Precision transmission line liquid level sensors

A low cost, fast yet highly accurate method for determining liquid level inside small containers, e.g. vials is described. The non-invasive technique can easily resolve sub-1% levels in vials containing a few ml or less of liquid and potentially be order of magnitude more accurate than capacitive level sensors. The vial is placed inside an electrically conducting holder to guide a radio-frequency wave inside the vial. The holder-vial guide acts like an open circuited transmission line whose reflection coefficient is nominally unity but phase angle is a function of the liquid level. By performing a vector measurement of the reflection coefficient, it is possible to achieve sub-wavelength resolution and thus measure the level highly accurately. An intentional impedance mismatch between the holder-vial guide and the measurement system can be used enhance the sensitivity to level variations, the optimum sensitivity occurring at multiples of quarter wavelength. The implementation details in the form of a reflectometer and results from an actual level measurement are presented.