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2.3.7 Scaled Pulse Output
The Scaled Pulse Output is used to transmit the totalised volume at a user selectable
resolution; this is programmed as a volume quantity per output pulse (e.g. 10Litres/Pulse or
100mL/Pulse). The frequency of the Scaled Pulse Output signal automatically adjusts according
to the input frequency and the scale factor, up to a maximum limit of 100Hz.
The Scaled Pulse signal can be transmitted in the same way as the Unscaled Pulse signal
above, either through the passive or isolated output.
The pulse width of the Scaled Pulse output is fixed at 300ms for output frequencies below
1.67Hz, however once above 1.67Hz the instrument adjusts the pulse width according to the
frequency to give a 50:50 duty cycle up to the maximum output frequency of 100Hz. Systems
should be designed to stay below the 100Hz limit, however if 100Hz is exceeded the
instrument will output at 100Hz and store any excess pulses in memory. Pulses stored in
memory will continue to output after the flow has stopped until the stored pulse count
reaches zero.
2.3.8 Alarm Output
Flow Rate Alarms may be programmed for High, Low, or High/Low; the digital output will
trigger when the registered flowrate passes the configured set-point(s). For a Low Alarm the
output will trigger once the flowrate drops below the set-point, a High Alarm will trigger with a
flowrate over the set-point, and a High/Low alarm will trigger if the flowrate is either over the
upper set-point or below the lower-set point.
Each set-point is configured with its own dead-band percentage which allows the user to set
a buffer zone (hysteresis) to avoid alarm “chattering” (switching quickly on-off-on-off) with a
flowrate hovering around the set-point. The dead-band setting is entered as a percentage of
the set-point value; e.g. a 5% dead-band with a 100L/min High Flow Alarm will trigger ON at
100L/min but will not turn OFF until the flow drops to below 95L/min.
Alarm Outputs can operate as an NPN signal via the passive digital output, or as a PNP or
NPN signal via the opto-isolated output. For relay switching be sure to follow the requirements
in the relevant wiring diagram in section 4.3.
2.3.9 Passive NPN Transistor Output
The passive NPN transistor output is the default method of transmitting the pulse or alarm
outputs, as it is the easiest and lowest cost solution where an NPN signal below 30VDC is
suitable. The passive NPN output shares a common ground inside the instrument, so may not
be suitable for installations requiring an analogue output in combination with a digital output,
as it may cause analogue signal issues due to the formation of a ground loop.
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General