MEASUREMENT OF VOLUME FLOW-THROUGH A MICROPOROUS MEMBRANE BY A NTC THERMISTOR MINIATURE BEAD

We propose an application for the NTC (negative temperature coefficien t) thermistor miniature bead as a sensor to measure the volume flowing through a microporous membrane over a 0-15 cm(3)/min range. Only one thermistor is used and small temperature changes that can occur during the experiment can be ignored. No compensation or linearity circuitry is used [C. Yang, M. Kummel, and H. Soeberg, Sensors and Actuators 15 , 51 (1988)]. Assembly is easy and it can be adapted to automatic cont rol, for example, to control the flow of a peristaltic pump by connect ion to an A/D DIA board and computer. When the experimental data was f itted to a potential type curve (y = ax(b)), a sensitivity between 5% and 8% and a correlation coefficient of 0.99 were obtained. The respon se of the sensor when the flow changes is immediate, although the time taken for the measurement to stabilize is dependent on the size of th e variation (between 4 and 7 s). The fitting curve is used to determin e the volume flow that is caused by a pressure gradient applied to the membrane placed between two bulks with the same fluid.