Var Capacitance

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Hartini Binti Abdul Rahman Bach. of Engineering (Hons) Electronic 4101006181 1/2011/2012 Group 3

TITLE : LEVEL MEASUREMENT TRANSDUCERS EXPERIMENT 2 : THE VARIABLE CAPACITANCE LEVEL TRANSDUCER SYNOPSIS/INTRODUCTION This lab presents the transducer which consists in two comb-shaped copper electrodes coated with an insulating layer. In this manner the two electrodes behaves like the armatures of a capacitor, whose capacity value depends on the dielectric constant of the material in the surrounding space. This space is partially occupied by the thin fiberglass substrate holding the electrodes and for the rest by air/water as is the case. When the electrodes are emerged(low level), the dielectric consist is that of the air (approx 1), while when the electrodes are fully mersed (high level) the dielectric constant is that of the water (approx 78 to 81). For all the intermediate cases the transducer displays a capacity value which is a nearly-linear function of the sinking level.

OBJECTIVE To study the principle of operation and construction characteristics of variable capacitance level transducer.

THEORY The capacitance sensor, consist of a target plate and a second plate as the sensor head. These two plates are separated by an airgap of thickness h and from the two terminals of a capacitor, which exhibits a capacitance C given by

Where C is the capacitance in picofarads (pF) A is the area of the sensor head (πD2/4) K is the relative dielectric constant for the medium in the gap (K=1 for air) K is proportionality constant; k = 0.225 for dimensions in inches and k = 0.00885 for dimension in millimeters

Universiti Selangor(Unisel) Faculty of Engineering KES 1281 Laboratory 2 Mdm Ibtisam bt Ismail

Hartini Binti Abdul Rahman Bach. of Engineering (Hons) Electronic 4101006181 1/2011/2012 Group 3

The transducer of the trainer has electrodes obtained from PCB etching for construction simplicity and room constraints. In the industry field these sensors are more often constructed as two coaxial pipes places placed vertically. The outer one is normally grounded and acting as a shield for the inner armature.

The two electrodes of the transducer must be well insulated from the liquid in order to behave as the armature of a low-leaking capacitor. This requires that the terminal strip holding the electrodes, the connecting cord, etc. are perfectly dry and clean. It may happen during the work on the trainer that water is let to enter the cover of the tank from the slot of the float assembly. This may wet the holder of the electrodes and cause poor or no operation for the oscillator. The correct situation is soon restored as this piece dries out.

The capacitance sensor has several advantages. It is non-contacting and can be used with any target material, provided the material exhibits a resistivity less than 100Ωcm2. The sensor is extremely rugged and can be subjected to high shock loads and intense vibratory environments. APPARATUS  The hydraulic arrangement of the training module  Level measurement transducer of the training module  Power supply  Jumper METHOD/PROCEDURE 1. The electronic module was connected to the +15V/-15V DC supply by three plug-in cables. 2. The J1 connector of the electronic module was connected to the corresponding one of the transducers tank by the proper connector cable. 3. A V-measuring instrument(DC multimeter or oscilloscope) was connected at the output sockets of the signal conditioner. 4. The two tanks bases were placed aside. The tap between the two tanks were closed. The tank which has no sensor was filled with 750cc of clean water.
Universiti Selangor(Unisel) Faculty of Engineering KES 1281 Laboratory 2 Mdm Ibtisam bt Ismail

Hartini Binti Abdul Rahman Bach. of Engineering (Hons) Electronic 4101006181 1/2011/2012 Group 3

Setting the zero of the output characteristics

5. The tap was opened in a manner to let enough water flow in to the point where the float just starts floating. This level was measured by the mill metric rule. P1 was adjusted for a null indication at the measurement instrument. 6. P3 was placed half-run. A precise setting of P3 is non-important for the execution of the experiment since its effect is only to shift at higher or lower frequencies the response characteristic of the transducer and conditioner.

Recording of the level/frequency characteristic

7. Starting from the situation of tank empty was was let come in step, in a manner to have measurement levels rising of approx 5mm at each step. 8. At each step, the actual level was measured with the rule and the indication of the frequency meter. The values were recorded.

Universiti Selangor(Unisel) Faculty of Engineering KES 1281 Laboratory 2 Mdm Ibtisam bt Ismail

Hartini Binti Abdul Rahman Bach. of Engineering (Hons) Electronic 4101006181 1/2011/2012 Group 3

RESULT/DATA/CALCULATION 1. Plot the results of your measurements in a graph showing frequencies on the vertical axis and levels on the horizontal one. 2. In order to have a graph clearly readable, the vertical axis (frequencies) should have an expanded scale in the range of operation of the transducer. 3. Once constructed the graph, comment its linearity and explain why it appears as a negative-slope line in discussion part.

Level (mm) 0 5 10 15 20 25 30 35 40 45

Frequency (Hz) 48.42 48.29 46.11 44.30 43.36 42.25 41.12 39.83 38.50 39.42

Universiti Selangor(Unisel) Faculty of Engineering KES 1281 Laboratory 2 Mdm Ibtisam bt Ismail

Hartini Binti Abdul Rahman Bach. of Engineering (Hons) Electronic 4101006181 1/2011/2012 Group 3

DISCUSSION

The transducer of the trainer has electrodes obtained from PCB etching for construction simplicity and room constraints. In the industry field these sensors are more often constructed as two coaxial pipes places placed vertically. The outer one is normally grounded and acting as a shield for the inner armature.

The two electrodes of the transducer must be well insulated from the liquid in order to behave as the armature of a low-leaking capacitor. This requires that the terminal strip holding the electrodes, the connecting cord, etc. are perfectly dry and clean. It may happen during the work on the trainer that water is let to enter the cover of the tank from the slot of the float assembly. This may wet the holder of the electrodes and cause poor or no operation for the oscillator. The correct situation is soon restored as this piece dries out.

The capacitance sensor has several advantages. It is non-contacting and can be used with any target material, provided the material exhibits a resistivity less than 100Ωcm2. The sensor is extremely rugged and can be subjected to high shock loads and intense vibratory environments.

The graph shows that it is nearly-linear and is negative-sloped. As the water level increases, the frequency decreases. This decreament is due to the water as another medium that could affect changes in the frequency. And thus, the water level is indirectly proportional to the frequency.

Universiti Selangor(Unisel) Faculty of Engineering KES 1281 Laboratory 2 Mdm Ibtisam bt Ismail

Hartini Binti Abdul Rahman Bach. of Engineering (Hons) Electronic 4101006181 1/2011/2012 Group 3

CONCLUSION The capacitance sensor has several advantages. It is non-contacting and can be used with any target material, provided the material exhibits a resistivity less than 100Ωcm2. The sensor is extremely rugged and can be subjected to high shock loads and intense vibratory environments.

As the water level increases, the frequency decreases. This decreament is due to the water as another medium that could affect changes in the frequency. Therefore, the water level is indirectly proportional to the frequency.

REFERENCE http://en.wikipedia.org/wiki/Transducer http://en.wikipedia.org/wiki/Variable_capacitor

Universiti Selangor(Unisel) Faculty of Engineering KES 1281 Laboratory 2 Mdm Ibtisam bt Ismail

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