The controller decides on a 'normal mode' when the amount of change in water level, ∆H, and the amount of change in pressure, ∆P, transmitted by the pressure sensor unit and the level sensor unit fall within reference value ranges (±Hr, ±Pr) over a set time T, decides on 'safe mode' when the amount of change in water level, ∆H, and the amount of change in pressure, ∆P, fall within set value ranges (±Hs, ±Ps) over the set time T, uses the respective set values to operate the exhaust valve and the air compressor connected to the air chamber during 'normal mode' or 'safe mode', and operates the exhaust valve or the air compressor only when 'safe mode' is departed from in order to control the water level and pressure in the air chamber.
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