ABB SM500F User Manual page 172

SM500F
Field mountable paperless recorder
Fig. E.2 Typical Steam Sterilization Cycle
It is important to note the large change in equivalent sterilizing time which results from a small increase in
the sterilizing temperature. Going from 121°C to 122°C, an increase of only 1°C, reduces the time needed
to kill an equal number of organisms by a factor of 26%. Likewise, a measurement error which results in the
set point being 1°C 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.
Table E.2 shows the resultant error in the Fvalue resulting from various measurement errors with a Z value of
10°C.
Temperature Error (°C)
Table E.2 Fvalue Accuracy
The recorder 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 further, the 'Input Adjustment' facility (see Section 7.10.1, page 117) 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.
Fig. E.3 is an example of configuring a math block to perform a sterilization Fvalue calculation.
170
0.1
–0.1
0.5
–0.5
1.0
Appendix E – Math Equations
Fvalue Error (F0)
2.3%
–2.3%
12.0%
–11.0%
26.0%
IM/SM500F Rev. Z