Here we design, construct, and characterize a compact Raman-spectroscopy-based sensor that measures the concentration of a water-methanol mixture. The sensor measures the concentration with an accuracy of 0.5% and a precision of 0.2% with a 1 second measuring time. With longer measurement times, the precision reaches as low as 0.006%. We characterize the long-term stability of the instrument over an 11-day period of constant measurement, and confirm that systematic drifts are on the level of 0.02%. We describe methods to improve the sensor performance, providing a path towards accurate, precise, and reliable concentration measurements in harsh environments. This sensor should be adaptable to other water-alcohol mixtures, or other small-molecule liquid mixtures.

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