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AQUATRAN 3/38 Operator's Manual
Measuring Water Vapor Transmission Rate
To make an accurate transmission rate measurement a known concentration Test Gas (usually 100%
Water vapor) is applied to one side of the barrier material to be tested and the other side is swept with a
water vapor free Carrier Gas (nitrogen). This is accomplished by mounting the sample in a device called a
Diffusion Cell. For convenience a diffusion cell is often referred to as a "test cell" or as a "cell".
This process is illustrated in Figure 1-2 which is a simplified depiction of a film sample mounted in a
diffusion cell. The Test Cartridge used in the AQUATRAN 3/38 contains two diffusion cells.
As shown in the illustration the film is mounted between two parts of the cell. The o-rings and sealing
surfaces in the cell prevent outside air from affecting the measurement.
The Test Gas is continuously applied to bottom half of the cell. When performing a test using a 100% RH
test gas, the water vapor is supplied by a saturated sponge. To perform a test using a Generated RH the
sponge is removed and a gas at the specified RH is supplied to the Test Gas side of the cell.
The Carrier Gas (nitrogen) enters the top half of the cell. As water vapor permeates the sample barrier it
mixes with the Carrier Gas. The output side (exhaust) of the test cell is routed to the water vapor sensor.
The amount of water vapor in the carrier gas is measured using the water vapor sensor.
Seal leakage can be a significant source of error when measuring the transmission rate of a film sample.
The TruSeal™ flush ring at the perimeter of the cell minimizes the effect of ambient water vapor on the
measurement. During a test the TruSeal™ flush ring is purged with carrier gas minimizing the possibility of
leakage past (or permeation through) the seals.
MOCON, Inc.
Figure 1-2: The AQUATRAN 3/38 - 100% RH Test Cell
Revision G
Introduction
1-5
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