Long - Capacitive Level Sensor Combined

7/28/2019 Long - Capacitive Level Sensor Combined

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Forrest Long

MECH 207Fall 2009

Images courtesy of Omega corp, discovercircuits.com, andMagnetrol Corp.

Capacitive Level Sensor


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Introductiony Level Sensor, used for a variety of liquids or solids

(powder, granular, aggregate, etc) in a storagetank/vessel

y Fuel levels, Water levels for process, waste, treatment,

Raw materials (alcohols, oils, chemicals)

y Plastics or solid powders for raw materials

y Can be integrated with electronics for continuous or

point (switch) level indications


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Typical sensor and installation


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Principle of Operationy Basic principle is that of the capacitor

y

Capacitor is defined as follows:


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Principle of OperationDistance over which charge is stored is shown in diagrams below anddepends on the medium measured

If the medium is non conductive then it provides a large change in the dielectricconstant

If the medium is conductive then an insulated probe is used and the capacitor isformed by the outer surface of the insulation (shorted to the wall) and theelectrode


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Operationy For the insulative medium case, As level rises then the capacitancechanges approximately as follows

y C =0.225 (Kair x Aair) +0.225 (Kmaterial x Amaterial )

Dair Dmaterialy C = (Kair x Aair) + (Kmaterial x Amaterial ) when Ds are the

same

y

For the conductive medium case, As level rises then the capacitancechanges approximately as follows

y C =0.225 (Kmaterial x Amaterial )Dmaterial

y

WithAmaterial defined by the height of the substance and the airterm eliminated since Dair>> Dmaterial without conductivemedium to connect

y The capacitance is directly proportional to the

height of the medium


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Dielectric constants


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One typical implementation


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Enhancementsy Measure the complex impedance of the circuit rather

than just the capacitance which can easily, as in the

above implementation generate a 4-20 mA signal other

analog transmittable signal

y Impedance measurement is a standard analog signal

processing method, easier to implement than highprecision measurements of femto-, pico-, nano-, or

micro-Farads


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Typical installation concerns


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Issues in usey Coating or build-up of salts or other process chemicals

y

Wicking and un even coating of the electrodey Corrosion

y Non-conductive tanks

y

High temperatures or pressuresy Signal noise particularly in using RF near other

sources

y

Splashing or bubbles/foamy Incompatible with very low density or very low

dielectric constant materials


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Pricing Synopsisy Per Omega:

y $850 - $1200 for probe and signal processing head

(starting)y Up to $1200 for remote (if required) signal processing

and control box

y $150 - $300 for isolation and power supply and otherinterface eletronics

y Per Magnetroly Ca. $1500 for integral mount continuous level transmitter

with 4 relays and 4-20 mA ouput (4-wire or 2-wire)y Ca. $800 for remote probe with ca. $1200 additional for

display, calibration, control, and transmitter electronics


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10 20 30 40 50 60 70 80

10 4.0

9 3.6

8 3.2

7 2.8

6 2.4

5 2.0

4 1.6

3 1.2

2 0.8

1 0.4

0 0

pFperinch cm

Dielectricvalue of media

Tank diameterinches (cm)

4 (10)

12 (30)24 (60)

48 (120)

96 (240)

Stillwell

Capacitance gain for Teflon and Halar coated probes

8XA-1AXX-XXX or 8XA-4AXX-XXX

8XA-2AXX-XXX or 8XA-5AXX-XXX

10 20 30 40 50 60 70 80

10 4.0

20 8.0

30 12.0

pFperinch cm

Dielectric value of media


4 (10)

12 (30 )24 (60 )

48 (120)

96 (240)

Stillwell40 16.0

0 0

Capacitance gain for Kynar coated probes

8XA-3XXX-XXX or 8XA-6XXX-XXX

7

C A P A C I T A N C E P I C O F A R A D ( P F ) C O N T I N U E D

G A I N G R A P H S

Capacitance gain chart for bare probes

8XB-XXXX-XXX or 8XC-XXXX-XXX

2 3 4 5 6 7 8 9 101

20 8.0

18 7.2

16 6.4

14 5.6

12 4.8

10 4.0

8 3.2

6 2.4

4 1.6

2 0.8

0 0

pFperinch cm



Stillwell

4 (10)

12 (30)24 (60)48(120)96(240)

Capacitance gain chart for bare flexible probe

8X2-AA1A-XXX 0.19" (5 mm) probe diameter

2 3 4 5 6 7 81

4 1.6

3 1.2

2 0.8

1 0.4

0 0

pF perinch cm



4 (10)

12 (30)

24 (60)48 (120)96 (240)

8

C A P A C I T A N C E P I C O F A R A D ( P F ) C O N T I N U E D

G A I N G R A P H S

Capacitance gain chart for standard flexible probe

8X1-5A1A-XXX 0.19" (5 mm) probe diameter

10 20 30 40 50 60 70 80

7 2.8

6 2.4

5 2.0

4 1.6

3 1.2

2 0.8

1 0.4

0 0

pF perinch cm



4 (10)12 (30)24 (60)48 (120)96 (240)

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Long - Capacitive Level Sensor Combined