US Patent 6927681 Mixed air amount alarm device

A mixed air amount alarm device is provided which determines the amount of air present in a hydraulic circuit between a hydraulic pressure source and an instrument driven by hydraulic pressure, and produces an alarm if the air amount is excessive, a situation is prevented in which an alarm is produced or not produced according to change of the air amount (volume) resulting from temperature change, and it is possible to stop an alarm if potential danger disappears and a safe state is recovered due to spontaneous release of air out of the hydraulic circuit. If the air amount is determined excessive, this is stored in a nonvolatile memory, and even if air amount excessiveness determination is not made at the next activation of the hydraulic device, the content of the nonvolatile memory is checked and an alarm is produced if air amount excessiveness determination has not been made before. It is preferable to add a function of erasing the air amount excessiveness determination history when specific conditions are met, for example, if the fluid temperature of hydraulic fluid or its inferred value is equal to or higher than a predetermined value, and in this state, the air amount is determined to be at a permissible level.