Permeation and Diffusion Devices
(Traceable to NPL/NIST standards)
Permeation tubes are made by sealing a liquid chemical in a tube made of fuoroplastic, nylon or silicon rubber depending on the permeant substance. It is essential that a liquid part is maintained in equilibrium with the gaseous part permeating through the plastic. After an initial stabilisation period of one to three weeks the chemical permeates at a uniform rate through the walls of a tube or through a membrane or nipple sealed in an impermeable container. The permeation rate is a function of temperature and to ensure stability of weight loss the temperature in the chamber containing the device must be better than 0.1 oC. The permeation rate remains constant until the liquid has been exhausted and the necessary partial pressure can not be sustained.
In order to generate a concentration of gas in a calibration system the permeation chamber is purged with a low flow of pure gas. The purge flow transports the chemical that has premeated from the tube until it is mixed with the dilution flow calculated to achieve the required concentration. The diluent air flow can be varied to produce a series of different concentrations for e.g. linearity testing. The concentration of the gas standard generated in the system is calculated knowing the permeation rate of the tube and the sum of the purge and diluent gas flow using the formula:
C=( R x K)/F
where: C = Gas standard concentration in ppm
R = Permeation rate in ng/min
K = Permeant gas reciprocal density in nl/ng
F = Total gas flow in ml/min
Europa in conjunction with Caltech UK supplies a wide range of tubes suitable for use in almost any manufactuers monitors and a wide range of calibartors too. We can produce devices to generate almost any concentration of gas from ppt to ppm and at oven temperatures from 30 to 70 degrees C. Please contact us for a list of devices available or call with your specific requirements.
