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Temperature measurement using data loggers

1. Introduction

Some service technicians try to determine the successful steam penetration inside hollow devices by temperature measurement in the hollow instruments. This method is suitable to test the steam penetration of porous loads (e.g. BD-cotton pack). However it is impossible to transfer this approach to hollow instruments as demonstrated by the following measurements.

2. Material and Methods

As a test object a stainless steel tube of 1 m length with an inner diameter of 6 mm and a wall thickness of 0,5 mm has been used. A biological indicator (spore strip) is placed at the end of the tube and the tube is closed with a silicone plug. The plug is cut in the middle and a thermocouple element is put through it and fixed at the biological indicator. Another thermocouple element is fixed at the outside wall on a level with the biological indicator. A third thermocouple element is recording the chamber temperature.
3. Test Procedure

Test cycle 1 – successful sterilization:

An air removal process with 4 x 250 – 1000 mbar allows air removal and steam penetration of the tube to inactivate the biological indicator successfully.

Test cycle 1 – unsuccessful sterilization:

The second air removal process with 4 air removal cycles 600 – 1000 mbar is not sufficient and is run afterwards. The biological indicator was not inactivated and grew.

Comparison of the test cycles:

The temperature curves inside and at the wall of the test tube showed an identical gradient for the good and the bad air removal phases, both during the air removal and during the plateau period. Therefore they do not provide any information about the success of sterilization inside the tube.

4. Conclusions

Temperature sensors can only record the temperature and are not able to differentiate between steam and air or other non-condensable gases. In case of insufficient steam penetration inside a cotton pack (e.g. Bowie-Dick-Test) the temperature inside the pack rises by far slower than outside the pack. This is not the case with hollow devices. Metal tubes like MIS instruments have an extreme high thermal conductivity and the inner volume is extremely small. In a tube of 2 mm diameter 0.3 ml would be enough to fill up 10 cm of the tube. Hollow instruments however have a very small wall thickness, extreme tiny lumina and are mostly made of metal having an extreme good heat conductivity and no thermal isolation. Because this small amount of NCG is heated by condensation at the outside of the walls, the temperature inside is heated up extremely quickly, even in absence of steam. Therefore there is no temperature difference between the two test cycles. The temperature gradient inside of a hollow device does not allow any conclusion about the success of steam penetration. As a consequence biological (BI) or chemical indicators (CI) must be placed at the critical locations inside tubes or MIS instruments, to differentiate between noncondensable gases and steam which has to condense to water. If the use of BIs or CIs is not possible because of a too small geometrical design, routine monitoring is only possible to use a validated PCD as a surrogate.