Thermal flow meter with drainage passage at inlet of bypass passage

Abstract

Provided is a thermal flow meter through which a measurement with high accuracy and has a discharge function. In a thermal flow meterof the invention, a part of a measurement target gasflowing through the main passageflows into a bypass passage. A drainage passageis provided in the bypass passage to communicate a bypass passageon the inlet port (between the inlet portand the measurement surfaceserving as the air flow sensing portion) and a bypass passageon the outlet port (between the measurement surfaceserving as the air flow sensing portion and the outlet port). The drainage passageincludes a through hole. The through holeincludes an inlet portwhich penetrates a wall surfaceforming the bypass passageon the inlet port and is opened in the bypass passageon the inlet port, and an outlet portwhich is opened in a rear surfaceof the wall surface. Waterflowing into the bypass passage is guided to the through hole, and guided to the bypass passageon the outlet port through the drainage passage, and discharged to the main passage. Therefore, an influence of the water flowing into a measuring portion can be reduced, and the measurement accuracy can be improved.