Xylem SDI-12 Data Logger iRIS 220, iRIS 320, iRIS 350, iRIS 150, iRIS 300 - Manual (Page 71)

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8 Analog Input Scaling
This section explains the recommended procedure to use when scaling an analog input (voltage or current).
It makes use of the scaling calculator provided in the iLink program. However, the calculation can also be
done manually using this formula. V = Input V, EU = Engineering Units (scaled output e.g. metres).
Multiplier = (Maximum EU – Minimum EU) / (Maximum V – Minimum V)
Offset = Maximum EU - (Multiplier * Maximum V)
8.1 Example: A 4-20mA Water Level Sensor
In this example, we have a water level instrument connected to the iRIS Analog Input 1. This instrument is
designed to provide a nominal 4-20mA signal for a 0-10 metre water level range. However, in the real world,
most instruments are not exact and a small difference in actual signal may occur. In addition, the internal
current sink resistor in the iRIS is 100
Ω (theoretically giving 2V at 20mA). There is a very small reduction in
resistance caused by the internal input impedance of the iRIS input channel to take into account (see
Section 0 Analog Inputs). Therefore the voltage measured by the iRIS for a given water level is slightly
different.
1. Power up the installation and allow it to stabilize.
2. Set the instrument to zero water level (using a calibrator)
3. Note the actual voltage measured by the iRIS (view this on the LCD on “System Status 3”
screen). See Section 5.2.5. The LCD screen example below shows 0.3932V for a 0m input level.
4. Now increase the instrument input to full scale (10m) using the calibrator. Again, note the
measured voltage relating to this input level. This time it is 1.9617V for a 10m level input.
5.
Using the iLink Scaling Calculator, enter the minimum and maximum measured voltages and the
levels they represent in the appropriate fields. Click the “Calculate” button to generate the
correct multiplier and offset parameters to use in the sensor setup menu for Sensor 1 as shown.
AI1: 0.3932 V
AI2: 0.0000 V
AI3: 0.0000 V
AI4: 0.0000 V
AI1: 1.9617 V
AI2: 0.0000 V
AI3: 0.0000 V
AI4: 0.0000 V
* Sensor 1 Cfg
(Now: 0.000)
0 Exit
1 Source [1: Analog 1]
2 Name [Water Lvl]
3 Mode [Period Avg]
4 Multiplier [ 6.40082]
5 Offset [ -2.55649]
6 Log Multiplier [1000]
7 Log Rate [15min]
8 Alarms
9 Data
>
Min V
Max V
Max WL
Min WL