PURPOSE:To conform the response time constants of both elements to each other, based on heat conduction, by a method wherein wet-bulb temperature detecting element is located at a forward end of a single long and narrow protection tube which has at least a flexible central part, and a dry-bulb temperature detecting element is housed in a part except the central part and the forward end part. CONSTITUTION:The temperature detecting element 2 (thermistor or the like) for wet-bulb is housed in the forward end part 11 of a long and narrow flexible protection tube 1, and the outer circumference surface of the forward end part 11 is surrounded with a water absorption gauge 4 which feeds water properly. A temperature detecting element 3 for dry-bulb is located in the proximity of the other end, and lead wires 21, 22, 31, and 32 for the elements 2 and 3 are lead out at the other end through a sealing member 14. The protection tube 1 is filled with a powdery insulating substance or an insulation process is applied to the lead wires. The detector is located in a duct such that a central part 12 is bent and a relative position is properly set in a manner to position the wet-bulb detecting end diagonally to the lower of a dry-bulb element 3, and this conforms the response time constants of the both elements 2 and 3 to each other.
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