Tenney BTS - Page 7

TENNEY ENVIRONMENTAL
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TEMPERATURE CONTROL - WATLOW 942 CONTROLLER
1
Temperature conditions are attained and controlled by a Watlow 942 microprocessor based
controller. Temperature measurements are made utilizing a 100 ohm platinum RTD sensor.
This controller features dual outputs, auto-tuning control with 24 step program capability and
easy fixed set point operation.
Heating of the chamber is accomplished with the time proportioned (PID) Output
ICI-01 of the
controller, which
turns on a triac in the heater circuit supplying power to the heater elements.
Cooling of the chamber is accomplished with
ONIOFF Output ICI-02 of the controller, which
energizes a contactor supplying power to the refrigeration circuitry.
IMPORTANTI Output 1CI-02 is set up for ONIOFF control and the PID value is set to zero
Do not change the PID value for channel two under any circumstance!
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Corn~ressor Turn-On Loaic
In determining when to turn the refrigeration system on or off, the controller's logic will compare
the following signals and setpoints.
1. Actual Chamber Temperature
2. Setpoint Temperature
3. Hysteresis
4. Deadband
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Hysteresis (HYS) and deadband (db) are parameters entered at the factory. Hysteresis is the
temperature change necessary to change the output from full ON to full
OFF and is detailed in
the Setup Menu section in the Watlow 942 Manual.
Deadband is the span in which Output ICI-
02 will remain ON below the controller's low setpoint, and is detailed in the Operation Menu
section of the Watlow Manual.
HYS and
db are entered in degrees C. Deadband is always entered as a (-) minus. The logic is
as follows:
Measured temperature rises above setpoint: Refrigeration turn-on equals
hysteresis minus deadband.
Measured temperature falls below setpoint: Turn-on is always above setpoint.
Tum-off is always below setpoint:
Turn-on is always above setpoint.
EXAMPLE SETTINGS FOR HYS AND db
HYS db Turn-On (Above SP) Turn-OR (Below SP)
Standard 4
-3 1
5 -3 2
5 - 1 4
No Good
3 0 3