Troubleshooting My DIY Capacitive Transducer

[vk6kro]

Your A to D converter must get rectified and filtered DC and it must usually only have DC in the range 0 to +5 volts.

Check your Micro's instructions to check on this.

But why would it have AC output if it is an opamp using only DC and having a DC input?

 

[phyky]

just curious if pic can connect ac.thanks

 

[phyky]

sorry, i just realize that dc ll not pass through capacitor make my charge amp won't working for my capacitive transducer.so i change to ac supply now with 5V amplitude.but the output of charge amplifier will oso change to ac so how could i connect it to my microC to do ADC? do i need a ac-dc converter?

 

[NascentOxygen]

sorry, i just realize that dc ll not pass through capacitor make my charge amp won't working for my capacitive transducer.so i change to ac supply now with 5V amplitude.but the output of charge amplifier will oso change to ac so how could i connect it to my microC to do ADC? do i need a ac-dc converter?

How rapidly can your PIC take repeated samples of the analog signal? It may be sufficient to add a DC offset in your "charge amplifier" so the voltage fed to the PIC alternates from 0 to +10V rather than from -5V to +5V, and make allowance for this in your PIC coding. (Should 10V exceeds the PIC specs, obviously attenuate it, say by 50%.)

 

[phyky]

Your measurement is not stable since the DC current through the water is decomposing it and you are getting bubbles of gas forming on the electrodes.

To measure the resistance of the water between the electrodes, you could use the "OHMS" function on a multimeter or you could use an opamp circuit like this:http://hampatong.com/wp-content/uploads/2010/04/Wheatstone-bridge-single-op-amp.png

Many opamps would work OK in this circuit.

i try did it as resistive water level sensor,it did quite well. but what is the reason of water level increase and resistance decrease?is it because the conductivity of water?because i never heard of resistive water level sensor. can u explain it for me?

 

[phyky]

How rapidly can your PIC take repeated samples of the analog signal? It may be sufficient to add a DC offset in your "charge amplifier" so the voltage fed to the PIC alternates from 0 to +10V rather than from -5V to +5V, and make allowance for this in your PIC coding. (Should 10V exceeds the PIC specs, obviously attenuate it, say by 50%.)

thanks for ur help.i tried with ac already.it did working but the output vary is so small and unable to do ADC. i can only accept it as the resistive transducer.haha

 

[vk6kro]

i try did it as resistive water level sensor,it did quite well. but what is the reason of water level increase and resistance decrease?is it because the conductivity of water?because i never heard of resistive water level sensor. can u explain it for me?

Most water sensors would possibly be resistive although it may add copper ions to the water if copper electrodes are used.

As the water rises, the resistance of the water between the electrodes decreases. You can think of this like a single resistor which then has other resistors placed in parallel with it.

Or, you can predict it from the resistivity formula:

where R= resistance of the water between the plates... ρ is the resistivity of the water (a constant) ... L is the length of the resistor (in this case, the spacing between the plates, also constant) ...and A is the area of the water beween the plates.

This last area is the property that varies as the tank fills up.
So, the resistance decreases as the tank fills up.

To be complete, there is current through the rest of the water in the tank, but the resistance of the water between the plates has the greatest effect.

 

[phyky]

i notice there is some blue compound in the water due to the electrolysis.but this procces can also occur to same plate material?how to avoid it?coated with somthing?

 

[vk6kro]

i notice there is some blue compound in the water due to the electrolysis.but this procces can also occur to same plate material?how to avoid it?coated with somthing?

Copper ions are blue in water and they are toxic in sufficient concentration. The toxic dose depends on the person's body weight but for a 200 lb person the toxic dose for Copper Sulfate is about 1 gram.

The way to reduce them is to take brief measurements of the water resistance and then display that for the next 5 minutes.

The water level should not change in that time and the total copper ion production should be reduced.
You would have to use a micro to do this easily.


Another way is to have a float on the water and a weight, joined to it by a nylon fishing line, rising and falling outside the tank. At the top of the tank, you have a pulley which turns a multi-turn potentiometer.
The line passes over the pulley and rotates it as the float rises and falls.

The position of the slider in the potentiometer is used to give a reading of how much water is in the tank.

 

[Mike_In_Plano]

Interesting problem.
If you separate the water from your probe by an insulator, than the insulator may act as the dielectric, and water (having a very high dielectric and some conductivity) would be seen as a varying surface on the outside of the insulator.
A simple embodiment of this may be a easily made from a piece of wire-wrap wire that is bent into a hair-pin shape, such that the inner conductor is never exposed to the water. The insulation is very thin, which helps by increasing the capacitance per unit length.
The difficulty I perceive is that it would tend to give false readings based on any water that wicked up along the outside. Thus, if the sensor built up contaminates, adhesion of water would be more of an issue.
To offset this, you could use a higher excitation frequency when measuring the capacitance. Since the impedance associated with the capacitance goes down with frequency, you may find that this somewhat overcomes the effect of the conductivity of the water that wicks up the probe wall.
It escapes me at the moment, but there is an old NASA circuit, which may be used to measure capacitance at a few MHz. It uses a 4 diode bridge, a reference capacitor and unknown capacitor, and is simply driven by a square wave through two large value capacitors (i.e. .1uF). A pair of lark value resistors (i.e. 470k) sample the DC difference that builds between the .1uF capacitors, and that represents the delta between the reference and test capacitance.