What are they and where did they come from?
Air detectors were first developed in the textiles industry where autoclaves are used in a process called ‘heat setting’ for yarns and synthetic fibres such as nylon.
In the textiles industry, autoclaves are used to improve and/or add certain properties to fibres, such as dimensional stability, elastic properties, heat resistance etc.
Similar to their use in pharmaceuticals; the textile autoclave works in three phases:
- a vacuum is applied to the chamber to remove the air from the chamber,
- heat is added through pressure and steam
- material is cooled using a vacuum.
To achieve this ‘heat setting’ effect, certain temperatures have to be achieved to attain the desired properties of the material.
Most autoclaves are controlled using pressure and saturated steam conditions. The most frequent causes of temperatures not being achieved include the incorrect removal of air from the chamber, or a leak causing air to enter into the chamber.
Therefore the textile industry needed to develop a way to ascertain if air was present in the chamber prior to reaching the temperatures at which the properties of the material would be altered permanently. A simple solution was required, and the Air Detector was created.
- Air Detectors are devices that are fitted to autoclaves to detect the presence of air in saturated steam.
- While they have been around a while now, Air Detectors haven’t changed much over the years.
- They still work on the same scientific principles and can be grouped into two basic types of Air Detectors:
Temperature-based Air Detectors
In its simplest form, an air detector is a temperature probe located in a tube attached to the drain line or chamber of the autoclave. A pressure set point is set on the HMI to check the temperature using the air detector probe. Once this pressure is achieved post-air removal and during heat up, a temperature reading is taken from the probe. If air is present in the chamber, air will collect in the tube where the temperature probe is located.
If the temperature is less than the temperature of saturated stream at this pressure, the system will alarm, as this indicates that air is present in the steam and that saturated steam conditions have not been met.
Pressure-based Air Detectors (Condensing)
The second type of Air Detector is the Pressure-Based Air Detector or the Condensing Air Detector. Similarly to the temperature-based design, these are located in a tube in the drain line of the autoclave. In this case, a pressure transducer is located in the tube. However, this tube is capable of isolating itself from the drain, usually using a pneumatic valve, and is fitted with a jacket capable of supplying cooling water to the outside of the tube. A sample of steam is collected post the air removal phase by isolating the tube from the drain. The cooling water condenses the steam, as this steam condenses it should produce a vacuum on the Pressure transducer.
The level of this vacuum indicates if air is present in this sample, as air is a non-condensable gas a small vacuum would indicate that there is air present that cannot be condensed by the cooling water, however a larger vacuum indicates that saturated steam is present in the tube and all the contents of the sample have been condensed.
What are the advantages of fitting an Air Detector?
The Air Detector provides in-process monitoring that gives additional assurance that an autoclave is operating correctly.
Due to the nature of autoclave cycles, the constant heating and cooling can lead to the loosening of connections to the chamber and the drain line, and this can lead to the ingress of air during the vacuum and cooling phase. This is a particular risk during the cooling phase, as ‘dirty’ air present in the chamber during this phase may come into contact with the sterilised product contact components, which could lead to failures in sterility testing or even worse.
Without the presence of an Air Detector, the cycles in between leak tests could be called into question as a result of a failed leak rate test.
While Air Detectors are not mandatory under European (EN285) and TGA Requirements (AS/NZS 4187 and AS/NZS 1410), you must show correct air removal has occurred, it is easier to do this showing that the air detector is installed and is functioning correctly than trying to show a correlation of chamber pressure and temperature for each cycle, similarly under AS/NZS 4187, daily leak rate tests have to be carried out on the autoclave unless an air detector has been fitted, in which case they can occur weekly along with an Air detector function tests (Inducing a leak and showing the Air Detector aborts the cycle). This can lead to reduced downtime on the autoclave.
I perform daily leak rate tests, so why would I need to install an Air Detector?
Even in the absence of a gross leak in the autoclave, it is possible that the design of an autoclave cycle will not be sufficient to remove air trapped in a porous load, loads containing long tubing are more susceptible to these issues. Even in cases where a load pattern has been designed and validated, air can become trapped as a result of an operator placing the items incorrectly on the shelving, for example in a kinked tube, leading to insufficient air removal. In this scenario, the Air Detector should abort the cycle due to the presence of the air in the saturated steam.
What set-up is required?
Once installed, the setup of the Air Detector can be difficult, and should be carried out only by someone experienced in conducting Air Detector set-ups. Once set up, however, the air detector requires minimum intervention to operate correctly. Care should be taken not to alter the Air Detector set-up once this has been completed.
Is there any additional validation required for Air Detectors?
Yes, two additional tests are needed during your initial chamber validation and yearly revalidation. These tests are:
- Air detector performance test: Here we are trying to show that a known leak into the chamber will not impact on the performance of the autoclave, the leak is normally the minimum amount that will cause the Air Detector to abort a cycle and as such the Air Detector is disabled for this test. These are normally carried out on a small load test and a large load in terms of mass that we have in the autoclave.The acceptance criteria for this test is that the probe located in the centre of the load must not be depressed by more than 2°C, once the drain achieves sterilisation conditions. The induced leak is normally in the region of 10 mbar/min and is induced using a variable flow valve attached to the side of the chamber of the autoclave. This test is carried out during the initial qualification of the chamber and then on an annual basis thereafter.
- Air detector function test: This second test is carried out to show the correct functioning of the Air Detector, once we have completed the Air Detector performance test, the Air Detector is enabled again and using the same leak that we used in the previous test, we ensure that the cycle aborts as a result of the air detector. This test is carried during the initial qualification and can be carried out weekly to ensure the correct operation of the Air Detector.
Air Detectors have been employed in pharmaceutical technology to provide additional assurance and defence for use in aseptic technology. They are a great example of how technological advances can come from unlikely sources and can be achieved in low-cost and effective ways.