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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
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