Calibration-free continuous bin level sensor
A sensor assembly is described herein that can automatically calibrate itself upon installation into an empty bin, eliminating the need to actually fill the bin to calibrate the level reading. The sensor will provide consistent measurement regardless of material properties (permittivity, density, temperature or moisture content). The capacitive nature of the sensor means that in some circumstances, it will sense the material through plastic/glass/fiber glass thereby allowing the sensor assembly to be mountable on the outside of a bin or container. The electrodes of the sensor system are designed to provide a continuous level reading.
1. A method for calculating an empty container sensing electrode capacitance and an empty container reference electrode capacitance comprising:
providing a sensing capacitive element configured from two parallel sensing electrodes positioned adjacent a container or vessel such that changes in a material level cause a proportionate change in a first capacitance of the sensing electrodes, wherein said sensing electrodes have a length LL and a nominal capacitance per unit length C 0 and wherein the capacitance of said sensing capacitive element varies in accordance both with the extent of the immersion of the parallel electrode in the fluid or material and a dielectric constant of the fluid or material;
providing a reference capacitive element configured from two parallel reference electrodes positioned adjacent to a bottom of the container or vessel, said reference electrodes having a length LR and a capacitance per unit length C 0 , and wherein the capacitance of said reference electrodes is a function of the dielectric constant of the fluid or material;
measuring, at some first non-empty fill level (H 1 ) at a first frequency (f 1 ) a measured capacitance of the sensing element at the first frequency (C tot@H1 ) and a measured capacitance of the reference element at the first frequency (C ref@H1 );
measuring, at some second non-empty fill level (H 2 ) differing from H 1 by at least 10% at a second frequency (f 2 ) a measured capacitance of the sensing element at the second frequency (C tot@H2 ) and a measured capacitance of the reference element at the second frequency (C ref@H2 );
generating the empty container reference electrode capacitance from a ratio of:
a. a difference of the measured capacitance of the sensing element at the first frequency (C tot@H1 ) and the measured capacitance of the sensing element at the second frequency (C tot@H2 ) added to the product of H 2 /LR and the measured capacitance of the reference element at the second frequency (C ref@H2 ) less the product of H 1 /LR and the measured capacitance of the reference element at the first frequency (C ref@H1 ) as a numerator;
b. a difference of H 2 /LR and H 1 /LR as a denominator;
generating the empty container sensing electrode capacitance from a sum of the measured capacitance of the sensing element at the first frequency (C tot@H1 ) and the product of H 1 /LR and the difference of the empty container reference electrode capacitance and the measured capacitance of the reference element at the first frequency (C ref@H1 ).
2. The method according to claim 1 additionally comprising:
determining a level of a fluid or material within the container by measuring a total capacitance of the sensing capacitive element and a total capacitance of the reference capacitive element; and
generating the level of the fluid or material within the container from a product of:
a ratio of LR (reference electrode length) and LL (sensing electrode length) and a ratio of:
a. a difference of the measured total capacitance of the sensing capacitive element and the empty container sensing electrode capacitance as a numerator;
b. a difference of the measured total capacitance of the reference capacitive element and the empty container reference electrode capacitance as a denominator.
3. The method according to claim 1 wherein the first frequency (f 1 ) and the second frequency (f 2 ) are the same.
4. The method according to claim 1 wherein the first frequency (f 1 ) and the second frequency (f 2 ) are different.
5. The method according to claim 1 wherein the second non-empty fill level (H 2 ) differs from the first non-empty fill level (H 1 ) by 10%-25%.
6. The method according to claim 1 wherein the sensing capacitive element and reference capacitive element are mounted internally to the container or vessel.
7. The method according to claim 1 wherein the container or vessel comprises plastic, glass, or fiberglass and wherein the sensing capacitive element and reference capacitive element are mounted externally to the container or vessel.
8. The method according to claim 7 wherein the sensing capacitive element and reference capacitive element are each covered by metal shield tape members.