Computer / Control System

The Computer/Control System performs all control functions required for humidity generation, as well as displaying, printing, and storing system parameters in real time. The computer/controller is made up of several main components, each with individual yet cooperative functions. The Computer/Control System utilizes a Windows based computer system, consisting of a 5x86 @133MHz CPU, flat panel VGA color liquid crystal display with built-in touch screen, PC-104 bus, IEEE-488 interface, keyboard, cables, 16+ MB DRAM, 500+ MB hard disk, 3.5" floppy disc drive, power supply, HP-IB interface, and our new ControLog ^TM software. The computer system communicates with an HP3852A data acquisition/control system that consists of an integrating 5-1/2 digit volt/ohmmeter employing: multiplexed inputs to read transducers and PRTs; digital outputs for control of temperatures, pressures, and mass flow; and relay outputs for control of system power, heaters, compressor and circulation pump.

Temperature Controlled Bath: The 9000 humidity generating system incorporates a computer controlled temperature bath. Bath temperature is digitally controlled by the computer at any value between 0°C and 70°C using PID (proportional-integral-derivative) algorithms. The test chamber, saturators, heat exchangers, and connecting tubing are immersed in approximately 20 gallons of distilled water that is circulated at the rate of 50 gallons per minute by a magnetically coupled centrifugal pump. Fast fluid circulation provides the temperature conditioning of these components, resulting in long term bath stability and uniformity. This allows a very stable humidity to be generated.

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

Calibration: Proper calibration of the temperature and pressure transducers ultimately determines the accuracy of the generator. The 9000 employs an integral programmatic calibration scheme allowing the transducers to be calibrated while they are electrically connected to the humidity generator. Coefficients for each transducer are calculated by the computer and stored to memory.

Test Chamber

The 9000 humidity generating system incorporates a completely immersed test chamber, with internal dimensions of 12" x12" x12" (305 mm x 305 mm x 305 mm). Test chamber pressure range is ambient to 20 psiA. The main chamber cover is removable utilizing quick release hold downs. Removal of the chamber cover allows a full 12 inch by 12 inch (305 mm x 305 mm) access to the test space. Access is also available through two 3.65" (93 mm) diameter ports in the chamber cover or two 1.125" (28.6 mm) inside diameter port cover adapters.

Applications for Use

The test chamber can accommodate various solid state sensors, chilled mirror hygrometers, psychrometers, hygrothermographs, and material samples for environmental testing. Virtually any humidity and temperature may be generated, for any length of time, within the operational limits of the generator. The output or recording of the device under test may then be compared with the generators printed data for analysis.

Chilled Mirror Hygrometers: Install the actual chilled mirror head into the chamber or insert a sample tube through the test port and draw a sample through the chilled mirror head and you can: verify mirror temperature measurement accuracy (calibration) when the hygrometer is in thermal equilibrium with its environment; perform operational checks of the heatpump and optical components before and after mirror cleaning and balancing; determine whether the hygrometer is controlling the mirror deposit in the liquid phase or ice phase when operating at dew and frost points below 0°C; determine if the hygrometer is correctly calculating other humidity parameters; determine hygrometers repeatability, stability, and drift characteristics.

Humidity Sensors And Chart Recorders: Insert your humidity probes through a test port in the chamber or install the hygrothermograph into the chamber and you can: determine humidity calibration accuracy and/or characterize humidity sensitivity by subjecting the humidity sensor to a variety of humidity levels; perform operational checks such as the sensing systems capability to correctly calc