OX-TRAN Model 2/22 Operator’s Manual
Theory of Operation
MOCON, Inc.
Revision R
D-3
How a Humidified Test Gas is Generated
Humidified Carrier and Test Gases are created by moving pressurized gas through a humidifier filled with
HPLC-grade water and mixing the wet gas with a dry gas in the appropriate ratios.
A separate humidifier is provided for each of the gases (Test and Carrier). A mixing valve is used to “mix”
the saturated gas (~100% RH) with the dry gas. A mix-ratio of 50% yields a gas with an RH of
approximately 50%.
A RH sensor is used to measure the actual RH delivered to the Test Cell. The instrument uses the
measured RH value to adjust the mixture ratio to maintain the RH of the gas delivered at the specified
value.
The RH that the sample will be tested at can be changed from wet-to-dry or dry-to-wet by simply changing
the RH set points. The “Sequential Test” feature can be used to automatically perform a sequence of tests
at different RH levels.
How the ReZero Cell Works
The transmission rate measured for samples mounted in the Test Cells is the sum of the transmission rate
due to permeation through the barrier material and all other sources of oxygen ingress. This includes the
system baseline. If the system baseline can be measured, subtracting it from the apparent transmission rate
of the Test Cells will give a more accurate representation for the transmission rate of the barrier material.
During a ReZero State the instrument routes carrier gas (which is at the same temperature and RH as used
in the Test Cells) to the oxygen sensor. The instrument components used to perform this function are
referred to as the ReZero Cell. The ReZero Cell is different from the Test Cells in that it does not contain a
barrier material that is exposed to a Test Gas. This means that any residual oxygen in the carrier gas is not
due to permeation through the barrier materials mounted in the Test Cells. The residual oxygen measured
in the ReZero State is therefore considered to be a good representation of the system baseline.
Periodically the instrument baseline is measured using the ReZero Cell. The ReZero Interval and
Examination Time is determined the by currently active Test Method. The resulting data is used to correct
the transmission rate data for any active tests. This ensures that changes in the baseline do not affect the
accuracy of the transmission rate data.