User Manual & Datasheet V 2.1
0%
43.75%
100%
4.0mA
11.0mA
20.0mA
14 L (Offset)
72+6=78 L
=28/7=4 L /mA (Linear)
Volume/mA
Change of volume per mA is linear from bottom to mid level, while it is non-linear at top
(volume per mA decreases as level approaches 20mA)
PLC thus should convert level (mA) to volume from 4 to 11.0 mA linearly as :-
Volume =Offset Change of Volume per mA x (measured mA - 4)
However, as soon as non-linear range starts (11.0mA) PLC should begin level to volume conversion differently.
The tank now should be strapped into a number of small zones (of level) where volume within each zone could be safely
considered linear. Then as long as level stays between those individual straps, volume can be calculated as
Volume =Previous (difference of volume / difference of mA) x (measured mA - 4)
Previous level is volume measured before existing strap
Difference of volume is Maximum Volume - Min Volume within the strap
Difference of mA is Maximum mA - Min mA within the strap
Measured mA is mA just received by PLC
For example shown above, Volume of tank at various mA is:
= 14L+(28L/7mA)
x(measured mA-4) if measured mA is between 4.0mA & 11.0mA
11.0mA & 13.25mA
= 54L+(10L/2.25mA)x(measured mA-4) if measured mA is between 13.25mA & 15.5mA
= 64L+(8L/2.25mA) x(measured mA-4) if measured mA is between 15.5mA & 17.75mA
= 72L+(6L/2.25mA) x(measured mA-4) if measured mA is between 17.75mA & 20.0mA
= 42L+(12L/2.25mA)x(measured mA-4) if measured mA is between
Volume at various strapping level can be found by taking notes of amount of fluid needed to raise the level, or it can be
calculated mathematically from tank dimensions.
Also, the more the number of straps the better the approximation is.
Strap1
Strap2
Strap3
Strap4
Offset Volume
57.8125%
71.875%
85.9375%
28+14=42 L
Diff of mA =2.25mA
Diff of Vol=12 L
Diff of mA =2.25mA
Diff of Vol=10 L
Diff of mA =2.25mA
Diff of Vol=8 L
Diff of mA =2.25mA
Diff of Vol=6 L
13.25mA
15.5mA
17.75mA
42+12=54 L
54+10=64 L
64+8=72 L
Volume/mA =12/2.25=5.33L/mA
Volume/mA =10/2.25=4.44L/mA
Volume/mA =8/2.25=3.56L/mA
Volume/mA =6/2.25=2.67L/mA
Diff of mA =7 mA
Diff of Vol=28 L
Measurement
begins from here
Tank
Sense Probe
Method of approximating volume of fluid in
tanks
for P LC/SCADA using
4-20mA output
irregular shape
CAP VE L
Caution :-
Devices that work on capacitance principle for measurement of level of fluids, are for indicative
purpose only. These devices should never be used for inventory control, weights and measures or any other
place where highly accurate measurement of fluid level/volume/weight etc is needed.
Figure 4: Tank Strapping
Sapcon Instruments Pvt.Ltd.
R
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