HA243821
Issue 5 Mar 98
Section 4
Page 4 - 23
MODEL 346 INSTALLATION AND OPERATION MANUAL
4.10 ALARM CONFIGURATION (Cont.)
4.10.3 ALARM TYPE (Cont.)
APPLICATIONS EXAMPLES
1. RATE-OF-CHANGE, RISING
When a large piece of rotating machinery is cold started, it is important to know if any of the bearings are overheat-
ing. An absolute alarm could be used but, during start-up, it would not give a warning until the bearing reached
its normal operating temperature, which could be too late.If however, a rate-of-change type of alarm is used, it
can be detected that the bearing is heating up more quickly than it should, thus giving advance warning of
possible failure, allowing corrective action to be taken before any major damage is done.
2. ABSOLUTE HIGH, WITH HYSTERESIS
If a tank temperature absolute alarm is set to become active at say, 80 ˚C, then the alarm may become ‘jittery’ at
80 degrees, since a fall of only 0.1 degrees is sufficient to reset the alarm. Thus, any turbulence within the tank
contents could cause the alarm to move continuously into and out of its active state, until the tank contents
reached a temperature well above the set point. If the temperature of the tank is actually to be controlled at the
setpoint temperature, the situation becomes even more unstable.
This problem is overcome using ‘hysteresis’, which effectively defines a deadband below the setpoint. In this
example, a hysteresis of 5 degrees would still allow the alarm to become active at 80 degrees, but it would not
reset until the temperature had fallen to 75 degrees.
4.10.4 ALARM SETPOINT
AXY Setpoint
X.XXXX Units
An alarm setpoint is the value against which the measured value is compared. The up/down/side arrow keys are
used to edit or enter a setpoint.
Notes...
1
Absolute High and Low setpoints are defined in terms of the measured value and represent levels beyond
which an alarm becomes active.
2. A deadband (deviation) setpoint defines the centre of a band of measured values, within which the alarm is
not active.
3. A rate-of-change setpoint represents a change in measured value which may not be exceeded (in the period to
be configured in the RATE page described below) without the alarm becoming active
4.10.5 ALARM HYSTERESIS
AXY Hysteresis
X.XXXX Units
If an absolute or deadband alarm resets exactly at the setpoint or edge of band, it can cycle in and out of Alarm
frequently with a very slight change in measured value. Hysteresis prevents this by establishing the number of
Units by which the Measured Value must pass the setpoint in the ‘out-of-alarm’ direction, before the Alarm resets.
The up/down/side arrow keys are used to enter the value.
