Contd...

5. Since hot-wire is based on heat transfer principles the calibration curve changes if either the wire temperature or the ambient temperature is altered. If the velocity is large ( m/s in air), heat transfer from the wire is primarily due to forced convection and hence for a given velocity the heat transfer coefficient is independent of the temperature difference . At smaller velocities additional heat transfer mechanism such as conduction to the prongs and free convection become important. When is independent of changes in can be accounted for by changing to keep and hence the calibration curve unchanged. Since the wire resistance and are related as

adjustments in can be made by a suitable setting of the operating resistance . For tungsten the value of is about C.