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Vacuum measurement in a plasma autoclave: from problem to solution

The idea of measuring a vacuum might sound strange. How is it possible to measure something that, by definition, does not exist? Yet, in the hospital environment, vacuum measurement plays a key role in everyday sterilisation activities.

In this article, we will explore the topic by starting with the basics and then focus on one of the most relevant applications, which not only has a great practical impact, but also challenged us with an unexpected challenge that we successfully solved.

 

Measuring vacuum: what does it mean?

Vacuum is not an abstract or philosophical concept: it is a physical, well-defined and quantifiable reality. It is the absence of atoms in a space. Absolute vacuum corresponds to zero atoms, but this condition is practically unattainable. In technological applications, a space with an extremely small amount of air molecules can also be considered empty.

However, even if there are very few molecules, it is not always possible to neglect them. Therefore, it is essential to monitor their presence by measuring pressure.

Measuring vacuum means dealing with pressures so low as to seem almost unrealistic, on the order of ten thousandths of a bar. These levels of accuracy are impossible for a pressure gauge, but can be achieved thanks to sensors based on the Pirani Vacuum principle, named after its discoverer.

 

The sensor consists of an electric wire in a controlled environment that is passed through by a constant electric current. It does not measure pressure directly but through a system that exploits knowledge about the behaviour of matter in a rather ingenious way:

  • We know that atoms exchange heat with each other. So the more atoms there are in space, the greater the heat exchange. In our case, the more air molecules there are, the greater the heating of the wire.
  • We know that the increase in temperature of the wire leads to an increase in its electrical resistance.
  • We can deduce that the electrical resistance is directly proportional to the amount of molecules present.
  • By applying certain formulas, it is possible to calculate the atmospheric pressure from the resistance value with great precision.

If you want to delve deeper into this fascinating topic, we have described the Pirani Void in detail in this post.

 

A concrete application: the plasma autoclave

What is the purpose of vacuum measurement?

A practical example is the operation of the plasma autoclave, which is mainly used in the medical field for sterilisation. The process is based on two fundamental steps:

  • Air elimination: the vacuum serves to ensure that no air stagnation remains, allowing an even temperature distribution.
  • Use of hydrogen peroxide: this is a particularly powerful sterilising agent, more effective than traditional steam, which is brought to the boil and spreads over all surfaces subjected to treatment.

To ensure that the plasma autoclave functions properly, it is essential to measure the vacuum with extreme precision. This is where the Pirani Vacuum Logger comes in, an instrument designed to monitor infinitesimal pressures reliably and robustly even under extreme thermal conditions.

Several years after the development of this data logger, we came across an unexpected critical issue that forced us to rethink our solution...

 

The challenge: protecting the logger from hydrogen peroxide

One of our customers contacted us reporting a malfunction of our vacuum measurement system used in its plasma autoclaves.

Although switching on correctly and connecting to the data collection systems, the Pirani Vacuum Logger could no longer detect pressure, showing a flat reading.

But if the logger was working, how come it was no longer tracking? It was obvious that there was a malfunction in the sensor.

  • Considering that the logger can withstand ranges from -80°C to +85°C, we ruled out the possibility that the problem was in the temperature. The plasma autoclave in fact never goes beyond this threshold.
  • We have never experienced similar problems with the Pirani Vacuum Logger in other conditions, for example in installations for monitoring freeze-drying processes.

We immediately suspected that the problem was hydrogen peroxide.


Why hydrogen peroxide is a problem for sensors

Hydrogen peroxide (H₂O₂) is a very effective sterilising agent, which is why it is used in processes where thorough sterilisation is required. But it is also an extremely aggressive molecule on metals, so much so that it can compromise precision electronic components.

And indeed, once we examined our sensor, we noticed that the internal circuits were ruined, with loose wires and broken tracks. The hydrogen peroxide had therefore compromised the measuring system.

It was clear that the Pirani Vacuum Logger needed to be protected from hydrogen peroxide. But how?

 

A filter for measuring vacuum in an autoclave

To address this problem, we developed a filter to be applied to the Pirani Vacuum Logger.

Conceptually, the filter we needed had to function as a catalysing system for hydrogen peroxide. In practical terms, ‘catalysing’ means accelerating the decomposition of H₂O₂ molecules into less reactive elements, thus reducing the number of aggressive molecules that could come into contact with the logger sensor.

Not only did we achieve this goal, but we did so without any energy consumption, using only the physical and chemical properties of the materials employed.

The filter consists of a robust steel casing and a silicone membrane that seals the chamber where catalysis takes place. Its design is based on three key principles to ensure maximum effectiveness and durability:

  1. Suitable material. After thoroughly investigating the chemical properties of H₂O₂, we selected manganese dioxide, a material that, combined with epoxy resin, offers a high catalytic capacity without ever becoming saturated. This choice ensures constant effectiveness over time.
  2. Screw-on cap design. The filter is designed as a screw-on cap, which easily connects to the Pirani Vacuum Logger via a thread. This design makes the filter modular, allowing it to be fitted and removed as required.
  3. Internal labyrinth to maximise catalytic surface area. Inside the filter, the hydrogen peroxide-laden air is forced to follow a long, winding, labyrinth-like path. This configuration increases the exposed surface area of the catalytic material, further enhancing the effectiveness of the filter.

 

Installation

The filter prototype, once developed in our laboratories, was tested directly on the customer's autoclaves, which proved to be very cooperative.

The results were immediate and significant: once the filters were applied, the Pirani Vacuum Loggers no longer encountered any problems. Damage due to hydrogen peroxide was averted!

Thanks to the new filter, our Pirani Vacuum Logger gained additional protection, ensuring optimal monitoring even in a particularly difficult environment such as a plasma autoclave.

 

Tecnosoft's philosophy: innovation and collaboration

The Pirani filter is the result of a company philosophy geared towards experimentation and collaboration.

Our products are born from the real needs of our customers and from an ongoing commitment to the search for tailor-made solutions.

Collaborating with customers and addressing complex challenges allows us to create reliable and customised measuring devices.

If your company needs reliable measuring instruments in technically demanding situations, contact us. We are ready to work together with you to find the best solution for your measurement needs.