1. Field of the Invention
This invention relates to an apparatus for measuring the water vapor transmission rate through plastic films or like structure which can also be used for measuring water content of solid, liquid and gaseous materials, with particular reference to a cell used in said apparatus for rapidly absorbing and electrolyzing the water vapor transmitted from a test sample. 2. Description of the Prior Art
In general, there are many known devices employing a variety of principles for determining the water vapor transmission rate through plastic films or like structure which may also be useful for measuring the water content of solid, liquid and gaseous materials.
The simplest device used is a standard metal cup having an opening with a definite area. The cup is partially filled with a desicant which will reduce the relative humidity in the cup to ten percent when a sample to be tested is secured over the opening. The cup and sample are weighed and placed in an oven set at a specific temperature, usually 100.degree. F. The oven is also maintained at a specific relative humidity, usually 100 percent. After several days the cup is removed from the oven and weighed. The water vapor transmission rate, hereinafter referred to as W.V.T.R., is then calculated in units of grams of water permeating the test material per hundred square inches of sample per 24 hour period.
A second device employs a cell having a moisture sensitive crystal wherein the resistance to an electrical current passing through the crystal is proportional to the humidity surround the crystal. The relative humidity on one side of the sample to be tested in usually fixed at one hundred percent and the relative humidity on the other side is set at 10 percent. Again, the temperature is normally maintained at 100.degree. F. The time needed for an incremental increase in humidity, normally 1 percent, is then obtained from the calibrated change in current passing through the crystal. The cycle is repeated until a relatively constant time is established from which the W.V.T.R. is calculated.
A third device incorporates a cell which works on the same principle as the present invention but is of a distinctly different design. This type of cell and principle are illustrated in Analytical Chemistry, Vol. 21, No. 12, pp. 2043-2048 (December 1959 ). The principle employed by the illustrated cell provides for the absorption and electrolysis of the water vapor that permeates through a test sample which has 100 percent relative humidity on one side and zero percent on the other. The W.V.T.R. is calculated from the electrical current required to electrolyze the absorbed water vapor which are related with primary accuracy.
The major problem with known devices and methods for measuring W.V.T.R. through plastic films or like structures is an inability to make rapid uncomplicated tests with accurate reproducible results. Sensitive calibrations, slow response and high experimental errors result in steady state rate calculations which may not reproduce. Even though the transmission rate of most gases such as oxygen, nitrogen and carbon dioxide through plastic films can be easily and rapidly measured, the transmission of water vapor is an exception. Water is a highly polar molecule which clings to most surfaces, thus causing water vapor to transport very slowly and unevenly through test equipment. It, therefore, takes a long time to reach steady state conditions when water vapor transmission is directly measured. Accuracy and reproducibility is also effected for the same reason. Additionally, it has been found by mass spectrometer analysis that the third type of device described above does not always absorb all of the water vapor transmitted through a test sample and is relatively slow in reaching steady state conditions.