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Operation
4.6 iRIS 150FX Metering Mode
When the iRIS 150FX is used in a straightforward metering installation, it is often desirable to view the current flow rate
and volume totals on the primary LCD screen for ease of use.
There have been several requests for a customised version of software for the iRIS 150FX that provides just such a
display for the user. Also the calculation of either flow rate or volume total from a single sensor has been requested. The
iRIS 150FX already supports that feature. However, with the recent release of standard software V1.20, the metering
variant is now being addressed. As part of this, the calculation of the derived value will be reviewed. The custom meter
software variant will make the following assumptions:
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Flow rate will always be in litres per second.
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Accumulated volume (total) will always be in cubic metres.
Difference between “Pulse” and “Frequency” signal processing
Any pulse signal is of course a flow rate (volume over time). However, because of how the internal processing is done, the
iRIS 150FX treats pulse signals differently depending on the nominal pulse rate and the configured sensor source.
“Pulse” is used for slow pulses where each one represents a significant volume, typically 100 or 1000 litres. Therefore the
pulses are directly accumulated as an accumulated volume, like rainfall.
“Frequency” is used for fast pulse rates where each pulse typically represents one litre or less and at the once-per-second
processing rate of the iRIS 150FX is better managed as a frequency and therefore deemed a flow rate.
There are three specific installation types that the iRIS 150FX metering software variant will support:
1. The meter provides only a slow pulse type signal as an accumulated volume
In this case, the flow rate is calculated from the volume pulses. Because the rate at which pulses are received can vary, the
response and accuracy of the calculated flow rate may depend on various circumstances.
The meter’s pulse output must be connected to digital input 1 and sensor 1 configured as a “pulse” source and the
multiplier defined to give a total in cubic metres.
Actual accumulated volume: Array id 1
Calculated flow rate: Array id 81
2. The meter provides only a flow rate in the form of an analogue, frequency or other signal
The flow rate can be derived from any source – typically analogue (4 - 20 mA) or digital frequency signal, but also could
be from an intelligent sensor type such as SDI-12. The only constraint is the flow rate must be scaled to litres per second.
The accumulated volume is derived by integrating the flow rate signal to an accumulating register. The difference in this at
each logging interval is stored as the derived volume value.
Actual flow rate: Array id 1
Calculated accumulated volume: Array id 81
3. The meter provides both a pulse and a flow rate signal
This type of installation provides the best quality data, but is involves a more sophisticated flow meter. The installation
can be wired to suit as long as sensor 1 is the accumulated volume in cubic metres and sensor 2 is flow rate in litres per
second. There are no derived values calculated in this installation.
Actual accumulated volume: Array id 1
Actual flow rate: Array id 2
Calculated flow rate algorithm
Option 1
This is the way the iRIS 150FX currently works. The total number of cubic metres accumulated within the logging period is
divided by the number of seconds in that the logging period. This derives reasonably accurate flow rate over the logging
period, however it is only calculated at the time of logging and therefore does not vary throughout the logging period.
Option 2
This would measure the time between pulses and derive a basic flow rate. Depending on how quickly the volume pulses
are received will determine the rate at which the flow rate is calculated. Pulses received rapidly will provide a fast update,
but is likely to lead to poor accuracy especially if there are only a few seconds between each one. The other disadvantage
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