Response time of thermal flow sensors

AbstractThermal flow sensors are applied for measurement of instationary flow and for the control of dispensing very small fluid volumes. For these applications, the response time of the sensor must be determined accurately. We investigated three methods for response time determination: a jump of temperature induced by electric heating, a gas velocity step made by a membrane burst and acoustic phase shifts between sound velocity and sound pressure. The measurements have shown that the response time of thermal flow sensors is a function of flow velocity. For stagnant flow, the thermal response time is at about 4.5 ms. With increasing flow, heat transfer rises, thus response time is faster and moves down to about 1 ms