Seal leaks pressure-driven

Oil, used for lubricating the gears and bearings, is separated from the compressed airstream by labyrinth seals. Shown in Figure 3.31, the seals consist of two separate components, an air seal and an oil seal. In between the two, the seal is open to atmosphere. Any oil bypassing the oil seal leaks to atmosphere. The air seal is always at a higher pressure so that any leaks are always from air to atmosphere and oil cannot enter the process fluid. Oil is typically provided by a shaft-driven pump with a standby motor-driven pump available. 

A leak in the pressure vessel of the seismometer would allow air to pump in or out driven by atmospheric pressure changes, and this would cause the proof masses to rise or fall buoyed by changing air density inside the vessel. This creates a very pronounced noise that is strongly vertical. Pressure leaks mostly arise in seismometers that provide service access ports such as for manual mass centering that may not have been properly closed or where the seals have deteriorated. 

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