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28
Note
In order to have the Fail Safe feature activated, an external
power supply has to be connected to the alarm device.
CONTROL THROUGH ANALOG OUTPUT
Models
HI 23xy1
and
HI 24xy1
have a proportional ana-
log output signal (selectable among 0-1mA, 0-20mA,
4-20mA, 0-5VDC, 1-5VDC and 0-10VDC) at the analog
output terminals.
With this signal, the actual output level amplitude is varied,
rather than the proportion of ON and OFF times (duty cycle
control). A device with analog input (e.g. a pump with a 4-
20 mA input) can be connected to these terminals.
IDLE MODE
IDLE mode is entered through setup code 2.
During idle mode the device performs the same tasks as when
it is in control mode except for the relays. The alarm relay is
activated (no alarm condition), the control relays are not
activated while the analog output remains activated.
When the instrument is in idle mode the red and green status
LEDs are on.
Idle mode is useful to disable control actions when the exter-
nal devices are not installed or when the user detects unusual
circumstances.
Moreover the alarm signal is generated only after a 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 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 ex-
ternal 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.
