6.3 Volume measurement in a storage tank
The amount of grain filled into a silo is to be recorded, displayed and
retransmitted to a PLC. A 4-20 mA level sensor powered by the loop power
from the unit measures the filled height in the silo. The relationship between
filling height (m) and volume (m³) is known and the the sensor current is
proportional to the height. The calculated volume is retransmitted at the
analogue output as a proportional 0-20 mA signal. On system fault the
analogue output transmits a fault signal of 21.0 mA.
Example:
Empty container:
- Sensor signal 4 mA
- Filling height 0 m
- Numeric display is to
show 0 (m³)
- Bargraph is to show
0%
- The analogue output
is to be 0 mA
Full container:
- Sensor signal 20 mA
- Filling height 10 m
- Numeric display is to show 1500 (m³)
- Bargraph is to show 100%
- The analogue output is to be 20 mA
Further entries:
- The analogue output is peak at 21.0 mA in fault condition
- Linearisation table with 10 points
Sensor
value (m)
X1
0.0
X2
0.2
X3
0.4
X4
0.6
X5
0.8
X6
1.0
X7
1.2
X8
1.4
X9
1.6
X10
10.0
Display
value (m³)
Y1
0
Y2
20
Y3
50
Y4
85
Y5
115
Y6
160
Y7
210
Y8
280
Y9
400
Y10
1500
Process transmitter
70
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