An examination of the use of rotational viscometers for the quality control of non-Newtonian liquid products in factories
A frequent task undertaken by quality-control personnel in typical consumer-goods factories is the measurement of the viscosity of liquid products. The problem often faced in this task is how to strike the correct balance between the complete rheological characterisation of the non-Newtonian properties of the liquid of interest – which requires expensive, sophisticated equipment and can be quite time-consuming – and the dictates of production pressures that demand, as near as possible, an
... possible, an instant decision, and one usually based on a single number. Here we consider the rheological issues that arise in such a debate, which is aimed at finding what adequate characterisation would require. We will investigate the implications of liquids products being non-Newtonian for two of the most commonly encountered viscometers in factory quality laboratories, i.e. the simple 'dip-in' rotating spindle viscometer of the Brookfield type (with its different forms and many imitations) and the more sophisticated concentric-cylinder-type device typified by the Haake Rotovisco VT 550 range. Each is capable of giving a single-number answer for viscosity, but the implications of understanding this single number are different in each case, with the dip-in viscometer being in an infinite sea of liquid and the concentric-cylinder situation being narrow gap. We also investigate when the infinite sea of the 'dip-in' viscometer is effectively 'infinite' and when is a concentric-cylinder geometry really 'narrow gap'? We will use the power-law model throughout our discussions.