Permeation tubes

Permeation tubes are small containers filled with a pure chemical compound in a two-phase equilibrium between its gas phase and liquid phase. The containers are in suitable inert polymeric material and at a constant temperature, the device emits the compound through permeable wall with a constant rate. The permeate is mixed with a carrier gas at a controlled flow rate to obtain a known mixture used as reference in gas testing equipment. A wide range of permeation rates can be made, normally from 20 ng/min to 500 µg/min, and accurate, stable concentrations range from ppb to high ppm.

The gas concentration, in ppm, can be calculated using following equation:

C(ppm) = C(ng/sccm) x K

where

K = (24.46)/molecular weight, and 24.46 is the molar volume in liters @ 25°C, 760mmHg .
K for SO2 = 0.382,
        NO2 = 0.532,
        H2S = 0.719,
        NH3 = 1.436.

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