Unusual Autoclave Temperature Distribution Results

James Agalloco, Agalloco & Associates

One of the commonly required tests performed in conjunction with the operational qualification of a steam sterilizer is the empty chamber temperature distribution test.  In this test, thermocouples are positioned throughout the sterilizer, and the temperature range across the sterilizer measured (An acceptable distribution range might be anywhere from ±0.5 to ±2.0 C based upon the age and use of the sterilizer).  The original reference to this can be found in proposed but never official 21 CFR 212 CGMP Requirements for Large Volume Parenterals from 1976.
In recent years some seemingly unusual results have been observed that are easily understandable if the design details of the sterilizer are better known.  In these studies, thermocouples positioned near the door seals were found to have temperatures substantially higher than those observed in the rest of the autoclave chamber.  For many years, autoclaves utilized compressible seals that were installed on either the door or the autoclave wall.  Most recent designs have replaced these compressible seals with retractable ones installed in deeper slots on the autoclave wall (see Figure 1) and are commonly installed with automatic doors.  The seals are held in place against the door using a pressurized gas, and are withdrawn using vacuum to allow for easy door movement.  The gas utilized can be either compressed air or steam.  With either of these, the pressure behind the seal must be substantially greater than the internal pressure inside the autoclave in order to maintain the seal during the dwell portion of the sterilization cycle.  When steam is utilized for this purpose, the higher pressure required for a proper seal corresponds to a higher localized temperature in the door seal area.  The heat from the steam behind the door seals can raise the temperature inside the chamber adjacent to them and create the out of tolerance temperatures observed.
There are two solutions possible to resolve the difficulties with excessively high distribution temperatures in this region.  The first solution would be to replace the high pressure steam with high pressure air behind the seals.  This may necessitate piping and software changes to the sterilizer, and while certainly effective, it may not be a trivial change.  The alternative is to relocate the distribution thermocouples slightly farther away from the door seal area so that the observed temperatures are more in line with expectations.  The use of the autoclave cart as a means for supporting the thermocouples provides a reproducible means of defining their location without positioning them so close to the heated seal area as to provide non-conforming results.  The first method might be considered more correct as it eliminates the locally superheated area around the door seal, however it is decidedly more expensive and time consuming.  The second approach may sound like a work-around, but since many empty chamber studies use the autoclave cart as a support anyway, it merely reflects a common practice.
For those purists, that read the word “superheat” in the prior paragraph and became alarmed, recognize that this condition is present only very close to the heated seal area.  The laws of thermodynamics mandate that this is only a local phenomena in a part of the autoclave remote from any load items and thus of little real concern.  The temperature distribution data from thermocouples positioned on the autoclave cart support the relative unimportance of this anomalous, highly localized situation.