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1.6.2 Presaturator
The airstream of a two-pressure generator must be 100% saturated with water vapor at test
temperature on the high-pressure (saturator) side of the expansion valve. This is
accomplished by first passing the airstream through a "presaturator" (PSAT). The
presaturator is a vertical pressure vessel presenting a water surface to the incoming airstream
and is maintained constant at a temperature 15 to 20 °C warmer than the desired system
(chamber) temperature. After presaturation, the air exits the top of the presaturator and is
directed to the saturator.
Presaturatio
n temperature, sensed by a 10KΩ thermistor, is adjustable and controllable from
20 to 90 °C. Heating of the presaturator is provided by two 250-watt stainless steel
immersion type heaters, powered through a solid state relay (SSR9).
The presaturator water level is maintained automatically via computer control of the
presaturator liquid level control solenoid valve (SOL4). This valve controls the flow of
distilled water from the supply reservoir (RES) (refer to 1.6.5) to the presaturator,
maintaining the level constant to within approximately 1/16 inch (1 oz.).
1.6.3 Saturator
Saturation of the airstream to 100% relative humidity, upstream of the expansion valve, is
accomplished in a "tube in shell" type heat exchanger referred to as the "saturator". The
humidified airstream from the presaturator, humidified to an absolute moisture content
greater than saturation at final saturation temperature, flows through the saturator. Excess
water vapor is condensed from the airstream as it establishes equilibrium with the heat
transfer fluid, ensuring that the airstream has a value of 100% relative humidity at fluid
(saturation) temperature. The saturation pressure is monitored as the airstream exits the
saturator. This measurement is used in the calculation and control of the desired relative
humidity.
1.6.4 Expansion Valve
After exiting the saturator, the 100% saturated high-pressure airstream is reduced to chamber
pressure through the "expansion valve". The expansion valve (V2) is an electro-mechanically
actuated valve, controlled from the computer, using feedback from the saturator and chamber
pressure measurements. This valve is heated and thermally insulated to maintain the valve
body above the dewpoint of the adiabatically cooled airstream.
1.6.5 Reservoir
The reservoir is a pressure vessel, constructed of 300 series stainless steel, and holds a one
gallon supply of distilled water for the presaturator. This vessel is maintained at system
pressure and, upon demand, allows reserve distilled water to flow freely to the presaturator.
If a low level reservoir water condition is sensed during operation of the humidity generator,
a warning message will begin flashing on the display, accompanied by an audible warning
beep.
A one gallon supply of distilled water may last from 12 hours to 9 months of continuous
operation, depending on the temperature and RH being generated. High temperatures and
high humidities require large amounts of water, while low temperatures and low humidities
require very small amounts of water. See section 3.2.9 for reservoir filling procedure.
