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AQUATRAN 3/38 Operator's Manual
Appendix D: Theory of Operation
How the Water Vapor Transmission Rate is measured
When a film is installed in a Test Cartridge, it is exposed to a continuous flow of dry nitrogen gas across the
one side (the Carrier Gas side) and an RH on the other (the Test Gas side). The gas leaving the cell via the
exhaust port consists of a mixture of nitrogen and water vapor in a ratio determined by the nitrogen flow rate
through the cell and the rate of the water vapor transmission through the film barrier.
With the nitrogen flow rate set to a constant value, the resulting water vapor density at the Carrier Gas side
of the cell will be determined by the water vapor transmission rate of the film barrier. The water vapor
density of the Carrier Gas is measured by routing the cell exhaust to a Water Vapor Sensor.
How the Water Vapor Sensor Works
The Water Vapor Sensor in the AQUATRAN 3/38 uses a pressure modulated Infrared Detector. The
measurement system consists of a bellows pump, a sensing chamber, an infrared source, a 2.6 micrometer
infrared filter, a lead sulfide photo detector and an amplifier.
The pump varies the pressure and the density of the gas mixture in the sensing chamber. As pressure and
density of the gas mixture varies, the absorption rate of infrared energy by water vapor also varies. The
Infrared Detector senses the change in infrared energy reaching it and produces a low-level electrical
signal. The Infrared Detector is connected to a low noise amplifier.
The signal from the detector is amplified, filtered, and converted to a DC signal which is directly proportional
to the water vapor in the exhaust of the cell. The signal is therefore proportional to the water vapor
transmission of the barrier material.
Water Vapor Sensor Calibration Theory
The AQUATRAN 3/38 is a relative (not absolute) water vapor transmission rate (WVTR) measurement
system. To achieve accurate results, the instrument must be calibrated using a reference material with a
known water vapor transmission rate. In other words, the WVTR of the test cells must be calibrated to a
known constant of WVTR prior to testing.
MOCON, Inc.
Revision G
Theory of Operation
D-1
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