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7ML19981CM01.1
MERCAP – INSTRUCTION MANUAL
Page 19
Applications Examples
Generic Application Calculations
The capacitance expected in a cylindrical tank with a probe centrally mounted is estimated
using the following formula:
C=
ξ
r
_air 24x0.95 pF = 12.7pF
Log (1/0.016)
In which
C = capacitance value in pF
ξ
r
= relative dielectric constant
L = active measurement length in
meters
D = internal tank diameter in meters
d = electrode diameter in meters
ξ
r
= 1 (air)
ξ
r
= 2 (oil)
24 = a K constant (can be substituted for 7.32)
For Vessels Filled with Oil
The following equation applies to oil-filled vessels matching the dimensions shown above.
Please note that the probe must be properly mounted and the metal tank is grounded.
C
increase for oil
=
ξ
r
_oil-
ξ
r
_air 24x0.95 pF= 12.7pF
Log (1/0.016)
OR
C
increase for oil
=
ξ
r
_oil-
ξ
r
_air 7.32x3.12 pF= 12.7pF
Log (1/0.016)
This means that the capacitance value for 0% to 100% changes from 12.7 to 25.4 pF. After
calibration then:
12.7 pF
≅
0%
≅
4 mA or 20 mA
25.4 pF
≅
100%
≅
20 mA or 4 mA
Mercap
D
2
=1.0m
L =0.95m
0.25m
a
d = 16mm
Siemens Automation