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An analysis of the physics of liquid expansion in fire sprinkler blubs is presented to characterize the variation in void size and pressure with temperature. Two types of bulbs are considered, one filled with liquid and air, and the other filled with liquid and its vapor. The analysis predicts that the void fraction decreases linearly with temperature until the void disappears or the bulb fractures. The increase of pressure at increased temperature is much steeper when there is no air in the bulb. Observations generally support the predicted void sizes, with some commercial bulbs found to contain liquid and air, and others found to contain liquid and its vapor.