Even calibration gas of very low concentrations can be continuously generated.
A permeation tube is a fluororesin tube that contains high-grade liquefied gas. When the tube is exposed to a certain temperature, liquefied gas penetrates the tube wall by osmosis in time-increments enabling a specific quantity of gas to be retained in the tube . Consequently, a carrier gas flowing at a constant speed (for calibration purposes) can be continuously generated by keeping the tube at a fixed temperature and infusing a constant quantity of dilution gas. This method is highly reliable because the concentration of the calibration gas is determined by calculating the decrease in weight of the permeation tube and the quantity of dilution gas. This dynamic method also enables highly reactive gases such as chlorine, hydrogen sulphide or ammonia to be generated. As shown in the table below, there are many different kinds of permeation tubes and they are widely used because they can provide a stable supply of calibration gas.