A tiltmeter is an instrument designed to measure very small changes from the horizontal level, either on the ground or in structures. Tiltmeters are used extensively for monitoring volcanoes, letting us know if the dams are filling, the small movements of potential landslides, the orientation and volume of hydraulic fractures to various influences such as loading and foundation settlement.
Tiltmeters work by measuring small changes in the ground. The very first tiltmeter was a long-length stationary pendulum. These were used in the very first large concrete dams, and are still in use today, amplified with newer technology such as laser reflectors. Although they had been used for applications such as volcano monitoring, they have distinct disadvantages, such as their huge length and sensitivity to air currents. Even in dams, they are slowly being replaced by the modern electronics tiltmeter.
Volcano and earth-movement monitoring then used he water tube, a long baseline tiltmeter. This was a simple arrangement of two water pots, connected by long water filled tube. Any change in tilt would be caused by a difference in fill-mark of one pot compared to the other.
Although extremely used throughout the world for earth science research, they have proven to be quiet difficult to operate. For example, due to their high sensitivity to temperature differentials, these always have to be read in the middle of the night.
The modern electronic tiltmeter, which is slowly replacing all other forms of tiltmeter, uses a simple bubble-level principle, as used in the common carpenter level. An arrangement of electrodes senses the exact position o the bubble in the electrolytic solution, to a high degree of precision. Any small changes in the level are recorded using a standard data logger. This is quiet insensitive to temperature, and can be fully compensated, using built-in thermal electronics. A newer technology uses MEMS electronics, it is not known if this can eventually displace the common bubble.
The most dramatic application of tiltmeters is in the area of volcanic eruption prediction. The main volcano in Hawaii had a habit of filling the main chamber with magma, and then discharging to a side vent. This repeated action, with a pattern of swelling of the main chamber, draining of that chamber, and then an eruption of the adjoining vent. Each number at the park of tilt, on the graph is a recorded eruption.