SM500F
Field mountable paperless recorder Appendix E – Math Equations
IM/SM500F Rev. Z 169
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:
Where
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.
F
val(t) = Current Fvalue
Fval(t–1) = Fvalue at last sample
x = Actual temperature
y = Target temperature
z = Z-factor (i.e. the temperature interval representing a factor of 10 reduction
in killing efficiency)
F
val t
F
val t 1–
10
xy–
z
----------
60
---------------------+=
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