No one has actually answered my question about air removal from a prevac and the chemical intergrators.

I have had some people trying to explain the difference from a downward and a prevac steriliser. I understand the difference.

I have been running little tests with chemical indicators. 3M and GKE.

Also testing of air removal.

I placed two lots of steripeel with instruments in on the trolley one paper side down the other plastic side down.
Result: paper side down you could see that the air had been pulled from the bag as the plastic had been sucked into the instruments.
Plastic side down came out like a little balloon you could see the bag was full of air.
Checked with our engineer who agreed with me that in a prevac air is mechanically pulled from the chamber and will pull a certain amount sidways but will not pull air up then down.

this morning I placed a GKE with the end clamped so air could not be removed and steam could not penetrate inside a steripeel bag and plastic down and one inside a bowl upright with the end clamped.
Result full colour change. The steripeel bag came out with air in it. When I was first introduced to GKE I was told that the coil was designed to give the toughest test of air removal and steam penetration.

I place to indicators inside a jug and placed upright on the trolley. again full colour change.

so this is still telling me that these indicators are changing whether there is air present or not. Are they doing what the companys claim they do. Or have I got it totally wrong.

for indicators to change your steriliser must meet all 3 conditions, temperatur,time and saturated steam. For all these 3 conditions to be reached you must have complete air removal from the chamber.

Regards
Judy Rowson
Clinical Educator
D.A.S.S.
New Zealand

Dear Judy,

I am sorry that obviously my answer was not meeting your expectations - let me try again and thanks for the additional information.

Pre-vac sterilizers are in principle capable of removing air from any point in the chamber - even from cups or bottles with the opening to the top. Basic principles of physics are working here:

• the pressure in a closed vessel is the same in all areas
• by lowering the pressure (i.e. creating a vacuum) the remaining air (or any other gas) expands homogeneously in all directions
• a certain volume of the air is drawn out of the chamber and replaced by water steam during the following pressure rise
• now a steam/air mixture is present in the chamber (the deeper the vacuum, the more air is removed)
• the next vacuum pulse removes again a certain amount of the steam/air mixture, which is replaced by steam during the next steam injection
• thus the steam portion of the mixture increases while the amount of airdecreases
• this applies to ALL areas in the chamber, as long as they are accessible to the pressure changes (again, open bottles standing upright can be cleared of air)

Sorry to say, but your engineer was somewhat wrong with his statement.

With wrapped goods, some other factors have to be taken into consideration: e.g. the permeability of the packaging material in regards to air and the sterilant.
During the vacuum phase, the air entrapped inside the package has to penetrate through the packaging material. This happens in most cases at a slower rate thus leading to a relatively higher pressure inside the pack (hence it "blows up").

Side comment: it is a common misunderstanding that packages "shrink" during evacuation. Just the opposite: they enlarge as the enclosed gases try to get out! This effect becomes stronger with the next evacuation steps, as condensate which has formed on the contents during the steam admissions, boils off again at the low pressure. The change of state from fluid to gaseous increases the volume dramatically!

A potential explanation to the different "filling grades" of the two bags.
Case 1, plastic side up: the condensate which had formed inside the pouch on the instrument has accumulated in the bottom paper due to gravity. During the final vacuum phase ("drying") this could easily boil off again.
Case 2, plastic side down ("balloon"): most of the condensate sits on the inside plastic (again due to gravity it will run down there) - during final evacuation it needs to penetrate through the upper paper layer. Again the inside pressure is higher than the chamber pressure leading to the "ballooning". Depending on the length of this phase the entrapped condensate might not have completely left the pack at the end - thus the pouch is "frozen" in a blown up status.

I leave it to Ulrich Kaiser to comment on his product, but the "helix" test (tube with one side closed) is not necessarily the "toughest" test. There are many different test models discussed and available where different physical dimensions present different penetration resistance levels. Many reasons influence the use of a specific test method/model, e.g. the process to be tested, the load items present, etc..


Conclusion: the indicators in the upright jug have been exposed to steam as there was no air present - the colour change is OK!


I hope my somewhat lengthy explanations are helpful - if not let us know!


Kind regards

Klaus Hahnen
Senior Technical Service Specialist
Sterilization Assurance Products
3M Laboratories (Europe), Neuss, Germany



P.S.:
to all physicists who read the above: yes, I know that there are exceptions, many influencing factors and an abundance of gas and pressure laws! ;-)

Dear Judy

We had already an intensive email exchange, which has not been published in this forum.

We informed you that an chemical indicator may be tested in an dry heat oven and you could determine, if he changes which dry heat as well. Have you done this test with a gke indicator? If you would have done it, you could have noticed, that the indicator can differentiate between steam and air.

You have tested our indicator inside of our PCD and closed the end of the PTFE-tube, Your judgement is correct, that the indicator is not allowed to change the colour indicating steam penetration, when the tube is closed and the test is carried out correctly..

In fact you reported a colour change even with the closed tube. We got this information a few times from other customers as well. After looking in the details we found always, that this test was not made correctly. Reasons for the wrong result have been:

1. The tube was not sealed carefully at the end. PTFE becomes very soft in 134 °C steam and a nail inside for example cannot seal the tube.
2. The tube may not be completely dry inside from the last application and due to the sealed end small amounts of water are not anymore able to be removed during the next prevac phase and during the heat up process this water is vaporated creating steam which is changing the indicator. This effect does not happen, if the tube is open and remaining moist from the previous application is removed before the sterilizer starts sterilizing again.

Our PCD indicator system sees more air failures in the day to day practice than our customers like to see, because this indicates a potential problem to their process.

Please ask our customers around the globe, if they see insufficient air removal, leaks or non condensable gases coming from the steam generator.

We recommend you repeat the test with the closed PTFE-tube under the following precaution:

1. Blow air through the PCD and make sure it is dry inside.
2. To seal the end of the tube using a clamp and make sure the seal remains in the process.

We have made this test several times in our lab. Under those conditions we never see a colour change in a steam process.

Also we would recommend you do also the recommended indicator test at home in your oven at 140 °C for 30 minutes.

We would be happy that you would report the results of the above recommend tests inside this forum as well.

On top you are any time invited to visit our application labs in the Frankfurt area in Germany, we are 30 car minutes from the airport away, where we can demonstrate above mentioned tests. Just jump in a plane, we pick you up at the airport.


With Best Regards
gke-mbH

Dr. Ulrich Kaiser
Application Lab