CAPACITIVE SENSOR FOR BUBBLE MEASUREMENT IN CRYOGENIC LIQUID 1. 2. 3. 4. 5.

PRINCIPLE .................................................................................................................................2 EXAMPLE OF OUTPUT vs % OF BUBBLES..........................................................................3 SENSOR FOR CRYOGENIC LIQUID ......................................................................................4 COMMUTATION MEASURE / NO MEASURE PRINCIPLE.................................................9 CONDITIONNING UNIT.........................................................................................................10

CAPAAB 22 / 32 Sentier des Torques BAT 8 92290 Chatenay Malabry Tel / Fax : 01 71 22 73 72

www.capaab.fr

CAPAAB

CAPACITIVE SENSOR FOR BUBBLE PERCENTAGE MEASUREMENT 1. PRINCIPLE In principle, the sensor consists of an outer electrode (usually the pipe in which the fluid flows) and a concentric inner electrode. The capacitance created between the two electrodes depends on the dielectric constant of the medium. When bubbles are present in the flow, this reduces the overall dielectric constant. The rate of bubbles is proportional to the capacitance change induced. In order to accurately define the volume measurement, the measuring electrode is extended to its two ends by a guard ring. The flow velocity of the fluid does not affect the measurement

External electrode Guard ring

Guard ring

Central electrode External body

BUBBLE

%

SENSOR

2

CAPAAB

2. EXAMPLE OF OUTPUT vs % OF BUBBLES

Capacité simulée =f(taux bulles) 10,15 10,1 10,05

Capacité (pF)

10 9,95 9,9 9,85 9,8 9,75 9,7 9,65 0

2

4

6

8

10

12

14

16

18

TAUX %

The sensor is held in a capacitive conditioner by a flexible cable (length up to several meters). Useful information is given in the form of a signal type 0 / 10V.

Each sensor is suitable for actual use conditions. If necessary, a 3D modeling electrostatic performed to optimize the sensor size and number of electrodes

View of the probe during validation

BUBBLE

%

SENSOR

3

CAPAAB

3. SENSOR FOR CRYOGENIC LIQUID In order to linearize the response of a given diameter range, it is necessary to limit the ratio between the diameter of the electrodes. A practical way to achieve this linearization is to play different roles in the concentric electrodes making up the sensor. Thus, it is possible either to a given electrode is grounded, or in the custody or able. This provision limits the number of electrodes. By the action of switching, it is possible depending on the applied potential, is to measure a single crown at a time (about 4 possible) or to group two adjacent rings to see if the signal is the sum of the individual.

BUBBLE

%

SENSOR

4

CAPAAB Probe with 3 electrodes (+ 2 grounded electrodes) : Black = ground

red = measure (or HP)

NB : HP = Hot point

Outer electrode

CENTRAL electrode

4 1 1 & 4 measured

HP Channel 3 or INT HP HP Channel 2 or MIDDLE Channel 1 or EXT Sequence order

BUBBLE

%

SENSOR

5

CAPAAB

4 2

2 & 4 measured

HP

Channel 3

HP Gnd

Channel 2 channel 1

sequence

BUBBLE

%

SENSOR

6

CAPAAB

3

1

1 & 3 measured

Conditionning unit are not shown

BUBBLE

%

SENSOR

7

CAPAAB VIEW OF THE % BUBBLE SENSOR :

BUBBLE

%

SENSOR

8

CAPAAB

4. COMMUTATION MEASURE / NO MEASURE PRINCIPLE Measure : the two generators V1 and V2 are strictly at the same frequency, amplitude and phase (for the guard to be effective). The measured current is proportional to the capacitance formed between the measuring electrode and the outer electrode Ces 3 fils constituent le câble triaxial Guard Electrode

V2

External Electrode grounded

Measuring electrode

V1 Current mesured

Guard Electrode

No Measure : the two generators V1 and V2 mode "measure" are replaced by short circuits. The measured current is zero because the capacitance formed between the measuring electrode and the outer electrode is subject to no potential difference (and guard)

Guard Electrode

External Electrode grounded

Measuring electrode Current mesured = 0

Guard Electrode

BUBBLE

%

SENSOR

9

CAPAAB

5. CONDITIONNING UNIT By default (ENABLE command to 0V), the cycle of four states as alternate. We must place the order at 5 V ENABLE for the manual mode is accessible. In this mode, a manual rotary switch selects one of four states. This mode is used during the verification phase of the operation or calibration. In order to be able to associate, in automatic mode, the voltage measured in the active state, the state of the sequencer is copied and available at the output of a sequencer sub connector D.

ENABLE 4 states états

Enable = 0 STOP = 1

From control room

Four state output 1 2 3 4

Clock

To control room

1 2 3 4

Frequency adjust Manual channel

selection

of

1 2 3 4

Logic for driving

Channel command 1

2

3

3 channels conditionning unit Sensor input

Measurement Output 1

BUBBLE

2

%

3

SENSOR

10