Experiment 9(b): Measurement of Water Level using Capacitive transducer

Setup:

• The capacitive transducer used here is cylindrical in shape. It has an inner metallic rod insulated with Teflon covering and an outer stainless steel pipe is acting as an outer electrode.
• There are some small holes at the bottom of the outer pipe through which the water enters in to the cylindrical capacitor. 
• Capacitor is formed with inner rod acting as one electrode, water acting as the dielectric and outer stainless steel pipe acts as other electrode.

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• The water acts as the dielectric, which gets changed when water level is changed.  
• The cylindrical capacitor is placed inside a plastic container, where water is poured for level measurement. There is an mm marking scale pasted over the container and a pipe is fitted at the bottom of the container with a flow control switch attached with it, to drain the water. 
• Two knobs are there in the signal conditioning board for calibration purpose (zero and span adjustment).
  

Working:

• When the water is poured in the plastic container it goes inside of the capacitive transducer through the hole at the bottom of the transducer. 
• As per the level of the water it occupies the space as dielectric in the transducer. 
• As the level increases or decreases the dielectric area occupied by water changes. Hence the capacitance of the transducer gets changed with the level of the water. 
• The signal conditioning and processing circuit used to detect the change in capacitance is same as in case of angular displacement measurement. The DC output voltage after the filter circuit is directly calibrated in terms of level of the water inside the plastic container.
  

Apparatus Required:

• DSO.
• Multimeter.
• Trace sheet/ Pen drive of 4GB or less.

Procedure:

• Connect the terminal of the capacitive transducer to the signal conditioning board at the place named “Transducer”. Connect a patch cord between T4 and T5. Switch ON the board.
• Drain all the water from the container by the water flow control switch connected to the outlet pipe.Tune the zero adjust pot to show zero reading on the display.
• Pour the water inside the container up to 200mm level and adjust the adder pot to show 200 on the display.
• Connect a dual trace oscilloscope at T1 and T2 and a multimeter at T3 in DC range.
• Now change the water level 10/15mm in step and note down the data. 
• Trace or save three to five wave forms. Follow the link how to save wave form in pen drive.

Observation:

Sl no	Level over the container	Level on Display	Pulse width/ duty cycle at T2		DC voltage at T3