15
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.
