Each tube is calibrated according to “EPA TRACEABILITY PROTOCOL FOR ASSAY AND CERTIFICATION OF GASEOUS CALIBRATION STANDARDS” U.S. EPA-600/R-97/121, Section 3, procedure P3.
during the calibration the permeation tube is kept, in an inert dry gas stream, at a constant temperature controlled with accuracy of ±0.05°C. The tube is periodically weighed with a semi-microanalytical balance (accuracy of ± 0.01 mg ) until the standard error reaches 95% confidence levels. The weights are plotted versus time and slope of the best fitting straight line gives the permeation rate.
Each tube is certified by a Chemist indicating:
- calibration temperature;
- permeation rate;
- average useful life expected on the bases of the gas content and of the permeation rate.