9.1 BASIC BLOCK DIAGRAM DESCRIPTION
See the Basic Block Diagram, Figure 9-1.
Room air enters the instrument, via the fitting on the panel. (Note: See the Installation
chapter regarding use of an instrument air line.)
A built-in Vacuum Pump is used to draw in the air sample, and to provide a positive
pressure in the Test Chamber.
The air flow is than divided and sent to a pair of finely matched volumetric Control
Valves. The RH CAL independently modulates the “Dry” and “Wet” valves from full-
open to full-closed, or any points between. The “Wet” valve feeds the heated
Saturator, which has a water fill mounted on the panel. The “Dry” valve feeds the Air
Dryer. When a desired RH setting is programmed into the instrument by the user, the
Control Valves automatically control the proper mixing of the wet and dry air in the
Test Chamber.
A Dew Point (DP) sensor is mounted within the Test Chamber. Control circuits on the
Circuit Board control the sensor mirror temperature, which tracks the Dew Point up
and down as it changes. An Air Temperature (AT) sensor is also installed in the Test
Chamber. Mounted in each of these sensors is a precise platinum thermometer to
provide AT and DP information. When the desired RH is selected by the user, a
microprocessor on the Circuit Board converts this information to Percent Relative
Humidity (RH). The proportion of wet and dry air in the chamber is automatically mixed
to keep it at the programmed RH at all times. In addition, the chamber Air
Temperature(AT) will also be controlled at any point programmed by the user.
An LCD Digital Display is mounted on the panel. It provides all pertinent information to
the operator, and allows programming of all parameters, including real time data. The
Keypad is used to enter desired programming information to the RH CAL.
The Analog Output (selectable 0 to 5 Vdc or 4 to 20 mA) is provided at a connector on
the panel. The Digital Output (RS-232C Serial Port) is also mounted on the panel.
A Power Supply module converts the AC power line voltage to DC to power the
electronic circuits.
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