The Model 3900 Low Humidity Generator is an extremely accurate means of producing known humidity values for calibrating and verifying humidity instrumentation. Based on the combined, NIST proven, fundamental “two-pressure” and “two-temperature” principles, this system will automatically supply a continuous humidified gas stream, within the frost/dew point range of –95.0 °C to 10.0 °C, for days or even weeks unattended. Simply apply power to the system, and the 3900 will power-up ready to purge and/or generate. Humidity setpoint values are input by the operator from the front panel keypad and are limited only by the operational range of the 3900 humidity generator.


  • Traceable to NIST
  • Based on NIST Proven Two-Pressure Principle
  • Push Button (Keypad) Control
  • Automated Control of User Setpoints
  • 3900 ControLog® Automation Software
  • HumiCalc® with Uncertainty Software
  • Automatically Applies Enhancement Factors
  • Computerized Internal Transducer Calibration
  • RS-232C Serial Interface
  • Only Nine Square Feet of Floor Space
  • Timed/Formatted Output to Printer
  • Battery Backed-up Real Time Clock
  • Backlit Liquid Crystal Display


Principle of Operation

The “two-pressure two-temperature” generation process involves saturating a continuous stream of air or nitrogen with water vapor at a known temperature and pressure. The saturated high pressure air then passes through an expansion valve where it expands to a lower pressure. The 3900 generates a particular humidity by first selecting a suitable saturation temperature, Ts. It then determines the saturation pressure, Ps, required to establish the correct saturation vapor pressure.

The precision of the system is determined by the accuracy of the temperature and pressure measurements and on the constancy of them throughout. When setpoint equilibration has been reached, the indication of saturation temperature, saturation pressure, test temperature, and test pressure may be used in the determination of all hygrometric parameters. Furthermore, because the humidity generated is based solely on the fundamental principles of temperature and pressure, no humidity sensing is used to measure or control the amount of water vapor produced by this system.

The 3900 operates using an on board multifunction CPU in conjunction with other peripheral cards to perform calculation and control functions. The embedded computer control system allows the 3900 to generate known humidity levels completely unattended with visual indications of system status displayed in real time on the Liquid Crystal Display.


Control / Display Screen

This frees the operating technician from the task of system monitoring and adjustment. A computer and/or printer may be connected via the bi-directional RS-232C interface ports allowing remote setpoint control and continuous system data retrieval.


Temperature Control

Temperature setpoint control is attained by controlling the temperature of a circulating fluid medium that jackets the saturator of the generator. The saturation temperature is governed by the temperature of this medium, which is digitally controlled by the computer at any value between –80 °C and 12 °C through the use of PID (proportional-integral-derivative) algorithms.


Pressure and Flow Control

Pressure control and mass flow rate control are accomplished through computer actuation of electromechanical valve assemblies. Saturation pressure and mass flow are measured continuously and controlled using PID algorithms similar to those employed in temperature control.


Two-Pressure Two-Temperature Generator

Regulated compressed air or nitrogen is directed through the saturator, which is a fluid encapsulated heat exchanger containing several planes of pure ice or water. The saturator is maintained at the required saturation temperature and saturation pressure. As the gas thermally equilibrates, it becomes saturated with water vapor. The saturation temperature, Ts, and saturation pressure, Ps are measured at the point of final saturation. The saturation pressure is then reduced to test pressure, Pt, and the conditioned gas is admitted to the device under test (DUT) at the desired humidity conditions. The final pressure, Pt, and temperature, Tt, of the gas is measured within or just after the DUT. The DUT is then exhausted to atmosphere or to a back pressure regulator to achieve pressure control.



Proper calibration of the temperature and pressure transducers ultimately determines the accuracy of the generator. This system employs an integral programmatic calibration scheme allowing the transducers to be calibrated while they are electrically connected to the humidity generator.


Calibration / Display Screen

This approach helps eliminate systemic errors that might be induced by removing the transducers from the generator. All calibration is performed mathematically by the computer so manual adjustments are not needed. Coefficients for each transducer are calculated by the computer and stored in the system’s nonvolatile memory until the next calibration is performed.

The main panel has easy access to the test temperature and the test pressure probe conectors, and you can see your results promptly on the liquid crystal display.

Specifications at a glance:-

Manufacturer reserves the right to change the design or specification without prior notice.