Optiquest iRIS 320, iRIS 220 manual Analogue Input Scaling, Example A 4-20mA Water Level Sensor

iRIS 220 / iRIS 320, V1.19 User Guide - 60

7 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 utility. However, the calculation can be done manually using this formula. NOTE: mV = Input mV, EU = Engineering Units (scaled output e.g metres)

Multiplier = (Maximum EU – Minimum EU) / (Maximum mV – Mi nimum mV)

Offset = Maximum EU - (Multiplier * Maximum mV)

7.1 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. In addition, the current sink resistor should ideally be 250￿ (giving 5V at 20mA), but only a 249￿ part was available. There is also the small reduction in resistance caused by the internal input impedance of the iRIS input channel to take into account (see Section 3.4.4 Analogue Inputs). Therefore the voltage measured by the iRIS for a given water level is slightly different.

1.Power up the installation and allow 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 993mV 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 4961mV for a 10m level input.

AI1: 993 mV

AI2: 0 mV

AI3: 0 mV

AI4: 0 mV

AI1: 4961 mV

AI2: 0 mV

AI3: 0 mV

AI4: 0 mV

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.

Min mV

Min WL

Max mV

Max WL

*Sensor 1 Cfg (Now: 0.000)

0 Exit

Source [1: Analog 1]

2 [Water Lvl]

3 Mode Avg]

4[ 0.00252]

5Offset [ -2.50252]

6Log Multiplier [1000]

7Log Rate [15min]

8Alarms

9Data

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