Fixed-roof liquid hydrocarbon storage tanks are a recognized source of volatile organic compounds. However, current emissions estimates suffer from a lack of reliable models. This work presents the development and characterization of an intrinsically safe, low velocity mass-flux sensor for monitoring storage tank venting. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation targeting oxygen within the tank vent line. Similar sensors have been successfully used to measure high velocity flows. Sensor design, optimum operational parameters, and indoor characterization results are presented. An oxygen concentration precision of 0.11% absolute was achieved over the tested range of 2.5-21%. For measurements at 21% O2, a velocity standard deviation of 0.24 m/s for 30-s average measurements was achieved, which could be improved to 0.07 m/s with a 10-minute interval drift correction. However, velocity measurement accuracy was poor at lower O2 concentrations.