Fluids (and freeze-dried products) in small  glass vials and ampoules can usually withstand the rigors of saturated steam sterilisation. The critical stage comes after sterilisation, during cooling. Vial contents and any enclosed air will still be hot, generating internal pressure. To prevent vessel breakages this internal pressure typically needs to be countered by an applied external pressure. The requirement for air-free saturated steam ends with the sterilisation phase, so during cooling sterile filtered compressed air is used to “ballast” the load to a controlled overpressure. This is “Saturated Steam sterilisation with Ballasted Cooling”.

Hot, molten media in open containers can also be sterilised in saturated steam autoclaves. Gravitational displacement of air by steam avoids the deep vacuum pulses which optimise air removal vacuum pulses but risk over-boiling the media.

Fluids in larger, fragile glass vessels or deformable containers – plastic bottles, LVPs, blister packs, pre-filled syringes etc.  - present a more challenging problem.  The pressure outside the container must be accurately matched to the pressure inside during the whole process including sterilisation, not just during cooling. This is “counter-pressure” sterilisation.

• In “Air-Steam” counterpressure sterilisation, steam is used as the heating medium, sterile filtered compressed air is used to equalise the pressures throughout the cycle.  Chamber fans and ducting must be used to keep air/steam mixes homogenous, dual control techniques are needed as temperature and pressure controls are no longer synonymous.
• In “Superheated Water” counterpressure sterilisation, compressed air is used for pressure management, but the steam is replaced by a recirculated water spray, heated by plant steam. The water spray is itself sterilised during the process. This technique offers faster heating (and then cooling), requires no Pure Steam but the resulting wet load rules it out of some applications.