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Operation
4.9
Analogue Input Scaling
This section explains the recommended procedure to use when scaling an analogue 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)
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Example: A 4-20mA Water Level Sensor
4.9.1
Example: A 4-20mA Water Level Sensor
In this example, we have a water level instrument connected to the iRIS Analogue 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.
The internal current sink resistor in the iRIS is 100Ω (theoretically giving 2V at 20mA). An external resistor of a
different value can be used if desired. E.g. 250Ω will give 5V at 20mA.
for more information for connecting the internal resistor to the input.
When using the internal resistor, it is important to configure the iRIS’ input to 4-20mA mode, see section
on how to do this. The iRIS is calibrated to account for the manufacturing tolerance of the
internal resistor and uses this to convert from volts to milliamps.
1. When an analogue input is select as a sensor source the Scaling Calculator button should be displayed:
2. Click this button and the Scaling Calculator will be displayed,
note
the input range should be automatically selected.
3. Now simply enter the minimum and maximum water levels, in this example 0 and 10 respectively, into the Scaled
Output Range Min and Max fields.
4. Click the Calculate button, this will generate the Multiplier and Offset. See Figure below.
5. Click the OK button to accept the calculated values, iLink will automatically copy the calculated multiplier and offset
into the given sensors’ configuration fields.
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