Mechanically simple, reliable devices for measuring pressure or detecting pressures above a certain limit are disclosed. A representative device includes a magnet surrounded by one or more bands of a ferrofluid, and which is free to move within a nonmagnetic, generally elongated housing. In operation, the housing is oriented vertically, and the lower aperture exposed to the region of pressure to be sensed. The opposite end, which preferably contains another aperture, is exposed to a reference pressure (ordinarily the atmosphere) that remains isolated from the region of pressure to be sensed. Gravity draws the magnet downward, toward the bottom of the housing, while pressure at the lower aperture tends to force the magnet upward. When the force exerted by the pressure source exceeds the downward force of gravity, the magnet travels upward through the housing; this movement may be detected by an appropriate sensing arrangement, and is used to provide a warning signal or a quantitative pressure measurement.