Velocity of sound measurements in gaseous per-fluorocarbons and their mixtures

An inexpensive sonar instrument was prepared for measurements of sound velo city in two fluorocarbon vapors; per-fluoro-n-propane (C3F8), per-fluoro-n- butane (C4F10), and their mixtures. The apparatus, measurement principle an d instrument software are described. All sound velocity measurements in per-fluorocarbons were made in the low p ressure range between 0.01 and 0.4 MPa, and at temperatures between 253 and 303 K. The purity of the C3F8 and C4F10 samples was checked using gas chro matography. Uncertainties in the speed of sound measurements were arround /-0.1%. Comparisons were made with theoretical predictions of sound velocit y for the two individual components. The instrument was then used for conce ntration monitoring Of C3F8/C4F10 mixtures. This approach is based on the s ensitivity of sound velocity to variations in the composition of binary mix tures of gases with disparate molar mass. Verification of mixtures was made along the saturation line over a range of temperatures, and also over the P-T region, where both components were superheated. The instrument was calibrated with sound velocity measurements in xenon; a gas having the combination of a well-defined specific heat ratio and high m olar mass. The temperature sensitivity of the instrument for the required m ixture measurement precision of 1% was determined to be 0.9 degreesC; the m easured temperature stability of the instrument was of this order. (C) 2001 Elsevier Science B.V. All rights reserved.