid phase. At a constant temperature, the device emits the compound through its permeable portion at a constant rate. Accurate, stable concentrations range from PPB to high PPM.
The purpose of a calibration gas standard is to establish a reference point for the verification of an analysis. The permeation tube rates can be certified using standards traceable to NIST by the most basic and accurate laboratory procedure – measuring the gravimetric weight loss over a known period of time at a known temperature.