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.  They are used to generate a stable concentration of a specific trace chemical. At a constant temperature, the device emits the compound through its permeable portion at a constant rate.

They are typically inserted into a carrier flow to generate test atmospheres for calibrating gas analyzer systems, testing hazardous gas alarms, or conducting long-term studies of effects on materials or biological systems.

A recent application is their use as explosives, narcotics, and chemical warfare dopants for TSA airport security (ammonia, DCM), law enforcement, border patrol, military, and other trace detection industry professionals.