A PC based photo-acoustic instrumentation system

The development of a compact real-time photoacoustic system is described. The system is designed to measure hydrocarbon concentration levels in the range 0-200 ppm in oil industry process discharge water. A developmental system using the output from a Q switched Nd:YAG laser, frequency shifted using Raman scattering and a monochromator, allows spectral measurements in the range 1-2 μm to be made. A PC controlled acquisition system, with a 25 MHz sample rate, is used to automate the measurements and subsequent data analysis. The raw photo-acoustic signal is immersed in random noise and the fast acquisition system enables 4096 samples to be taken within the pulse repetition rate to obtain an adequate signal to noise ratio in real-time. Programmable electronics have been developed to reduce the digitisation errors to less than 0.5%. Further off line analysis using neural networks demonstrates the potential of the technique