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1 Introduction
Viscosity is of paramount importance to lubrication engineers, rheologists, chemical engineers, and many more besides. This report describes the background, development, operation and testing of a portable automatic instrument for the measurement of bulk viscosity.
The requirement for such an instrument arose during oxidation studies of new synthetic flourosilicone lubricants, which were carried out by bubbling oxygen through a 20 cm3 quantity at a temperature of 250C. These lubricants have some advantages over conventional hydrocarbon oils: for example they are better suited to harsh conditions such as the extraterrestrial environment. However their high temperature stability is inferior and the oxidation studies were aimed at develeping additives to improve penformance in this respect.
The condition of the lubricant was monitored by taking hourly samples and measuring the viscosity using a Ferranti-Shirley cone-on-plate viscometer. As the oxidation progressed over several hours or sometimes days, the measured viscosity increased, finally resulting in the formation of a stiff gel. Unfortunately lack of regular data, for example during the night, often resulted in considerable uncertainty regarding the course of the experiment. An automatic viscometer was thus almost essential, particularly for the longer tests. No such instrument was found to be available commercially and so a suitable device was designed and built.
Briefly, this original mechanism stirred the fluid using a glass rod, indirectly driven via a conventional tension spring by a small motor. The extension of the spring was kept constant by means of an electrical contact. Brushes made connection to a simple electronic control circuit which increased the speed while the contact was broken, and decreased it while closed. Thus the rotation rate oscillated around a certain mean value, which was proportional to the viscosity. (See further details on the theory in section 2.) A timer switched the device on at regular intervals for a few seconds, and an output voltage was produced indicating motor speed. This could be plotted on a chart recorder, to obtain a direct graph of relative viscosity against time.
The viscometer just described satisfied the requirement although the accuracy of the measurements was poor. Several possible improvements to the mechanism were foreseen, and in particular the rough nature of the control electronics was an obvious area for further development. In view of this the current project aimed to develop an improved viscometer mechanism and control system. Calibrations and comparisons were also necessary to verity correct eperation.
This report considers the theory of fluid viscosity measurement (section
2). The viscometer mechanism, microprocessor control system and operating
program are described (section 3). The results of
several experiments that were conducted are presented (section
4). Many ideas for further development have occurred during this work,
some of which are discussed (section 5). Detailed
circuit schematics, program
listing, numerical results and the engineering
drawings are shown in full in the appendices.
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