Application of the Hygrometer (900-901D1) A-27
4/9/98
Liquid Applications
Theory of Operation
The direct measurement of water vapor pressure in organic
liquids is accomplished easily and effectively with GE
Panametrics’ Aluminum Oxide Moisture Sensors. Since the
moisture probe pore openings are small in relation to the size of
most organic molecules, admission into the sensor cavity is
limited to much smaller molecules, such as water. Thus, the
surface of the aluminum oxide sensor, which acts as a semi-
permeable membrane, permits the measurement of water vapor
pressure in organic liquids just as easily as it does in gaseous
media.
In fact, an accurate sensor electrical output will be registered
whether the sensor is directly immersed in the organic liquid or it
is placed in the gas space above the liquid surface. As with gases,
the electrical output of the aluminum oxide sensor is a function of
the measured water vapor pressure.
Moisture Content Measurement in Organic Liquids
Henry’s Law Type Analysis
When using the aluminum oxide sensor in non-polar liquids
having water concentrations ≤1% by weight, Henry’s Law is
generally applicable. Henry’s Law states that, at constant
temperature, the mass of a gas dissolved in a given volume of
liquid is proportional to the partial pressure of the gas in the
system. Stated in terms pertinent to this discussion, it can be said
that the PPM
W
of water in hydrocarbon liquids is equal to the
partial pressure of water vapor in the system times a constant.
As discussed above, a GE Panametrics aluminum oxide sensor
can be directly immersed in a hydrocarbon liquid to measure the
equivalent dew point. Since the dew point is functionally related
to the vapor pressure of the water, a determination of the dew
point will allow one to calculate the PPM
W
of water in the liquid
by a Henry’s Law type analysis. A specific example of such an
analysis is shown below.
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