Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

262 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSA dilatometer constructed by Saue? is shown in Fig. 45. The sensing elementis a linear variable differential transformer, whose rectified output is recordeddirectly. A pivot arm connects the transformer (A) to the bearing arm (B) and amicrometer screw allows for calibration and adjustment of the transformer. Byadjustment of the compensating screw, the pivot arm can be moved so that thecontact point (B) is exactly one sample length from the fulcrum. Temperaturesabove and below ambient can be obtained by circulating gas round the specimen.Volume expansion is most easily measured by fluid displacement and is particularlysuitable for organic materials which are not generally hard or highly oriented.In an apparatus devised by Loa~by~~~, the sample is enclosed in a bellows and theintervening space filled with oil. Expansion is recorded as a linear motion of thebellows. The sample size is made as large as possible so that it is just out of contactwith the bellows at the point of maximum expansion. Smaller pieces can be usedinstead of a single specimen. This allows a more rapid approach to thermal equilibrium,but at the expense of sensitivity.,Cradle slideadj. screwExhaustFig. 45. Programmed dilatometer providing sub- and supra-ambient temperatures. H. A. Sauer,from ref. 348..,To avoid the need for calibration, Lieberman and Crandal1364 have devised adilatometer which operates by measuring the change in light intensity after passingthrough two parallel diffraction gratings. As the sample expands, the gratings moveparallel to one another and the light recorded by a photocell passes through aseries of maxima and minima. Traverse through each maximum corresponds to amovement of one grating spacing. The accuracy of the measurement depends on theperfection of the grating and the accuracy of reading the maxima and minima.Danni~~~’ used a different optical method. The movement of a mirror, partially