LEVEL AND VOLUME MEASURING

 

Two of most reliable method to measure volume or level are capacitive and ultrasonic echoing.  They apply when:

 

Capacitive:  When the holding tank or recipient is non-metallic (glass, plastic, PVC, ceramic, etc.) Otherwise, the tank will be used as one plate, and a electric isolated probe (metal shaft) as the other plate, thus forming a coaxial capacitor.

 

Ultrasonic Echoing: When the holding tank is too high, surrounding EMI (Electromagnetic Interference) will affect the measuring, in which case the echoing technique is used.  A radar like-pulse is emitted and the time it takes to bounce back is measure and is proportional to the distance from the pulse source to the liquid surface. Due to the intrinsically delicate adjustments that must be done, it is quite more difficult to build a working echoing level transducer.

 

MASURING LEVEL OR VOLUME WITH A CAPACTIVE METER

 

The simplicity of the circuit I used is shown

 

The 10 MHz oscillator (like the one I used on my simple xtal inductive proximity switch) is the heart of the circuit.  The bottle itself is a two plate capacitor:

 

As the level increases, the dielectric increases, thus more current flows.  At a high frequency, the voltage drop across the capacitor is more sensitive to dielectric changes.  The resistor acts as a load for the changing current and provides an output reading for the serial DAQ or the parallel DAQ.  A Inverted Germanium diode in parallel with the resistor will dissipate the negative portion of the sinewave.  I omitted it for the sake of simplicity.

 

Here is a picture of the set up I used:

 

This is the setup I used:  A 386DX laptop, my  serial DAQ, a coffee glass bottle.