  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.
