3-10
3.2.5 Purging
The Purge mode is generally used to prevent icing within the saturator and dry the saturator
outlet after movement (transportation), storage (power off, no gas flow, etc.), after performing
the saturator fill procedure (section 3.2.8), or while transitioning the saturator from higher to
lower temperatures.
When the system is not being used (power off, no gas flow, etc.), the saturator is closed off and
the gas within is static. As thermal equilibrium is reached, water vapor will condense on all
inner surfaces between the saturator outlet and the expansion valve inlet. The Purge mode
counteracts this condition by allowing the carrier gas to flow in the opposite direction (expansion
valve to saturator), drying the affected sections of tubing. This is a necessary preparatory step in
any low humidity system.
As a general rule, when starting from an ambient condition the system should be purged for 24
hours or more before attempting to operate in the Generate mode. If sufficient purge time is not
allowed, condensed or trapped water will remain and system accuracy will suffer. Insufficient
purge time is usually indicated by higher than normal (wetter than normal) indications of the
device under test. These indications can be as little as a few tenths of a degree to as much as
several degrees frost point.
Purging should also be performed while transitioning from warmer to colder saturation
temperatures, and for approximately 5 hours after each 500 hours of continuous Generate mode
operation.
During Purge mode, both flow control and saturation temperature control are active, but
saturation pressure control is disabled. The generator will attempt to achieve the indicated flow
and saturation temperature setpoint values.
Notes
-1) When the saturation temperature is lowered, the fluid jacket
surrounding the saturator cools in order to reduce the saturation
temperature to its new setpoint value. As the saturator cools during
this transition period, temperature gradients will exist between the
inside of the saturator and the fluid jacket that surrounds it. The
saturator outlet passes through this fluid jacket and will also exhibit
temperature gradients along its length. If gas is allowed to flow
normally through the saturator during this cooling period, the
100% humidified gas of the saturator may condense at the colder
saturator outlet. Therefore, Purge mode should be used while
cooling the saturator to lower temperatures. For this reason, the
lowest humidity of a generation sequence or profile should be
performed first. This low to high order requires that a Purge be
performed only once prior to the generation sequence when cooling
to the lowest saturation temperature. Then as humidity values are
increased, warming the saturator to higher values, further purging
is not required.
2) During Purge, no gas flows to the 3900 Conditioned Gas Outlet,
and consequently no gas flows through the device under test if
connected.
