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A bellows gauge is a mechanical instrument designed to measure very low pressures or vacuum conditions. It operates using a thin-walled, accordion-like bellows that expands and contracts in response to pressure changes. This motion is transmitted through a mechanical linkage to a pointer, providing an accurate pressure reading even in extremely low ranges. Without a bellows-type design, small changes in low-pressure systems—such as in air, gas, or liquid level monitoring—can be difficult to detect. In these applications, bellows gauges deliver the sensitivity and accuracy required to maintain stable process conditions. Ashcroft is a global leader in pressure and temperature measurement instrumentation. This article is part of our Understanding the Basics series. Read it to learn how bellows gauges work, when to use them and what makes them essential for certain low-pressure applications.