I'm trying to design a 4 Pole probe to measure electrical conductivity and I having some doubts about the theoretical calculation of its cell constant.
I know the formula, K(cell constant)=L(Distance)/A(Sectional Area) and i how to calculate in a 2-Pole Probe, in which the L is the distance of the probes and the A is the sectional area of them.
My probe design is like the one in the figure in which the figure. The current sourcing electrodes are the external and are ring shaped and the voltage measurement poles are metallic tips.
So for the cell constant calculation, in the distance part, which distance should I consider, the distance between current electrodes or the voltage measurement poles?
And since the current poles are rings, what is cross sectional section, the section made by the voltage poles (rectangle) or the one made by rings (circle).
Thank you. 
Best Answer
You want to measure the conductivity of a liquid in the tube? If the current distribution inside the tube is homogenius, the resistor to be measured is a cylinder with a diameter of the inside diameter of the tube. The length of the cylinder is the distance of the two voltage measurement poles, this is the Distance L in the formula. The area A is calculated from the radius of the cylinder, the inside radius of the tube.