This thesis presents a comprehensive static model for a capillary type thermal massflow sensor for gases. Using such a sensor, a meter is constructed which measures gas mass flow by means of a bypass arrangement which ensures a constant flow ratio between the sensor and the meters’ mam flow element. The model equations are derived using steady state heat transfer theory for both a simplified one-dimensional and a multi-region two-dimensional case Consideration of a two-dimensional model allows for the analysis of additional model parameters such as gas conductivity and heat loss. However for such a model, the use of analytical methods to obtain a system solution is precluded due to the complexity of the system, and hence a numerical discretisation method, namely the finite difference scheme, is employed to compute the temperature field for the multiregional system. Heating coils are used to heat the sensor tube and are included in a bridge arrangement. The mass flow rate, as indicated by the instrument, is calculated as a function of the bridge voltage as detected by the coils, which also function as thermal sensors. A reduced model is developed by omitting negligible model terms Such a model may be then used as a design tool for the simulation and optimisation of the performance of the flow meter. Actual test rig results are used to validate the model and both sets of results correlate well for low mass flow rates However, there remains a noticeable difference between the two for the non-linear sections of the output curve and the reason for this remains to be ascertained. Nevertheless, the model has proved beneficial in showing the effect of parameter changes and the sensor’s susceptibility to ambient temperature changes. Recommendations for future development and improvement are listed.