Miniaturization of on-wall in-tube flexible thermal flow sensor using heat shrinkable tube

We previously proposed a novel type of on-wall in-tube thermal flow sensor, and experimentally evaluated its performance. The sensor has an advantage that it can measure the flow-rate both at near the tube entrance (hydraulically developing region) [1] and at bent tube (axially asymmetric flow) [2]. With the requirements of further miniaturization of the sensor in the fields of the portable fuel cells and chemical analyzers, we newly developed the fabrication process to miniaturize it less than 2.0 mm in external diameter by using a heat shrinkable tube. The film sensor fabricated by photolithography was inserted inside tube manually. By applying heat shrinking process, the film was automatically mounted on the inner wall surface, and the outer size of the tube was miniaturized to almost of the half size from its original. The final inner and outer diameters of the tube were 1.2 mm and 1.7 mm, respectively. We evaluated the fundamental performance of this flow sensor. The electrical resistance at the sensor linearly increased with temperature. The obtained temperature coefficient of resistance of the sensing element was 0.0023 K^-1. We measured the relationship between the input power consumption and the gas flow rate, and finally evaluated the response time. We obtained a value of 100 msec by forming a cavity structure under the heat element.