Some considerations when using a microwave oven as a laboratory research tool

Microwave ovens can be used in laboratories for the rapid heating of materi al - either to dry them completely or to subject a workpiece to sudden ther mal stress or electric field stress. Determinations of the moisture content levels in soil or leaf tissue samples, for example, can be made within ten s of seconds rather than hours. It is often assumed that placing a load wit hin a microwave oven will result in it being heated evenly as well as quick ly, but this is not always the case. This paper describes how a microwave o ven works and illustrates how the heating effect within a workpiece can var y. The size and shape of a sample as well as its physical properties determ ine the power absorption. Equal volumes of water in different shaped contai ners attain different final temperatures and a tall, cylindrical water load is shown to have different temperatures at different levels. Most microwav e ovens do not have a true, variable power capability, but rely on an on/of f timing ratio to vary mean power. If this is not appreciated, then erroneo us conclusions might be drawn from a set of experiments involving different power levels. Changes in mains supply voltage can affect the amount of ene rgy dissipated in a load. This may introduce variations in results if exper iments are conducted over a period of several hours. Experiments are descri bed which illustrate these effects and some criteria and working practices are suggested to improve the consistency and reliability of results when us ing a microwave oven as a research tool.