Simplified Electrical Impedance Tomography (EIT) setup

Question

We are working on a setup to measure the terminal settling velocity of objects (spheres, metal, for example) in a vertical tube filled with some sort of fluid (Mud, water, etc.). Preferably, we want to measure the velocity of the object using a non-invasive measuring technique. One of the options we are investigating, is Electrical Impedance Tomography (EIT). By measuring the passing of the object at several locations, we can determine the speed (distance between two measuring points divided by the time it took to travel between them). The reason we want more than two points, is to be able to make sure that the terminal velocity has been reached (no acceleration noticeable between two speed measurements).

Traditional EIT setups have two rings with around 16 or so electrodes each. These map the position of the object in a 2d plane as it passes through the planes made up by each of the sets of 16 electrodes. This traditional technique works quite well, but really only provides two position measurements: this is what we want to avoid, as we want quite some more.

We are looking at simplifying this setup greatly. One of the options is to reduce the amount of electrodes at each measuring point to maybe two or three. Notice that we really only need to know that it is passing through that plane, it does not matter where in the 2d plane it passes through. With a simple Arduino setup we could then connect some of these 'electrode rings', perhaps even have two 'slave' Arduinos that handle the electrodes, and one master that handles the rest.

However, we are in doubt whether this would be able to measure the passing of objects accurately. It of course depends on the dimensions of the tube (D) and the object (d), but if that is in the order of D/d = 30 or 40: will the simplified electrode rings even notice the passing of the object (by seeing a reduced resistance)? We shall assume that the sphere stays somewhat in the middle of the tube, and will not move sideways too much. We can later analyse the 'resistance plots' and take the peaks of these plots as the point at which the object passed.

So, to sum up, our questions are:

1. Will this simplified setup (2 or 3 electrodes) even be able to notice the reduced resistance through the fluid as the object passes through?
2. If not, is there a minimum for the amount of electrodes per ring? Does the current really only travel in a straight line to the other electrode, or would it 'notice' an object passing through just on the side of this straight line as well?
3. Do you have any tips for us as for how to solve this problem of simplifying the setup?

Source for EIT (that we have looked at as well): EIT with Arduino (Github)

Excuse us for our limited electrical engineering knowledge, we are mechanical engineering students ourselves.

Thanks in advance!

Answer 1

Neat problem.

Unfortunately, water-ish things with dissolved ions are quite conductive. If the diameter of the object is 30-40x smaller than diameter of the tube, I sincerely doubt you're going to be able to detect any significant resistance change as the water will be significantly more conductive than most things you pass through it. And, if you're talking metal, the drop in resistance over the small area is swamped by the larger resistance to get the current to the area.

Given that the stuff you are talking about seems to sink in water, this indicates that you've got something with a density quite a bit different from water. I would probably try to use sound/ultrasonic transducers of some form. Either put the transducer at the bottom/top of the tube and bounce sound up/down (looking for reflections and possibly a Doppler shift) or bounce the sound across the tube and look for reflection time changes to indicate a density change.

I'm thinking something along the lines of an ultrasonic anemometer but for liquids.

Good luck. Post back with whatever solution you come up with.