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An hysteresis will eliminate the possibility of continuous
sequences ‘energizing/de-energizing’ of the alarm relay
when the measured value is close to the alarm setpoint .
The hysteresis amplitude is 0 .2 pH for pH and 30 mV for
ORP .
Moreover the alarm signal is generated only after an user
selectable time period (alarm mask) has elapsed since
the controlled value has overtaken one alarm threshold .
This additional feature will avoid fake or temporary alarm
conditions .
Note
If the power supply is interrupted, the relay is de-energized
as if in alarm condition to alert the operator .
In addition to the user-selectable alarm relays, the meter is
equipped with the
Fail Safe
alarm feature .
The
Fail Safe
feature protects the process against critical
errors arising from power interruptions, surges and human
errors . This sophisticated yet easy-to-use system resolves
these predicaments on two fronts: hardware and software .
To eliminate problems of blackout and line failure, the
alarm function operates in a “Normally Closed” state and
hence alarm is triggered if the wires are tripped, or when the
power is down . This is an important feature since with most
meters the alarm terminals close only when an abnormal
situation arises, however, due to line interruption, no alarm
is sounded, causing extensive damage . On the other hand,
software is employed to set off the alarm in abnormal circum-
stances, for example, if the dosing terminals are closed for too
long a period . In both cases, the red LED will also provide a
visual warning signal . The Fail Safe mode is accomplished
by connecting the external alarm circuit between the FS•C
(Normally Open) and the COM terminals .
