Permeability Measurement
Quantitative assessments of gas diffusion through a barrier material indicate the volume of water vapor passing through a unit area over a defined time. The moisture vapor transmission rate determines the effectiveness of protective enclosures. Units are expressed as grams per square metre per day.
Diffusion Barrier
Polymer films and metallic foils act as the primary resistance to humidity ingress. A low moisture vapor transmission rate is necessary for maintaining the dry internal environment of a sensor housing. Material thickness and density directly influence this value.
Pressure Gradient
Driving force for the diffusion is the difference in partial pressure between the two sides of the material. When the moisture vapor transmission rate is measured, one side is exposed to a high humidity source while the other is kept dry by a desiccant or a carrier gas. Temperature must be held constant during the test.
Accuracy Verification
Gravimetric methods or infrared sensors detect the minute amounts of water that penetrate the sample. Determining the moisture vapor transmission rate requires a steady state condition where the flow of vapor becomes linear. Calibration involves using reference films with known permeability to check the sensitivity of the detector.
Small leaks in the test fixture can cause a substantial overestimation of the transmission value.
