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Refer to figure 5.1
765 VITO water probe. Can be delivered in three sensitive lengths for the water measurement: 0.5 m
•
11
(1' 7"
/
), 1.0 m (3' 3"
16
766 / 768 combi probe; lowest temperature element above water probe (position 8 of Model selection
•
code is a 'C'). There are three possible sensitive lengths for the water measurement: 0.5 m (1' 7"
6
1.0 m (3' 3"
/
) and 2.0 m (6' 6"
16
766 / 768 combi probe; lowest temperature element inside water probe (position 8 of Model selection
•
code is a 'D'). There are three possible sensitive lengths for the water measurement: 0.5 m (1' 7"
6
1.0 m (3' 3"
/
) and 2.0 m (6' 6"
16
5.1.2
Principle of measurement
Factory calibration values for the maximum capacity and minimum capacity are listed on the "CheckList
765/766/768 Final Assembly", delivered with every VITO probe.
The value at VR is the minimum capacity (capacitance measured in air). This is an approximate value and
depends on the dielectric property of the stored product. This value needs to be adjusted. Refer to : Calibration of
water probe in Appendix B.
The value at VT is the maximum capacity (capacitance measured in water) and needs no adjustement.
The measured capacitance by the water probe has a linear function with the water level. Hence, the water level is
calculated as:
If
LW >
WS
Where:
LW = water level
MX = measured capacitance
VR = minimum capacitance (probe fully covered by product)
VT
= maximum capacitance (probe fully covered by water)
WP = water probe length
WB = water probe bottom position
WS = alarm hysteresis
For example:
Minimum capacitance (item VR)
Maximum capacitance (item VT)
Measured capacitance (item MX) =
Water probe length (item WP) =
Water probe bottom position (item WB) = 0.0000 m (or 0.00 inch or 0.000 ft)
Then the water level (item LW) becomes: 0.2300 m (or 9.05 inch or 0.755 ft)
5.2
Commissioning of water bottom probe
5.2.1
Commissioning measurements
The purpose of these calibration measurements is to determine the water probe offset and a control
measurement for the water level.
With these measurements the product level is used as a horizontal line between the point where the tank zero
point is located and the point where the water probe is installed.
Instruction manual VITO
6
12
/
) and 2.0 m (6' 6"
/
).
16
16
12
/
).
16
12
/
).
16
otherwise
=
121.34 pF
=
432.49 pF
268.92 pF
0.4850 m (or 19.09 inch or 1.591 ft)
Water bottom measurement
LW = 0
11
/
),
16
11
/
),
16
Page 36
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