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IDLE MODE
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 acid and base 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.
Control actions are stopped as soon
as the user presses SETUP and enters
the password.
In order to reactivate the control mode, use code 02 of setup
(see “Setup” section). Otherwise, the meter remains in idle
mode.
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, 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’s will also provide a
visual warning signal.
The Fail Safe mode is accomplished by connecting the exter-
nal 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 HI 21523 has a proportional output signal (select-
able 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.
