蝴蝶風門是眾所週知的導管風量控制元件，因設計細節之不同而可細分為許多亞種。在局部排氣裝置之中，蝴蝶風門通常安裝於緊鄰氣罩的下游位置，以便調整氣罩的吸入風量。無論蝴蝶風門能細分成幾個亞種，不同的亞種之間總是會有一些必須共同遵守的物理定律，而蝴蝶風門造成的導管靜壓損失就是由這些物理定律共同決定。本篇將「氣罩以及緊鄰氣罩下游的附屬風門」視為－整個被動的工作元件以利用其物理特性：由於氣罩開口附近的氣壓與環境氣壓相同，元件造成的靜壓損失即是蝴蝶風門下游負壓，且其數值與氣罩下游導管動壓成正比。若能取得蝴蝶風門的特性函數，則可進一步評估氣罩靜壓損失，實現控制氣罩效能的目的。對於前述的蝴蝶風門特性函數，本篇提出新的方法，能在不需預知氣罩風量的情況下描述蝴蝶風門的靜壓損失與風門關閉角的函數關係。本篇同時也展示了取得此函數的實驗過程，最後再以具體案例說明如何利用此函數來實現快速調整氣罩風量的目的。利用本篇提出的方法，局部排氣裝置設計人員能在安裝試車之前預估各個蝴蝶風門的關閉角設定值，以縮短微調各氣罩性能所需的時間。

Butterfly vavle stands for a well-known category of throttling means ot control duct airflow rate. In most cases, a butterfly valve is attached downstream to a local exhaust ventilation hood. Despite of all the differences on valve body design, butterfly vaves always follow some fundamental physical rules, by which the pressure difference across a butterfly valve can be predicted. In this study, an ordinary installation set of an exhaust hood with its butterfly valve attached downstream, is taken for a working object. The working object, with room pressure or environmental pressure in the vicinity of hood opening, builds up a static pressure loss which is proportional to in-duct dynamic pressure. Since a working object is totally passive, the static pressure loss across a working object can be calculated by in-duct dynamic pressure and valve characteristic function. A new method to identify the above-mentioned non-dimensional characteristic function of butterfly valves, as well as a practical working example, is also presented in this study to show how the modulation of a local exhaust ventilation system can be simplified.