Eurotherm 394, User Manual

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MULTI-POINT CIRCULAR-CHART RECORDER: USER GUIDE
Page 49
HA250385
Issue 7 Nov 06
0 2 4 6 8 10
Litres x 1000
50
40
30
20
10
0
Minutes
Rate of change - rise
alarm active
Rate of change - fall
alarm active
Rate of change - fall
alarm active
In this example, both rise and fall
alarms are triggered at 200 litres
per minute
Earliest part of chart
Rate of change - rise
alarm active
Latest part of chart
Measured
value
4.5.2 ALARM CONFIGURATION (Cont.)
Rate-of-change alarms
With rate-of-change alarms, a value, a time period and
an averaging period have to be configured. In the ac-
companying sketch, the Value is 200 litres and the Time
Period is one minute.
The alarm is triggered if the PV changes by more than
the configured Value in less than the configured Time
Period (i.e. more than 200 litres/minute in the sketch)
The averaging period can be used to change the sensitiv-
ity of the alarm, such that noise spikes or normal oscil-
lations in the input signal do not trigger false alarms.
Figure 4.5.2e Rate-of-change alarm definitions
ALARM PARAMETERS
THRESHOLD
Sometimes called 'setpoint', this is the trip point for absolute alarms, entered using the up and down arrow keys.
REFERENCE
For Deviation alarms, this sets a 'central' value on each side of which the Deviation Value (see immediately below) is
to operate. The value is entered using the up and down arrow keys.
DEVIATION
For Deviation alarms only, this is a value each side of the reference value, within which a Deviation IN alarm is
active, and outside which a Deviation OUT alarm is active. See
figures 4.5.2c and 4.5.2d.
CHANGE
For Rate-of-Change alarms only, this allows a value (D) to be entered using the up/down arrow keys. If the change in
the channel value (
∆
PV) over the specified time period T (see below) is greater than D (
∆
PV/T > D) then the alarm is
tripped.
PER
For Rate-of-Change alarms, allows a time period to be selected for the above change value. The up arrow key allows
1 second, 1 minute or 1 hour to be selected as the period.
AVERAGE
Allows a period of 0 to 9 seconds to be entered for Rate-of-Change alarms. This has the effect of preventing spurious
alarms being triggered by transient changes in the PV value.
HYSTERESIS
Allows a 'deadband' to be entered for absolute and deviation alarms, to prevent alarms being continuously triggered if
the process variable value hovers around the trip point. The value is entered using the up/down arrow keys.
DWELL
This feature allows the triggering of any alarm to be delayed for a period configurable up to 2047 seconds. If the
alarm source returns to a non-alarm state during the Dwell period, the alarm is ignored. The dwell period is entered
using the up/down arrow keys.
ALARM JOBS
As shown in figure 4.5.2a , the jobs page is reached by operating the page key from the Alarm : Setpoint page. Two
jobs can be set up for each alarm, and the actions they can carry out are as shown in the Jobs description in section
4.1.5