There is demand for non-invasive, low-cost and accurate sap flow measurement in horticulture in order to optimize yield, heating and irrigation. The current state-of-the-art of sap flow technology and ultrasonic sensors do not fulfil this research gap. The novel concept for a non-invasive ultrasonic sensor for slow flow rates demonstrated by a TU Delft bachelor thesis group was a promising alternative solution. To find out if the novel concept could be developed in to a reliable alternative method for sap flow or slow flow rate measurement this thesis project sought to answer the following research question: What are the design sensitivities and parasitic sensitivities of the non-invasive ultrasonic flow sensors accuracy and how can the latter be mitigated. The experimental setup of the previous research was constructed and improved upon. A one-dimensional analytical model of the setup was composed to evaluate the contribution of the transducers oscillator dynamics to the output measurement. This research unfortunately found that the sensors seemingly high resolution to very small flow rates can be attributed to a consistent measurement error.