Page 171 - PBL614 IM/SM500F-EN SM500F ( WIRO)

SM500F
Field mountable paperless recorder Appendix E – Math Equations
E.3 Sterilization Fvalue Calculation
The ability of heat to kill micro-organisms varies with the type of organism and increases exponentially with rising temperature.
Therefore, the time taken in sterilization is reduced if the target temperature is increased and the time spent approaching and receding from the target temperature can be taken into account.
Example – an increase of 10°C from 121.1 to 131.1°C in the steam sterilizing temperature of the Bacillus stearo-thermophilus organism increases the death rate by a factor of ten.
The change in sterilization temperature which causes a factor of 10 change in the death rate is unique to each organism and is called the Z value.
Although 121.1°C is universally accepted as a reference for steam sterilization processes, the actual sterilizing temperature varies, depending on the products involved and on each sterilization process.
The Fvalue is calculated using the general formula:
10 z  Fvalt = Fvalt – 1 + --------------
Where
Fval(t) = Fval(t–1) =
x =
y =
z =
 x–y ------
60
Current Fvalue
Fvalue at last sample
Actual temperature
Target temperature
Z-factor (i.e. the temperature interval representing a factor of 10 reduction in killing efficiency)
Example – A typical steam sterilizing cycle – see Fig. E.2.
The period AB is the chamber evacuation part of the cycle, when the chamber is alternatively evacuated and purged with steam to remove air. The ramp up to final sterilizing temperature starts at B. The thermal conductivity of the load determines the time taken to achieve point D, but is typically 30% of the total cycle time. It is in the area, C D, and E F, that Fvalues make their contribution to shortening sterilization time, by accumulating credit for the time spent approaching and receding from the sterilizing temperature.
IM/SM500F Rev. AA 169