Successful development of a high pressure temperature sensor

Project supported by AiF, AiF 16114N

“Model based strategies for the optimization of automated high pressure treatment processes of food on the example of processed meat”

High pressure treatment for the pasteurization of food is gaining significance in the food processing sector. As a non-thermal pasteurization process, the application of high pressure prolongs the shelf life of food without the use of preservatives. Within the scope of the project, the German Institute of Food Technologies (DIL) has successfully developed and implemented a system for the wireless on-line transmission of temperature changes.

The special feature of this system is that the data can be collected even under extreme conditions. The measuring system will operate at an external pressure above 60 MPa. The hydrostatic pressure applied is about six times higher than the pressure at the deepest point in the ocean and corresponds to the pressure generated by a water column, 60 kilometers high. The fluctuating load caused by the repeated pressure build-up applies an almost unbearable strain on the specific steel materials. This means that cable feedthroughs in industrial high pressure tanks are currently not safe enough for use. The only way to transmit signals is via a wireless system.

SSince the high pressure tank is a Faraday cage, electromagnetic senders as known from mobile radio communications, for example, cannot be used. This measuring system uses ultrasonic technology (>20 kHz) for the wireless transmission of the measuring data. For this purpose, an ultrasonic sender-receiver system has been developed which operates in a frequency range above the spectrum of the high pressure pasteurization plant (35 kHz). It reliably transmits temperature changes in the range below 0.1 °C even during the pressure build-up.

This precision is made possible because the measuring values are compiled analogously and because a high frequency range is available for the transmission of the signal. Corresponding with the conversion of the measuring values, an external microphone developed within the scope of this project collects the acoustic signals sent. In total, a frequency range of more than 55 kHz can be used.

In terms of the evaluation technologies available, which are able to resolve frequency changes within a fraction of seconds, it seems feasible that this application can be expanded with several sensors. A commercially available PC with a 100 MHz USB oscilloscope is used for data collection. The measuring microphone fixed at the outer wall of the high pressure pasteurization plant is connected to this computer system. A signal transmitter integrated in the oscilloscope operates an external signal transmitter that can be used for calibration and testing of the system. PProprietary graphic software is used for the evaluation. The software controls the oscilloscope and the signal transmitter according to the operator settings.

The sound waves received via the measuring microphone from the inside of the high pressure tank are Fourier-converted and analyzed. The signals generated by the sensor are recognized within the Fourier spectrum and converted into a temperature value. The temperature data can be used on-line to control the plant and its pressure build-up according to the processing requirements. The underlying principle is the adiabatic heating of different substances when charged with hydrostatic pressure. For example, bacon under pressure heats up more than pork meat. In order to ensure a high product quality, holding times during the pressure build-up may be necessary for heat equalization purposes. The measuring data collected are stored in an intermediate file on the computer for documenting the pasteurization process.
The measuring instrument industry including SMEs could implement the results from this project within a short time. The application of this development is not limited to high pressure pasteurization alone; it can also be applied to all fields where measuring values have to be collected and transmitted under extreme environmental conditions, for example, in deep sea research.

The project was supported within the scope of the “program for promoting joint industrial research (IGF)” by the Federal Ministry for Economy and Technology (via AiF) through the Research Group of the German Food Industry (FEI) (AiF project no 16114 N).

The contacts for this project at DIL are

MBA Bernhard Hukelmann    Prof. Stefan Töpfl
b.hukelmann@dil-ve.de   s.toepfl@dil-ev.de