Sterilization Fvalue Calculation - ABB SM2000 User Manual Supplement

...2 MATH CONFIGURATION
2.9.5 Sterilization Fvalue Calculation – Fig. 2.8
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.
– an increase of 10ºC from 121.1 to 131.1ºC in the
Example
steam sterilizing temperature of the Bacillius Stearo-thermphilus
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 121.1ºC 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:
(x – y)
(10
Z
F = F +
val(t) val(t–1)
60
Where
F = Current Fvalue
val(t)
F = Fvalue at last sample
val(t–1)
x = Actual temperature
y = Target temperature
z = Z-factor
representing a factor of 10 reduction in killing
efficiency
– A typical steam sterilizing cycle – refer to Fig. 2.7
Example
below.
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 achive 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.
°C
140
120
100
80
60
40
20
A
Fig. 2.7 Typical Steam Sterilization Cycle
10
)
(ie. the temperature interval
B C D E F
Time (minutes)
It is important to note the large change in equivalent sterilizing
time which results from a small increase in the sterilizing
temperature. Going from 121ºC to 122ºC, an increase of only
1ºC, reduces the time needed to kill an eaqual number of
organisms by a factor of 26%. Likewise, a measurement error
which results in the set point being 1ºC too low could result in a
product not being sterilized properly.
As the Fvalue calculation is essentially a logarithmic function, the
effect of measurement errors is significant on the resultant
Fvalue.
The table below shows the resultant error in the Fvalue resulting
from various measurement errors with a Z value of 10ºC.
T e m p e a r u t e r E r r r o º ( ) C
0 1 .
– 0 1 .
0 5 .
– 0 5 .
1 0 .
Table 2.6 Fvalue Accuracy
The ScreenMaster can measure TC and RTD inputs with an
accuracy of better than 0.1%. This results in superior Fvalue
calculation accuracy.
To improve the accuracy even further the Scale Adjust facility
can be used to adjust the individual channel readings to be
correct at the sterilizing temperature.
As Fvalue calculation is an integrating function, the sample rate
has a direct effect on the accuracy when the temperature is
changing. With a steady state signal the sample rate does not
affect accuracy.
F v a u l e E r r r o F ( ) o
2 3 . %
– 2 3 . %
1 2 0 . %
– 1 1 0 . %
2 6 0 . %