1. Field of the Invention
This invention relates to the measurement of the rate of permeation of a gas or vapour through a sample of a material.
2. Description of the Related Art
Permeation describes the process of species entering, moving through and leaving a test sample. Measuring the rate of permeation (or strictly speaking the rate of transmission) of vapours and gases through materials, in particular barrier materials, is important in various fields. Examples are packaging of food, of medical supplies, of electronic components and as membranes in fuel cells. Barrier layers serve the purpose of preventing or restricting the passing of a gas or a vapour.
A number of methods are known to measure the rate of permeation.
U.S. Pat. No. 2,755,660, JP 63132137A and ASTM standard D1434-82(2003) describe a method where one side of the test sample is kept at a constant pressure and the pressure increase on the other side of the sample is measured.
U.S. Pat. No. 5,390,539 and ASTM standard F1249-05 describe the use of electrochemical and infrared sensors for the detection of oxygen and water vapour.
US2002/0173922A1 describes a sophisticated method combining an acoustic wave and optical detection method where the barrier layer for permeation measurements needs to be deposited onto a transducer, followed by an elaborate measuring procedure consisting of many steps.
EP 1373861A2 describes the “Calcium test” where the progress of corrosion of a thin layer of calcium deposited on the test sample is taken as measure for the amount of water or oxygen that has passed through the test sample.
U.S. Pat. No. 6,909,088 describes a mass-spectrometric method for measuring the rate of permeation.
U.S. Pat. No. 6,804,989 describes a very sensitive method to detect water vapour permeation by using radioactive water.
U.S. Pat. Nos. 4,858,461 and 5,081,863 describe a multi-chamber setup, which requires continuous gas flow; stack arrangements have been suggested too (U.S. Pat. No. 4,468,951).
U.S. Pat. No. 5,591,898 describes a method using a carrier gas to calculate the permeability P as product of the measured coefficient of diffusion D and the solubility S.
All methods described in the art have the detecting means attached to the assembly with the test sample. Despite all the effort that has gone in the invention of these methods and apparatuses they suffer from serious drawbacks. The proximity between test sample and sensor limits the parameter range of testing, because the sensors are usually not rated to high temperatures. Another disadvantage is the limited sample throughput as continuous measurement even in an arrangement with multiple cells requires the detector in attendance of the test sample over substantial periods of time. Using a carrier gas makes the experimental set-up more cumbersome and costly.