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Applications Examples
Generic Application Calculations
The capacitance expected in a cylindrical tank with a probe centrally mounted is
estimated using the following formula:
24 L
------------------------- - pF
C
=
Log D d
Where:
C = capacitance value in pF
= relative dielectric constant
r
L
= active measurement length
D = internal tank diameter
d = electrode diameter
24 = a K constant for dimensions in meters
7.32 = a K constant for dimensions in feet or inches
= 1 (air)
r
= 2 (oil)
r
Examples (using dimensions above):
Capacitance in air:
Dimensions in meters:
Dimensions in feet:
Capacitance in oil:
If the same vessel is filled with oil, relative dielectric constant for oil (2) replaces the
relative dielectric constant for air (1), and the resulting calculation is:
---------------------------------- - pF
C oil
=
oil
Log 1 0.016
or
--------------------------------------- - pF
C oil
=
oil
Log 3.28 0.05
The initial capacitance value at 0% (probe in air) is 12.7 pF, and the capacitance value for
100% (tank filled with oil) is 25.4 pF.
After calibration:
12.7 pF 0% 4 mA or 20 mA
25.4 pF 100% 20 mA or 4 mA
7ML19985GE04
SITRANS LC500 – OPERATING INSTRUCTIONS
(L in meters) or
C
----------------------------------- pF
C air
=
air
Log 1 0.016
C air
=
air
24 0.95
=
7.32 3.12
=
7.32 L
------------------------- - pF
=
Log D d
SITRANS LC500
D = 1.0 m
24 0.95
=
7.32 0.82
------------------------------- - pF
=
Log 1 0.06
(dimensions in meters)
25.4pF
25.4pF
(dimensions in feet)
(L in feet)
0.25 m (0.82')
L = 0.95 m
(3.12')
d = 16 mm
(0.05')
(3.28')
12.7pF
12.7pF
Page 101
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