A compact, robust, and inexpensive magnetically coupled pressure gauge includes a spiral-faced or helical bellows coupled for rotating a magnetic field source (permanent magnet) within a pressure vessel the orientation of which is externally sensed and correlated to pressure within the pressure vessel. Mechanisms of extraneous magnetic field compensation are discussed Applications contemplated include measuring pressure in pressure vessels and pressure of pneumatic vehicle tires without breaching the integrity of the particular pressurized vessels. Embodiments included a visual, manual tire pressure monitoring system.