GE Moisture Image 1 Series, Service Manual, Page: 84 / 107

June 2003
A-22 Application of the Hygrometer (900-901D1)
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.
For liquids in which a Henry’s Law type analysis is applicable, the
parts per million by weight of water in the organic liquid is equal to
the partial pressure of water vapor times a constant:
(a)
where, K is the Henry’s Law constant in the appropriate units, and the
other variables are as defined on page A-11.
PPM
W
K P
W
×
=