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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
circumstances, for example, if the dosing
terminals are closed for too long a period.
In both cases, the red LED’s will also pro-
vide a visual warning signal.
The Fail Safe mode is accomplished by connecting the
external alarm circuit between the FS•O (Normally Open)
and the COM terminals. This way, an alarm will warn the
user when pH goes over the alarm thresholds, the power
breaks down and in case of a broken wire between the
process meter and the external alarm circuit.
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
Model HI21523 has a proportional output signal (selectable
among 0-1 mA, 0-20 mA, 4-20 mA, 0-5 VDC, 1-5 VDC
and 0-10 VDC) at the analog output terminals. With this
output, the actual output level amplitude is varied, rather
than the proportion of ON and OFF times (duty cycle con-
trol). A device with analog input (e.g. a pump with a 4-20
mA input) can be connected to these terminals.
Note
Models with this feature don’t have the output relay for a
duty cycle control.
