Application of the thermal wave resonator cavity sensor to the measurement of the thermal diffusivity in air contaminated with vapours of different liquid hydrocarbons E. Marín † , * , J. A. P. Lima †† , M. G. da Silva †† , M. S. Sthel †† , S. L. Cardoso †† and H. Vargas †† † Facultad de Física, Universidad de La Habana, San Lázaro y L, Vedado 10400, Ciudad de La Habana, Cuba †† Universidade Estadual do Norte Fluminense. Centro de Ciencia e Tecnologia. Av. Alberto Lamego 2000, 28015-620, Campos dos Goytacazes, RJ, Brasil. * Now postdoctoral fellow of the Centro Latinoamericano de Física (CLAF) at ††. A measuring device based in thermal wave interference in a cavity is described. It was tested measuring the air thermal diffusivity with good accuracy. The device is further applied to the measurement of the thermal diffusivity of air mixed with the vapors of liquid hydrocarbons of the paraffin family (n-pentane, n-hexane and n-heptane). Our results illustrate how the diffusion of their vapors in air can be monitored by a simple method and how the thermal properties (i.e. the thermal diffusivity) of the gas mixture change as a result of this mechanism. Based in the former results a device suitable for in field measurements is predicted. (Received on June 29, 2000, accepted on October 22, 2000) Introduction Despite the increasing number of techniques and methods suitable for materials characterization, their applications to gases have been limited to a few examples, mainly spectroscopic in nature. These traditional techniques are based on the absorption of certain characteristic laser lines by gas atoms or molecules, typically in the infrared range of the electromagnetic spectrum, thus leading to the use of expensive signal generation and detection schemes used mainly for low concentration measurements [1]. As there is a well known expanding demand for reliable and precise measurement of basic properties of gases and as there exist some lack of information related to their thermal properties, such as thermal diffusivity [2], we discuss an alternative method for measurement of this parameter in gas mixtures. It is based on the phenomena of thermal wave interference. The method is another application of photothermal (PT) techniques [3], a field that has experienced enormous expansion in many directions as a result of its general applicability and adaptability to several areas of research. It was proposed recently by Shen and Mandelis [4], who’s succeeded to measuring the air thermal diffusivity with high accuracy by detecting the temperature resulting from the propagation of a thermal wave through the air filling a cavity formed between a PVDF pyroelectric sensor and an Al foil, namely a Thermal Wave Resonator Cavity (TWRC). In the present paper, the method is applied to measure the thermal diffusivity of air saturated with different concentrations of vapors of different liquid hydrocarbons of the paraffin family. Experimental The instrument designed in our laboratories consists of a chamber of total volume 210 cm 3 , containing the gas under investigation, in which a TWRC [3] is enclosed. This is shown in Fig. 1. The cavity, of variable length L, is formed between a 15 μm thick Al foil and a pyroelectric temperature sensor (a 25 μm thick Polyvinyledene Fluoride (PVDF) film with Al-coated surfaces). Light chopped at a frequency f illuminates the external surface of the Al foil, which is painted black and acts as a light absorber. The foil temperature therefore oscillates periodically and generates a thermal wave that diffuses through the gas filling the cavity and causes a temperature fluctuation at the surface of the pyroelectric detector. The temperature distribution T(x,t) within the gas region along the longitudinal x coordinate follows the periodic heating of the Al foil and can be obtained by solving the heat diffusion equation with the boundary condition that light energy is totally absorbed at the foil surface. The solution of physical interest for applications in PT techniques is related to the time-dependent Variable L Pyroelectric Signal V(L) Modulated Light Flux Gas Sample Thermal Wave 0 L x Al-Foil Metalized Pyroelectric Temperature Sensor Fig. 1. Schematic view of the TWRC sensor. 2001 © The Japan Society for Analytical Chemistry s475 ANALYTICAL SCIENCES APRIL 2001, VOL.17 Special Issue