i have made a sketch in eagle and would appreciate any of your advice. Basically i would like to measure the change of conductivity between two electrodes over time. I do have a couple of them mounted them on the wall of cylindrical vessel ( 4 cm diameter). The diameter of the electrodes is ~ 2 mm. While solids settle the conductivity changes (i hope so) with (1-Concentration)^n .
In order to avoid polarization i would like to use a square wave (e.g with tone() ) .
Since I am a chemist I am not sure if this would work. Would it ?
If i would use an external function generator, the + input of IC1 would have to go to the unknown Resistors RX and the ground potentials would not be the same for all grounds . Would that matter ? I guess not.
To get signals 180 degrees out of phase you simple invert them, so a H-bridge circuit will give you the AC you need. You must take the measurement with the voltage in one direction and then if possible in the other direction. Ten make the outputs the same until you next need a reading.
The schematic is new since my first reply can you talk me through it?
Thanks for your reply. Yes, i changed the figure since i made a mistake with the current sensor.
I would like to measure the voltage drop over my unknown RX. So i thought of using an instrumentation op-amp against ground. In order to calculate the conductivity i need the current, that is why i placed the current sensor in front of the DEMUX. Since my electrodes will be of steel, i would like to use a square wave with an amplitude from e.g +5 to -5V and a frequency of around 500 Hz. A diode at the end should make the signal readable for the AD-converter. The second one is for protection.
Yes but what are the 4051s doing.
That current sensor will produce 185mA per amp. What sort of current are you expecting? I can't see you getting enough current through the resistor to measure anything useful. Certainly if you have the 4051s in line have you looked up the transfer resistance of those, any the maximum current through them is only in the order of tens of mA.
I think this circuit is a non starter.
You mean the ACS712, yes you are right. I need a different one. I should measure the current after the DEMUX ? I thought the Voltage will drop but not the current. The transfer resistance is about 180 Ohm.
If i use a function generator as input signal i use a square wave from a function generator as input i do not have to make myself a bridge and the signal form would be better. Also i can generate e.g -5 to +5 V. I ordered one (UTG 9005 C).
I thought of using the ACS721 current sensing breakout board from Sparkfun. So i adjust the slope with the gain (not sure if slope or intercept). Of course i will amplify noise. But since i will see my signal to noise ratio I will know my uncertainty. Any suggestions about the current sensing part ?
Are these measurements to be conducted in an aqueous solution? If so, what are the solids? Are they to some extent soluble in water? The conductivity of pure water is extremely low and would not be measurable with the circuitry you are proposing.
It is a buffered solution. Conductivity is in the range of 1-20 mS/cm. The particles are made of polystyrene. I have to play araound with my cell constants but this is rather simple for me in comparison with the measuring curcuit.
So i will redraw my sketch and use the current sensing board from Sparkfun. Probably i have to switch to another IC and make my own op-amp curcuit.
Still i would like to know if the basic idea, to measure the voltage drop with an instrumentation op-amp across my electrodes while exciting them with an AC sqaure signal is worth a try ?
Would you put a resistor between GND and the electrodes ? Or would you make a bridge-like wiring ? (which i have never done for AC currents)
Would you put a resistor between GND and the electrodes ?
No.
Or would you make a bridge-like wiring
Sounds better.
and use the current sensing board from Sparkfun
I will say it again it is not sensitive enough for what you want.
Measuring current across an unknown resistor involves having a shunt resistor that you measure the voltage across and this is the current measurement. At the same time you measure the voltage across the unknown resistor. Having these two quantities you can calculate the value of the unknown resistor.
I don't see why the conductivity of the buffer would change, depending on whether polystyrene particles are suspended or not. Polystyrene is an outstanding insulator.
Light scattering is a sensitive indicator of particles in suspension.
Measurement of slurry solids content by electrical conductivity:
Light scattering would not work at the concentration range i would like to measure. Ultrasonic devices would, but not over different heights (at least for reasonable price). EIT or ERT would be an option. But 60 000 Euros for such a system ist too much at the moment.
Thanks Mike for pointing the direction to go.
I will try to measure the voltage drop with the equipment i have right know and if that does not work I will have to find another way.
If i use a wheatstone bridge with in instrumenation op-amp and an measuring rectifier (e.g AD630) it should be possible to measure a voltage signal.
Is it sufficient to measure just the signal or do i have to compensate the bridge? I guess it is not that easy. Do i have to take care about some capacitive effects ? I found an entry where someone said one should use a C-trimmer since both sides have different capacities. What does that mean ? Can someone recommend me Literature ?
