The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Instrument for Measuring pH with Optical Indicator Thin Film

Ville Voipio

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 7th of December, 2001, at 12 o'clock noon.

Dissertation in PDF format (ISBN 951-22-5727-0)   [8539 KB]
Dissertation is also available in print (ISBN 951-22-5726-2) and as a CD-ROM (ISBN 951-22-5728-9)


The aim of this thesis is to describe a novel pH measurement instrument and demonstrate its performance by measurements carried out with a prototype. pH measurement is one of the most common measurements in process industry, but despite the wide-spread use of the measurement, common pH measurement methods have several deficiencies.

The new instrument is based on measuring color changes of pH sensitive dye molecules which are trapped in a thin porous glass membrane. The main advantage of the dye measurement is the strong measurement signal, there are no noisy low-level electrochemical signals to be measured.

The dye color measurement is realized by depositing a porous dielectric mirror on top of the dye film. This mirror can be manufactured with the same methods as the indicator film itself. The reflection spectrum of this indicator film and dielectric mirror combination is then dependent on the external pH, and the pH can then be determined by a simple three-wavelength measurement.

A prototype was constructed to prove the viability of this optical measurement concept. New optical constructions were designed to meet the requirements of this instrument. The data processing methods required in the measurement interpretation were also developed.

This thesis describes a part of an industrial research and development project. The ultimate aim of the project is to produce a commercial measurement instrument which is manufactured in large volumes. Thus, economic viability and manufacturability play an important role in the constructions and methods described in this work.

Prototype measurements show the measurement principle is accurate enough to be used in an industrial instrument. The results also show that the optical properties of the medium under measurement can be compensated for with signal processing.

In addition to the pH measurement instrument itself, some new sol-gel thin film research methods were developed. Also, to enable accurate manufacturing control, a signal processing method for improving the resolution of a commercially available position sensor was developed.

Keywords: pH, sol-gel, thin film, dielectric mirror, process instrument

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© 2001 Helsinki University of Technology

Last update 2011-05-26