PROBLEM TO BE SOLVED: To provide a fire detector which prevents a false alarm or an alarm failure when an attenuation coefficient is changed due to a change in surrounding environment.

SOLUTION: A sound pressure ratio calculating means 40 calculates a sound pressure ratio between a sound pressure of a first ultrasonic wave propagating from a sound source 1a to a sound wave receiving element 3a and a sound pressure of a second ultrasonic wave propagating from the sound source 1b to the sound wave receiving element 3b through a propagation path longer than that of the first ultrasonic wave. A sound pressure ratio correcting means 45 corrects the sound pressure ratio for an output of the sound pressure ratio calculating means 40 so as to cancel a variation caused by the change in an attenuation coefficient of absorption of the ultrasonic wave in a monitoring space with no smoke. A smoke concentration estimating means 41 estimates smoke concentration in the monitoring space using the sound pressure ratio whose variation due to the change in the attenuation coefficient is canceled. A fire determination means 42 determines whether a fire has occurred or not by comparing the smoke concentration estimated by the smoke concentration estimating means 41 and a predetermined threshold.

COPYRIGHT: (C)2010,JPO&INPIT

【課題】周囲環境の変化に起因した減衰係数の変化の影響で非火災報や失報を生じることのない火災感知器を提供する。
【解決手段】音圧比算出手段４０は、音源部１ａから受波素子３ａに伝わる第１の超音波の音圧と、第１の超音波より長い伝播経路を通して音源部１ｂから受波素子３ｂに伝わる第２の超音波の音圧との比を音圧比として算出する。音圧比補正手段４５は、音圧比算出手段４０の出力について、煙がない状態での監視空間における超音波の吸収減衰の減衰係数の変化に起因した変動分をキャンセルするように前記音圧比を補正する。煙濃度推定手段４１では、減衰係数の変化に起因した変動分がキャンセルされた音圧比を用いて、監視空間の煙濃度を推定する。火災判断手段４２は、煙濃度推定手段４１にて推定された煙濃度と所定の閾値とを比較して火災の有無を判断する。
【選択図】図１