1-1
Section 1
GENERAL INFORMATION
1.1
INTRODUCTION
The Model 3900 low humidity generating system is a facility capable of producing known
humidity values using the combined fundamental principles of the "two temperature" and
"two pressure" generators developed by NIST. This system is capable of continuously
supplying accurately known humidity values for instrument calibration and evaluation.
When used within the specified frost point range of -95.00 °C to 10.00 °C, the system will
generate manually entered setpoints for days or even weeks unattended.
The 3900 operates using an embedded computer and control system to perform calculation
and control functions. The Computer Control System utilizes a multifunction CPU in
conjunction with other peripheral cards for control and is incorporated into the 3900 low
humidity generator. Peripheral equipment, such as a printer or computer, may be connected
using the bi-directional RS-232C interfaces.
Humidity and temperature setpoint values are input by the operator from the front panel
keypad. The system is then automatically controlled at a setpoint, with visual indications of
system status displayed in real time on the Liquid Crystal Display. The automatic features of
this system allow the 3900 to generate humidity and temperature setpoints completely
unattended, freeing the operating technician from the task of system monitoring and
adjustment.
1.2
PRINCIPLE OF OPERATION
1.2.1
General Description
The Model 3900 humidity generation system is based on the "two temperature - two
pressure" principle. This process involves saturating air or some other gas, such as nitrogen,
with water vapor at a given temperature and pressure. The saturated high pressure gas is then
reduced to test pressure and warmed to test temperature. The indication of saturation
temperature, saturation pressure, test temperature, and test pressure may be used in the
determination of all hygrometric parameters. Humidity generation by this system does not
depend upon measuring the amount of water vapor, but rather is dependent on the
measurements of temperature and pressure alone. The precision of the system is determined
by the accuracy of the temperature and pressure measurements, and on the constancy of them
throughout.
1.2.2
Humidity Formulas
The humidity (or water vapor content) of a gas may be expressed in a variety of ways. The
humidity parameters available with the 3900, and the formulas used to derive them, will be
expressed in terms of the two-temperature two-pressure generator. While some basic
understanding of humidity is helpful, thorough knowledge of the following formulas and
their relationships to the 3900 is not a requirement for successful operation of the generator.
