When propane escapes through a vent or nozzle, you can easily measure the orifice's size. A rectangular vent has an area equal to the product of its length and width. A circular nozzle opening has an area equal to the product of pi and its squared radius. Yet when propane escapes through an irregular orifice, you may find the orifice size more difficult to gauge. In these cases, you can calculate the orifice size from the propane's rate of flow.

Find the square root of the pressure drop across the orifice, measured in inches of water. For example, if propane leaks through an opening at a pressure of 0.8 inches of water: √0.8 = 0.894.

Multiply this answer by 4,005, a fixed conversion factor: 0.894  4,005 = 3,580.47.

Divide the rate of air flow, measured in cubic feet per minute, by this result. For example, if 360 cubic feet of propane leak through the orifice each minute: 360 ÷ 3,580.47 = 0.1. This is the orifice's size, measured in square feet.
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