Appendix D: Theory Of Operation; How The Water Vapor Transmission Rate Is Measured; How The Water Vapor Sensor Works; Water Vapor Sensor Calibration Theory - Ametek mocon AQUATRAN 3/40 Operator's Manual

AQUATRAN Model 3/40 Operator's Manual Theory of Operation
: Theory of Operation

How the Water Vapor Transmission Rate is measured

When a film is installed in a Test Cell, 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 test 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 Test 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 Model 3/40 uses an 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 Test Cell. The signal is therefore proportional to the water vapor
transmission of the barrier material.

Water Vapor Sensor Calibration Theory

The AQUATRAN Model 3/40 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.
Room temperature and humidity are just two of many factors that can marginally affect test results.
Consequently, a piece of film with a known WVTR will test higher or lower.
Any reference film, whose WVTR has been established, may be used for calibration. A reference film at any
arbitrarily constant test flow will produce an arbitrary constant amount of water vapor in the carrier gas. The
MOCON, Inc. Revision D D-1