Moisture detection device

Very small projections (-) are arranged on a mirror surface (-) of a mirror. The shape of a projection (-) is not limited to a circular cone, but it can be a hollow-cylindrical shape, semispherical shape, or square prism shape, and also, it can be a polyhedron with many faces. With a reduction in temperature of the mirror, water vapor contained in a gas to be measured condenses on the mirror surface (-) of the mirror. In this case, because of the very small projections (-) on the mirror surface (-), the condensation is promoted by the projections (-) serving as the cores. This facilitates condensation even at low dew points and improves response. Further, the size of condensation products does not easily vary relative to variation in flow speed of the gas to be measured, and this makes equilibrium of condensation less likely to break, increasing measurement accuracy.