1 servo working principle
TRANSCRIPT
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Servo
Radar
1950 ‘60 ‘70 ‘80 ‘90
PV70 800/801 802 811 854
872
‘00 ‘10 ‘20
873 973/971
970
990Flexline
TemperatureMRT Microlect, Analog
Spot Pt100 Ni191/Cu90
MITMTT
MESCMRT-MTT-MPT
MIRMRT/MPT
VITOMTT
MRT/MPT
DensityDip & Profile (Servo)
Hydrostatic (HIMS/HTG)
Tank GaugingHistorical overview Enraf instruments
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Servo level gaugingIntroduction
• Archimedes’ law
• S = [DW] - [DV] ·– [DW] = Displacer Weight– [DV] = Displacer Volume
– = product density
buoyancy
apparent weight S
weightmax.[DW]
min.[DW] - [DV] ·
appa
rent
wei
ght (
gram
s)
weight
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Servo level gaugingSetpoint [S1]
• For level measurement a certain apparent weight (setpoint) is required
apparent weightapparent weight
max.[DW]
min.[DW] - [DV] ·
appa
rent
wei
ght (
gram
s)
[S1]
max.[DW]
min.[DW] - [DV] ·
appa
rent
wei
ght (
gram
s)
[S1]
• Setpoint closer to displacer weight less immersion
• Advantages:– less influence of product density
(variations)– less contamination of displacer– less influence of turbulence
• [S1] = [DW] - 15
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Servo level gaugingInfluence of Product Density Changes
• Immersion i =
• Example– [DW] = 223 gr.– [DA] = 64 cm²– = 800 kg/m³ (0.8 gr/cm³)
– i = = 0.29 cm (2.9 mm)
• Influence of density variations 0.7 0.9
– = 0.9 i = = 0.26 (2.6 mm)
– = 0.7 i = = 0.33 (3.3 mm)
• Density change of 100 kg/m³ causes level change of 0.5 mm
[DW] - [S1][DA] ·
223 - 20864 · 0.8
223 - 20864 · 0.9
223 - 20864 · 0.7
i
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Servo level gaugingInfluence of contamination
• Immersion i =
• Displacer clean: i = = 0.29 cm (2.9 mm)
• Displacer contaminated:
i =
i =
i = 0.49 cm (4.9 mm)
• Level error = 2.9 - 4.9 = -2 mm
• Conclusion: contamination of 1 gr.causes level error of -0.2 mm
[DW] - [S1][DA] ·
223 - 20864 · 0.8
(223 + 10) - 20864 · 0.8
([DW] + contamination) - [S1][DA] ·
S1=208Displacement 15 gr.
[DW] = 223 gr.
Weight = [DW] + 10 gr. Contamination= 233 gr.
S1=208Displacement 25 gr.
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Servo level gaugingInfluence of turbulence
• Horizontal (error is significant)– [S1] = [DW] - 15: automatic correction– 854: positioning actions (up/down)– use of stilling well
• Vertical (S [S1], integration action)– adjustable integration time [T1]– stable read-out– accurate average
A B
A B
A BA < B A = BA > B
L
taverage
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Servo level gaugingServo sensitivity
• Minimum required level change for level measurement– instrument sensitivity S 854 0.3 gr.– level change i 0.1 mm (displacer size [DA] = 64 cm²)
• Sensitivity depends on– instrument sensitivity S (manufacturer)– displacer diameter (application)
• 811: standard 140 mmsupport pipe (if any) 8"
• 854: standard 90 mmsupport pipe (if any) 6"
• Servo on 2" stilling well– use displacer diameter 45 mm– level sensitivity i 0.2...0.4 mm ( = 0.8 gr/cm³)
• Sensitivity:
i i
i ± i =([DW] - [S1]) ± S
[DA] ·
i = ±± S
[DA] ·
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Servo level gaugingSummary
• Setpoint [S1] = [DW] - 15 [g]
• Immersion i [mm]
• Sensitivity i = [mm]S[DA] ·
[DW] - [S1][DA] ·