An inexpensive sonar instrument was prepared for measurements of sound velocity in two fluorocarbon vapors; per-fluoro-n-propane (C3F8), per-fluoro-n-butane (C4F10), and their mixtures. The apparatus, measurement principle and instrument software are described. All sound velocity measurements in per-fluorocarbons were made in the low pressure 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 chromatography. Uncertainties in the speed of sound measurements were arround +/-0.1%. Comparisons were made with theoretical predictions of sound velocity for the two individual components. The instrument was then used for concentration monitoring Of C3F8/C4F10 mixtures. This approach is based on the sensitivity of sound velocity to variations in the composition of binary mixtures 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 molar mass. The temperature sensitivity of the instrument for the required mixture measurement precision of 1% was determined to be 0.9 degreesC; the measured temperature stability of the instrument was of this order.