© 2011 Gastech Australia Pty Ltd
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4.2.5
Fan Control / Alarm Relays
When a gas reading reaches or exceeds the pre-set alarm points, the programmed relay contacts will activate
(see specification table for operating points). The corresponding LED will illuminate on the main control unit
to show that the contact is active. If fitted, the HMI screen will also indicate that a signal is active.
The low and high speed fan control signals can be used to drive the fan system between two fixed speeds
depending on the gas concentration detected.
4.2.6
Variable Speed Enable Signal
The Variable Speed Enable can be used to switch the fan system on and off based on a cycle time compliant
with AS 1668.2. Additionally the timing of this signal is designed to present the least stress to the fan system
by limiting the number of on/off cycles to no more than 4 per hour.
This signal is used in conjunction with the 0 - 10VDC output drive signal to control the fan speed during its on
cycle.
4.2.7
Fan Control Voltage
The main control unit produces a control voltage signal that can be connected to fan controller units to allow
the fans to be driven at variable rates. The control voltage range is the industry standard 0 - 10VDC.
The 0 - 10VDC is scaled such that 0V represents 8ppm of detected carbon monoxide and 10V represents
30ppm of detected carbon monoxide. Between 8ppm and 30ppm, the voltage will be set proportionately.
This allows the fans to work only as hard as is required to move the level of detected carbon monoxide back to
the accepted limits according to AS1668.2. This variable drive in conjunction with the timed enable signal
provide the most efficient form of fan control and is recommended above the use of basic two level on/off
type systems. The control voltage is set based on the highest input signal fed from any of the enabled
channels. For a gas sensor this represents the unit that is sensing the highest concentration level.
