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A simple thermodynamic model with temperature and volumetric flow rate measurements was developed to determine the mass flow rate of anhydrous ammonia (NH3) to a single knife on an applicator while operating in the field. The model did not predict the mass flow rate well. One possible reason for the model failure was the poor calibration of the volumetric flow meter. Also, the paddlewheel flow meter designed for use with liquids may not work well to measure NH3 flow rates where the NH3 may be 65 to 97% gas by volume. Another possible reason for the failure of the model is that the model may be too simple to adequately describe the actual behavior of the NH3 flow in this system.