Hanna Instruments HI 23 Series Instruction Manual page 28

Moreover the alarm signal is generated only after a user se-
lectable time period (alarm mask) has elapsed since the
controlled value has overtaken one alarm threshold. This
additional feature will avoid fake or temporary alarm con-
ditions.
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, all EC/TDS
controllers are 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, how-
ever, due to line interruption, no alarm is sounded, causing
extensive damage. On the other hand, soft-
ware is employed to set off the alarm in
abnormal circumstances, for example, if the
dosing terminals are closed for too long. In
both cases, the red LED will also blink pro-
viding a visual warning signal.
The Fail Safe mode is accomplished by connecting the exter-
nal alarm circuit between the FS•C (Normally Open) and
the COM terminals. This way, an alarm will warn the user
when EC exceeds the alarm thresholds, during power down
and in the case of a broken wire between the process meter
and the external alarm circuit.
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