Easidew PRO I.S. User Manual
12
97130 Issue 16.2, August 2021
GOOD MEASUREMENT PRACTICE
Temperature and Pressure effects
As the temperature or pressure of the environment fluctuates, water molecules are
adsorbed and desorbed from the internal surfaces of the sample tubing, causing small
fluctuations in the measured dew point.
Adsorption
is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved
solid to the surface of a material, creating a film. The rate of adsorption is increased at
higher pressures and lower temperatures.
Desorption
is the release of a substance from or through the surface of a material. In
constant environmental conditions, an adsorbed substance will remain on a surface
almost indefinitely. However, as the temperature rises, so does the likelihood of
desorption occurring.
Ensuring the temperature of the sampling components is kept at consistent levels is
important to prevent temperature fluctuation (i.e. through diurnal changes) continually
varying the rates of adsorption and desorption. This effect will manifest through a
measured value which increases during the day (as desorption peaks), then decreasing
at night as more moisture is adsorbed into the sampling equipment.
If temperatures drop below the sample dew point, water may condense in sample
tubing and affect the accuracy of measurements.
Maintaining the temperature of the sample system tubing above the dew point of the
sample is vital to prevent condensation. Any condensation invalidates the sampling
process as it reduces the water vapor content of the gas being measured. Condensed
liquid can also alter the humidity elsewhere by dripping or running to other locations
where it may re-evaporate.
Although ambient pressure does not change drastically in a single location, the gas
sample pressure does need to be kept constant to avoid inconsistencies introduced
by adsorption or desorption. The integrity of all connections is also an important
consideration, especially when sampling low dew points at an elevated pressure. If
a small leak occurs in a high-pressure line, gas will leak out; however, vortices at the
leak point and a negative vapor pressure differential will also allow water vapor to
contaminate the flow.
Theoretically flow rate has no direct effect on the measured moisture content, but
in practice it can have unanticipated effects on response speed and accuracy. An
inadequate flow rate may:
