Rotation: simple mesurement of a complex phenomenon

In our catalogue, along with loggers that measure temperature, pressure and other physical quantities, there is an instrument for measuring rotation.

This arouses curiosity in many customers. We therefore decided to write this article to explain the usefulness of this data logger. You will discover that measuring rotation is not as simple as it may seem at first glance but requires some concepts of physics, mathematics and geometry. Finally, we will tell you how our rotation data logger was invented, starting with the initial request and ending with the fine-tuning of the measurement system.

Rotation and sterilisation: a winning combination

Sterilisation is a process that aims to eliminate the bacterial load in a food by exposing the food to high temperatures (usually above 100°C).

In reality, the temperature inside a product is never uniform. Consider, for example, a jar of tomatoes. If it is heated in a thermal process, the glass or metal will initially heat up, since it is in direct contact with the outside. Subsequently, the heat will first spread to the tomato closest to the glass or metal, and then to the inner layers.

The logical consequence is that the tomato in the centre of the jar maintains a lower temperature for longer than the product near the walls, so the bacteria have a better chance of finding a survival oasis there from which they can spread again and contaminate the product.

One would think that the problem would be solved by increasing the temperature of the sterilisation process. Indeed, we would be sure to reach sufficiently high temperatures even in the innermost layers. But this can lead to degradation of the outer layers, change in taste, texture or loss of certain organoleptic properties of the food. This is why it is not always an applicable solution.

To overcome these drawbacks, there is a technique that is as simple as it is effective: stirring.

Stirring allows the heat to be distributed as evenly as possible. Anyone who has cooked risotto alla Milanese or heated up some sauce in a frying pan knows what we are talking about!

What is simple in the kitchen, i.e. stirring with a ladle or spoon, becomes impractical in sterilisation autoclaves.

However, there is a simple and effective alternative: rotation. In this way, the stirring of the product, resulting in even heat distribution, ensures uniform heating.

This is precisely what happens in rotary autoclaves. The product is placed in these special autoclaves with an internal body that rotates on itself, similar to the drum of a washing machine.

Just like temperature, pressure and humidity, rotation must also be guaranteed. At first glance, this may seem simple. In reality, the phenomenon of rotation is complex. It must be described in its mathematical foundations to be understood.

Let us consider a can of peas and see just a few axes of rotation of the infinite possible: an axis starting from the centre of the lid and going perpendicular to the base, an axis going from the middle of the can to the opposite side, and an axis parallel to the axial axis but outside the can (as if the can were fixed on the outer rim of a spinning wheel).

All these rotations (but not only these) have the property of mixing the contents.

At the root of it all is the force of gravity g=9.8m/s² which causes the higher parts to be attracted towards the ground (the lower parts are also attracted, but are blocked by the jar).

We can therefore say that mixing only occurs if the product is not solid and is subject to a force of attraction and rotates around an axis (with some exceptions). On the space base it would therefore not be possible because in the absence of gravity (gravity actually acts, but is compensated by the centrifugal force due to the rotation of the station).

However, we are on earth and consider gravity g.

In the space coordinates X, Y and Z we have the three components of g: gx gy and gz., and the vector sum of the 3 components must be 1 (if g=1), i.e. the square root of the sum of the squares of gx gy and gz must be 1.

There are cases in which the jar rotates, but the components do not vary: if the jar is standing on the table and we make a mark on the wall of the jar we have gz =1 and gx and gy are equal to zero whatever the orientation of the mark on the jar. In this condition there is also no mixing.

If the axis of rotation is tilted, the three components are non-zero and there is always mixing.

As can be seen, the problem is not trivial.

Rotation monitoring: how the data logger is born

We were contacted by a research and consultancy organisation that supports Italian food companies. We were asked for a system to monitor the number of rotations that canning jars undergo during a sterilisation cycle in a rotating autoclave.

It is important that the rotations are uniform. In this way it is possible to achieve homogenous sterilisation of the jar contents in all positions of the autoclave.

Until then we had never measured the rotations of an autoclave, so we had no logger ready to use.

But that wasn't a problem: Tecnosoft's mission is to provide useful data to the user through dedicated measurement systems for specific applications. So, we set about devising a solution.

As we have illustrated in the previous paragraphs, measuring rotation is not trivial. There are many variables at play and there is a lot of mathematics to be applied.

Consequently, we needed a sensor capable of recording a physical quantity from which we could derive all the required data through mathematical calculations.

The choice fell on the triaxial accelerometer.

Measuring rotation with the accelerometer

The accelerometer, which as the name suggests is an acceleration meter.

The sensor we chose, the triaxial accelerometer with MEMS technology, consists of three acceleration sensors, each oriented along the x, y, z cartesian axes.

All we needed to do was to combine the triaxial accelerometer with a logger for data monitoring. We chose S-MicroW, which lent itself well to the purpose because of its small size and its resistance to high temperatures and pressures.

We then developed ad hoc software that transforms the data obtained into a 3D simulation of the rotations undergone by the logger. It is possible to see the position in space of the canister acquisition by acquisition and to see the dynamic evolution of the rotations with different speeds chosen by the user.

This is the story of our Rotator. A logger that sees the light from the study of solutions for specific needs, in the tradition of Tecnosoft.

If you too need an ad hoc data logger for your measurements, contact us!