Transmission rate Helium

Standard Reference Material 1470 is a 23 micrometer thick polyester film whose gas transmission characteristics with respect to helium, carbon dioxide, oxygen, and nitrogen have been carefully measured. A completely computerized manometric permeation measuring facility developed at NBS was used for the measurements. The steps taken to characterize the gas transmission rate of this material over the range of pressures from 67.5 kPa to 135 kPa and over the range of temperatures from 18 °C to 31 °C are described. The results obtained in these measurements are compared with those in the literature. The role of Standard Reference Material 1470 in improving the repeatability and reproducibility of gas transmission measurements employing other instrumentation is discussed. 

To better estimate gas permeation for high quality barriers a novel technique has been developed [24, 25], which is based on the measurement with a quadrupole mass spectrometer of the helium transmission rate through the sample. 

Under equilibrimn conditions, which can be reached after some minutes or some hours, depending on the nature of the sample, the gas flow, F, provides the helium transmission rate. A typical graph showing the achievement of steady state conditions is given in Figure 4.7. 

The estimation of the transmission rates for gases other than helium can also be obtained, provided a preliminary calibration is carried out. This is particularly important for water, which is the main gas permeating through the barrier skin. To run the calibration procedure, the helium transmission rate is measured for various samples having different and known permeation rates for water. The linear correlation between helium and water transmission rates is then established, as shown in Figure 4.8. 

Polyvinyl alcohol resins n. A water-soluble thermoplastic prepared by partial or complete hydrolysis of polyvinyl acetate with methanol or water. These resins are mainly used as packing films since they are impervious to oils, fats, and waxes, and have very low transmission rates of oxygen, nitrogen, and helium. 

The H/C ratio of the coke deposits was quantified by temperature programmed oxidation (TPO) in a 1 % oxygen helium mixture. Temperature was raised to 850° C at a heating rate of 10° min 1. The calculations of the H/C ratio involved the results from the measurements of carbon dioxide production and oxygen uptake (according to Ref. [8]). Coke deposits were also characterized by thermogravimetry and transmission electron microscopy. 

---