Watlow Series 998 - Manual (Page 105)

Auto/Manual Operation
WATLOW Series 998 User’s Manual 7.3 Tuning, Alarms and Error Codes, Chapter 7
and [``rA] or [``dE] are inoperative, and the 998 functions as a sim-
ple on/off control.) The [``HY] (hysterisis) prompt determines the
switching differential value.
3. Reset/Integral Adjustment: Gradually increase [``rE] or decrease
[``It] until the upper display process value begins to oscillate or
“hunt.” Then slowly decrease [``rE] or increase [``It] until the upper
display stabilizes again near set point.
4. Cycle Time Adjustment: Set [``Ct] as required. Faster cycle times
sometimes achieve the best system control. See Chapter 8, “Burst
Firing” for more information. However, if a mechanical contactor or sole-
noid is switching power to the load, a longer cycle time may be desirable
to minimize wear on the mechanical components. Experiment until the
cycle time is consistent with the quality of control you want. [``Ct] will
not appear on units with a process output (option “F”).
5. Rate/Derivative Adjustment: Increase [``rA] or [``dE] to 1.00
minute. Then raise set point by 20° to 30°F, or 11° to 17°C. Observe the
system’s approach to the set point. If the load process value overshoots
the set point, increase [``rA] or [``dE] to 2.00 minutes.
Raise the set point by 20° to 30°F, or 11° to 17°C and watch the
approach to the new set point. If you increase [``rA] or [``dE] too
much, approach to the set point is very sluggish. Repeat as necessary
until the system rises to the new set point without overshooting or
approaching the set point too slowly.
6. Calibration Offset Adjustment: You may want your system to control
to a process value other than the value coming from the input sensor. If
so, measure the difference between that process value (perhaps at
another point in the system) and the process value showing in the
upper display. Then enter the [`CAL] offset value you want. Calibration
Offset adds or subtracts degrees from the value of the input signal.
Manual and Automatic Operation
To change from auto to manual, first select the desired channel using the
Display key. Press the Display ∂ key until the desired CH A or CH B
indicator light is lit. Only one should be lit. Then press the Auto/Manual
key å twice. Note: Both channels can operate in the manual modes
simultaneously.
Manual operation provides open-loop control of the outputs from a range
of -100% to 100% output for heat/cool operation, 0% to 100% for heat
only and -100% to 0% for cool only. The 998 allows a negative output
value only if [Ot1A], [Ot2A], [Ot1b] or [Ot2b] is set to cool. Automatic
operation provides closed loop on/off or PID control. When the operator
transfers from a closed loop to an open loop, the 998 sets the power level
to the setting of the [FAIL] parameter. If it is set to [bPLS] (bumpless
transfer) then it retains the power level of the closed-loop control. When
the 998 returns to closed loop control, it restores the previous set point
process value.
The Auto/Manual light (located at the key) indicates auto or manual oper-
ation. When it is on, the channel indicated by the CH A or CH B light, is