A method and an apparatus for accelerating the equilibration of a fluid, typically the reference fluid of an analyzer, are provided, the surface of the fluid being in contact with a gas phase. The device can include a cassette, which can be exchangeably inserted into an analyzer and which holds at least one flexible, gas-tight bag, in which the fluid and the gas phase are accommodated. The cassette or the analyzer can include at least one transmitting element, which can be caused to vibrate and which mechanically contacts the flexible bag in order to input mechanical energy into the liquid in at least one wall region.