Preliminary Study on Humidity and Gap Sensing Abilities of Planar Coil Made of SnO2

To use a single sensor to measure the humidity and the non-contacting gap, a planar coil made of SnO2 is designed. The spiral structure of the SnO2 makes it possess the capability to induce the eddy current effect, which results in the coil made of SnO2 being not only humidity sensitive but also gap sensitive. The humidity variation can induce variations in the impedance and the dc-resistance of the SnO2 coil. The gap variation can induce the impedance variation whereas cannot induce the dc-resistance variation. The dc-resistance decreases with the increase of the humidity. Under the invariant humidity/gap, the impedance amplitude and the impedance angle decrease with the increase of the gap/humidity. A way to solve the difficult problem to use the SnO2 coil to realize the humidity and the gap is proposed. The result that “the dc-resistance is changed” indicates that “there is a humidity variation,” and then the humidity variation can be gotten by using the humidity-dc-resistance-calibrating-relation. The impedance variation induced by the humidity can be gotten by using the humidity-impedance-calibrating-relation. On the basis of the overall impedance variation and the impedance variation related to the humidity, the impedance variation related to gap is achieved. Finally, the gap variation is gotten by using the gap-impedance-calibrating-relation. The aforementioned researching conclusions prove the feasibility of using the coil made of SnO2 to measure the gap and the humidity.