Preconditioning time

Multiprobe temperature and humidity distribution profiles should be obtained for empty preconditioning chambers, and temperature penetration profiles should be obtained with probes located within loads. The purpose of these studies is to demonstrate that the load is being uniformly equilibrated to the temperature (and by inference to the humidity) of the sterilizer within the proposed preconditioning time frame. Any serious lack of uniformity detected during validation studies should be investigated and corrected (even if Only by extending the time of the preconditioning cycle). 

The curve J(t,Ag) is the hypothetical creep curve that would be obtained if the age of the sample could be fixed immediately after the beginning of the creep experiment, i.e., A is equal to the preconditioning time, t, and Equation 7 can be written as ... 

In reality the shape of this creep curve is only measurable for creep times substantially smaller than the preconditioning time t, i.e., for t-values smaller than 0.1-0.3t [1],... 

This leads to the following important conclusion. On the supposition that shape and time position of the equilibrium creep curve, J(t,A ), as well as the time-age shift function, b (A,A ), are known at the aging temperature, we can construct the shape of the creep curve, J(t,t ), for any preconditioning times, t [9,17-19]. As a disadvantage of this procedure we have to determine the time-age shift function experimentally from very time-consuming aging creep experiments and that has to be redone for each aging temperature. 

Figures 4 and 5 [19,26,27] show creep measurements on PS N 7000 o.W. at the aging temperatures of T = 90°C and 60°C, respectively. The creep curves for PC 2800 at T = 120°C are given in Figure 6 [12,29]. The full curve in Figure 4 represents the measured equilibrium creep curve, where the preconditioning time was chosen long enough to reach equilibrium in volume. For PS N 7000 o.W. t is about 4 months to reach equilibrium at 90°C. For PC 2800 the equilibrium creep curve at T = 142.5°C is shown (Figure 6). After completition of the various preconditioning times indicated, the ereep experiments were started.

Figure 4. Creep curves of PS N 7000 o.W. under the influence of progressive aging at = 90°C after various preconditioning times full line represents the equilibrium creep curve at T. ...

A remarkable feature of these creep curves is the different initial plateau in the creep compliance at short creep times. The creep compliance is shifted to lower values in the short-time region with increasing preconditioning time, t. As a first consequence it means that the creep curves in the short-time region cannot be brought to coincidence merely by a horizontal shift parallel to the logarithmic time axis an additional vertical shift is necessary. 

Another problem arises in measuring the equilibrium creep curves at low temperatures, which has to be done if b (A,AJ is to be determined according to Equation 16. Within a reasonable experimental time scale the equilibrium state in volume can only be reached at temperatures near T [4,6,10,28]. At T = 90°C the preconditioning time for the equilibrium creep curve is about four months, at 85°C about 30 years, and at 70°C it would be thousands of years. The problem respecting the equilibrium creep curve is solved since it has been shown in [9] that for the evaluation of the time-age shift function it is not absolutely necessary to know the exact time position of the equilibrium creep curve at the aging temperature, but it is sufficient to know the shape of this creep curve. 

The reduced aging creep curves at T = 90°C are shown in Figure 12. As reference value J/ls) the Is value of the creep curve with a preconditioning time t = 2 s was chosen. All creep curves coincide in the short-time region, and the influence of the reduction vanishes for creep times longer 0.3t. These reduced creep curves... 

Figure 16. Comparison for PS N 7000 o.W. between creep compliance from the experiment (symbols) and calculated from theory (drawn lines) at T = 90°C for various preconditioning times.

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