Tuning and Operating, Chapter 6
43
WATLOW Series 942 User's Manual
A
Deviation alarm
alerts the operator when the process strays too far from set
point. The operator can enter independent high and low alarm settings. The
reference for the deviation alarm is the set point. Any change in set point causes a
corresponding shift in the deviation alarm.
Example:
If the set point is 100
°
F, and
a deviation alarm is set at +7
°
F as the high limit, and -5
°
F as the low limit, the high
alarm trips at 107
°
F, and the low alarm at 95
°
F. If you change the set point to
130
°
F, the alarms follow the set point and trip at 137
°
F and 125
°
F.
Both Process and Deviation alarms can be latching or non-latching. When the
alarm condition is removed a
non-latching alarm automatically
clears the alarm
output. The operator must
manually clear
a
latching
alarm
before the alarm
disappears. The alarm condition must also be removed before the alarm can be
cleared.
Flashing 'LO" or "HI" in the lower display indicates an alarm. The Lower display
alternately shows information from the current parameter and the "LO" or "HI" alarm
message at one second intervals. The alarm output is de-energized and the L3 or
L4 LED is lit.
To clear an alarm…
•
First correct the alarm condition, then…
• If the alarm is latching…
Clear it manually; press the HOLD/RUN key once as soon as the process
temperature is inside the alarm limit according to the HYSX parameter.
• If the alarm is non-latching…
The alarm will clear itself automatically as soon as the process temperature is
inside the alarm limit according to the HYSX parameter.
Alarms
Alarm Silencing
for auxiliary output 3 is available with the deviation alarm. If it is a
non-latching alarm, this overrides the normal alarm output functions only during
initial power up. If latching, press the HOLD/RUN key once to override the alarm
output function. With alarm silencing, the alarm message and alarm LED's show
the alarm condition but the output remains in a non-alarm state until the process
value is within the safe region of the deviation band. Any future deviation outside
this safe band triggers an alarm.
Press once -
Clear
a latched
and
corrected
alarm.
Figure 40 -
Alarm Display
Examples
Temp
SP1
Actual
0
°
C
Time
AL 2
AL 1
AH 2
AH 1
New SP1
Figure 39 -
Deviation Alarm
Example
WARNING:
An alarm display will
be masked by an
error condition or
when the control is
in the Calibration or
Setup Menus.
!
NOTE:
An X applies to
either Hysteresis 3,
or 4.
NOTE:
For Deviation alarms:
When the End step in
the Program menu is
OFF or OFFA, the
deviation alarms are
active (relay ener-
gized).
For Process alarms:
When the End step is
OFF, process alarms
are disabled in a de-
energized position.
When OFFA is
selected, the process
alarm works normally.
