Measuring MVTR and Oxygen Transmission
by David Vaczek
Mocon has developed the Aquatran Model 1 that employs a new proprietary Aquatran water-vapor sensor for moisture-vapor transmission rate (MVTR) testing of flat materials and packages. Using coulometric sensor technology, the unit is 10 times more sensitive than infrared sensor testing, with sensitivity down to 0.0005 g per sq m/day (5 × 10–4 g/m2/day) says Tim Ascheman, engineering manager.
Water molecules diffusing through a film sample that is clamped into a diffusion cell are conveyed by carrier gas to the sensor. A computer monitors the absolute increase in water vapor molecules in the gas stream, reporting the MVTR value. The sensor cell employs phosphorus pentoxide for absorbing incoming water vapor. When the water vapor is electrolyzed, two electrons are passed to an electrical circuit for each water molecule, producing an electrical current proportional to the number of water molecules.
Ascheman says Mocon’s unique sensor design provides a longer life and more sensitivity for measuring permeation through packaging material.
ASTM F1249 describes the procedure for MVTR testing using modulated infrared sensors. “With con- centration detectors, you have to put a known calibration standard into the system and determine the voltage-to-transmission-rate ratio for each sensor.
“A coulometric sensor requires no calibration. Vapor molecules entering the sensor produce an exact, predictable, quantifiable electrical charge. Since the coulometric sensor is absolute, every sensor gives you the same voltage output for every molecule that enters the sensor,” he says.
Mocon’s Ox-Tran Model 702 units feature coulometric sensors for measuring oxygen transmission rates in flat films and packages, following ASTM D3985 and F1307.
“There was a big push at the October ASTM meeting to create a new standard for methodologies that test oxygen transmission with various other sensors. It is our contention that if you make the standard too generic, it loses value and becomes more of a guide,” says Ascheman.
Round-robin testing in recent years has demonstrated that the coulometric detector produces significantly more-repeatable results. “The precision and bias statement for D3985 was recently updated with just the data specific to our instrument,” he says.
The sensor converts oxygen molecules into an electrical charge that is related to the amount of oxygen transfer.