Pressure; Response Time - GE Moisture Image 1 Series Service Manual

Moisture Monitor Hints
(cont.)

Pressure

Response Time

Application of the Hygrometer (900-901D1)
GE Panametrics hygrometers measure only water vapor pressure. In
addition, the instrument has a very rapid response time and it is not
affected by changes in fluid flow rate. If any of the above situations
occur, then they are almost always caused by a defect in the sample
system. The moisture sensor itself can not lead to such problems.
GE Panametrics hygrometers can accurately measure dew points
under pressure conditions ranging from vacuums as low as a few
microns of mercury up to pressures of 5000 psig. The calibration data
supplied with the moisture probe is directly applicable over this entire
pressure range, without correction.
Note: Although the moisture probe calibration data is supplied as
meter reading vs. dew point, it is important to remember that
the moisture probe responds only to water vapor pressure.
When a gas is compressed, the partial pressures of all the gaseous
components are proportionally increased. Conversely, when a gas
expands, the partial pressures of the gaseous components are
proportionally decreased. Therefore, increasing the pressure on a
closed aqueous system will increase the vapor pressure of the water,
and hence, increase the dew point. This is not just a mathematical
artifact. The dew point of a gas with 1000 PPMv of water at 200 psig
will be considerably higher than the dew point of a gas with 1000
PPMv of water at 1 atm. Gaseous water vapor will actually condense
to form liquid water at a higher temperature at the 200 psig pressure
than at the 1 atm pressure. Thus, if the moisture probe is exposed to
pressure changes, the measured dew point will be altered by the
changed vapor pressure of the water.
It is generally advantageous to operate the hygrometer at the highest
possible pressure, especially at very low moisture concentrations.
This minimizes wall effects and results in higher dew point readings,
which increases the sensitivity of the instrument.
The response time of the GE Panametrics standard M Series
Aluminum Oxide Moisture Sensor is very rapid - a step change of
63% in moisture concentration will be observed in approximately 5
seconds. Thus, the observed response time to moisture changes is, in
general, limited by the response time of the sample system as a
whole. Water vapor is absorbed tenaciously by many materials, and a
large, complex processing system can take several days to "dry
down" from atmospheric moisture levels to dew points of less than -
60°C. Even simple systems consisting of a few feet of stainless steel
tubing and a small chamber can take an hour or more to dry down
from dew points of +5°C to -70°C. The rate at which the system
reaches equilibrium will depend on flow rate, temperature, materials
of construction and system pressure. Generally speaking, an increase
in flow rate and/or temperature will decrease the response time of the
sample system.
June 2003
A-3