1. Providing the air is dissolved in the oil, it will not affect its characteristics.
2. Undissolved air, i.e. bubbles, cause the following:
- signs of corrosion on pumps and controls,
- increased compressibility and the associated potential for the output element to produce stuttering movements,
- premature ageing of oil,
- destruction of seals during relief.
Air dissolves in oil in proportion to the pressure (up to around 300 bar). This means that dissolved air at low pressure is released until the saturation limit is reached (e.g. after the throttles). Since the rate of release is greater than the rate of dissolving, air bubbles remain in oil for longer still, even when pressure increases again, with the consequences outlined above.
During assembly work, air infiltrates the circuit at places in the negative pressure (suction) area which are not properly tight and when oil is flowing back into the container. If it infiltrates the oil in the container, it will keep being drawn in as it only rises slowly.