13
3.3.2 Purge output
Used only when the probe is mounted in the blowdown line, Purge ensures the sensor measures
the conductivity at boiler temperature. The purge duration time is the time the valve is open to
enable a representative boiler sample to reach the probe. A purge occurs every 30 minutes
either independent of burner firing, or dependent on cumulative boiler firing time.
3.3.3 Pulsed output
For smaller boilers where the capacity of the blowdown valve is relatively high compared to
the boiler size, the blowdown may be set to pulsed, rather than continuous output, opening
for 10 seconds, and closing for 20 seconds. This slows the rate at which the boiler water is
removed so that the level is not unduly affected, avoiding the risk of triggering a low water alarm.
3.3.4 4 - 20mA retransmit
An isolated 4 - 20 mA output is provided as standard, and may be used for remote display
of the TDS level or as an output to a computerised management system.
3.4 Other features
To prevent unwanted or inadvertent changes being made, most commissioning parameters
are protected with a pass code.
The BC3150 can communicate via an infrared link between adjacent controllers – see
Section 7, Communications.
Fig. 3 Output with purge time set (>0s)
Low valve
open
Valve
closed
Interval
10s
Purge Blowdow n Blowdown Blowdown
Conductivity drops
below set point
Blowdown cycle
20s
20s
Purge Blowdown
Time
Fig. 4 Pulse output with purge time set (>0s)
Water
conductivity
SP
Hysteresis
Closed
High conductivity
Time
Purge Purge
Interval
Blowdown
Keeps valve open until
conductivity drops below
hysteresis
Valve
open
Water
conductivity
SP
Hysteresis
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