Sterilising
The time-temperature-pressure relationship

There are important relationships that need to be maintained between temperature, time and pressure (see table below) to ensure adequate steam quality. Deviations from these ranges may compromise the sterilisation process. Always check the cycle printout to ensure these parameters have been met.

The sterilising time for a particular load is calculated using the holding time (which includes a safety factor) plus penetration time.

STERILISING TIME = HOLDING TIME + PENETRATION TIME

The holding time is the time the whole load must be held at sterilising temperature. This time depends on the temperature selected, for example 121oC for 15 minutes holding time or 134oC for 3 minutes holding time (see table below).

Penetration time is the time it takes for the hardest to reach part of the load to achieve the required temperature or the time it takes for the whole load to reach that temperature. The penetration time is variable and depends on the size and nature of the load. The validation process will establish the penetration times for each type of load. The penetration time must be added to the holding time to ensure adequate sterilisation of the whole load.

The sterilising time is usually set automatically by the manufacturer of the steriliser, depending on the type of load selected. The importance of selecting the right cycle for the load type cannot be overstated as it is essential that there is adequate penetration of steam into the whole load to ensure sterilisation occurs. It is important that the cycle time and temperature reflect the type of load and packaging material used. Always follow manufacturers' instructions and have the operator's manual available at all times.

The internationally recognised relationship is shown in the table below.

Temperature(oC)Pressure(kPa)Pressure (mb)Pressure (psi)Holding time (in minutes) including safety factor*
121
126
132
134
103
138
186
206
1030
1380
1860
2060
15
20
27
30
15
10
4
3

*Note that penetration times need to be established and added to these.

See section 4.2 of AS/NZS 4815.