Operating Manual

Chapter 3 Set up Level, Level Alarms, and Volume Accu-Wave
3-8 TN Technologies
Figure 3.5 Set Point / Clear Point Alarm
High Limit and Low Limit Alarms
An alarm is activated when the measurement value reaches the specified set point. The
relative values assigned to the set point and the clear point determine whether the alarm is
a low limit alarm or a high limit alarm.
If the set point value is less than the clear point (or the dead band value is positive), the
alarm is a low limit alarm. In this case, the alarm is activated as the measurement value
decreases below the set point value. The alarm stays active until the measurement value
again increases to the clear point value.
Similarly, if the set point value is greater than the clear point (or the dead band value is
negative), the alarm is a high limit alarm. In this case, the alarm is activated when the
measurement value increases beyond the set point value. The alarm stays active until the
measurement value again decreases to the clear point value.
The terms low limit and high limit refer to the relative magnitude of the set point and the
clear point, not necessarily to a high or low level. The behavior of the alarm depends on
the measurement associated with the alarm. Thus, a low limit alarm (set point < clear
point) for a distance measurement is in fact a high level alarm. A low limit distance alarm
would be triggered when the distance to the process surface decreases below the clear point
distance and reaches the set point distance value. Since the distance between the material
surface and the sensor is decreasing the level is actually increasing. Specific examples of
high limit and low limit alarms are provided later in this chapter.
b. Modified
Set Point /
Dead Band
a. Initial Set
Point /
Dead Band
Initial Set
Point (2 m)
Implied Clear
Point (4 m)
Dead Band
(2 m)
Initial Set
Point (3 m)
Implied Clear
Point (5 m)
Dead Band
(2 m)