Document Type

Theses, Ph.D


Available under a Creative Commons Attribution Non-Commercial Share Alike 4.0 International Licence


The tensiograph drop analyser, operating in the mode of ultraviolet-visual absorption photometer, was re-engineered to improve measurement accuracy. As well as instrumentation re-design, the other objective of the research was to analyse tensiograph signals to explore how their features are defined by drop properties. Using fibre optic technology, the tensiograph emits a light beam into a growing pendant drop whereupon it undergoes a number of internal reflections. A portion of the reflected light energy is coupled to a photodiode producing a proportional current that is electronically processed and transferred to a computer for analysis. The signal produced, known as the tensiotrace, corresponds to drop transmittance varying with respect to drop volume. Originally the tensiograph was designed to perform qualitative analysis of drops since, albeit with complex interdependencies, the tensiotrace encodes information about several liquid properties such as refractive index, surface tension and absorbance. Based on the physics underpinning tensiotrace production, 3-D computer simulations visualising light transmission within the drop were produced. This inspired the development of an analytical ray tracing model for tensiotrace production explaining how key tensiotrace features are related to drop properties. Instrumentation error was systematically analysed to highlight dominant error sources. Expressions relating the sensitivity of absorbance measurements to a range instrumentation error were derived and the drop analyser performance was compared to standard absorption spectrophotometers. Based on this analysis, the tensiograph optoelectronic circuitry was redesigned, producing an improved signal-to-noise ratio of 98 dB as well as a substantial reduction in drift-both of which were shown to produce proportionate improvements in absorbance accuracy. The error performance of the optoelectronic circuitry was compared to the tensiograph optical/mechanical components and was shown to be superior. Based on the principles of adaptive Wiener filtering, a least squares estimate for drop absorbance was developed. The re-engineered drop photometer measures absorbance within a tolerance of 0.11% for absorbancies in the range 0.2