iTHERM TrustSens

Does a thermometer really measure the correct value? To answer this question, plant operators in the food and life sciences industries must regularly remove their measuring devices for calibration – in extreme cases, after each batch. Now, Endress+Hauser has developed the iTHERM TrustSens, a thermometer that continuously self-calibrates during ongoing processes. The innovative Industry 4.0 technology is based on the fixed-point calibration usually only performed in laboratories and uses the physical phenomenon of the Curie temperature.

"With the iTHERM TrustSens, we have made a real step forward in temperature measurement technology," says Marc Stahl, Global Product Manager Thermometers at Endress+Hauser Wetzer GmbH+Co. KG, the competence center of the group for temperature measurement and system components. In the food and life sciences industries, temperature is the parameter with the greatest influence on product quality. Only with the correct temperature, for example, does cooking oil not turn rancid when heated, milk remains germ-free, and cells in bioreactors used for pharmaceutical production can multiply optimally. To meet GMP (Good Manufacturing Practice) quality guidelines, thermometers must be recalibrated at regular intervals. In large plants, this is done at hundreds of measurement points several times a year or even weekly.

During calibration, the value displayed by a measuring device is always compared to a 'true' value, an external reference traceable to national or international standards, rather than just validated. Thermometers are calibrated in the field using block calibrators or calibration wheels in comparison with reference thermometers that are traceable to thermometers directly calibrated in the laboratory at fixed points of the international temperature scale. These are defined temperatures at which substances change their state or phase, such as the melting point or the triple point of water. "We have succeeded in integrating a phase transition point in the solid state into a thermometer for the first time," says Dr. Marc Schalles, who researched the fundamentals of TrustSens at TU Ilmenau.

For developing the functional principle, the engineer utilized the physical phenomenon of the Curie temperature: this is a temperature unique to each pure material at which its magnetic or electrical properties change abruptly and can be electrically detected. The Curie temperature is always constant. For example, iron is no longer attracted by a magnet above 768°C. If the temperature falls below this value, it changes again. "We found a material whose properties change at temperatures relevant to the food and life sciences industries, used during cleaning or sterilization," says Marc Stahl.

The reference sensor in TrustSens is made from this special material. It is closely connected to the actual temperature sensor Pt100, which measures process temperature via the electrical resistance of platinum. "This thermal coupling ensures that both sensors are ideally exposed to the same temperature at all times, such as in a calibration bath," says Schalles. After each cooling process from higher temperatures, the Pt100 is automatically calibrated during the process: when the Curie temperature and thus the physical fixed point are reached, the reference sensor delivers an electrical signal. If the Pt100 measures a value within the specified tolerances at the same time, it has been successfully calibrated – in compliance with GMP guidelines and the rules of the US Food and Drug Administration (FDA).

"The iTHERM TrustSens primarily helps plant operators minimize the risk of undetected measurement deviations of the Pt100 sensor," says Marc Stahl. All common Pt100 sensors are subject to aging effects that can lead to incorrect measurements, known as drift. If the sensor's recalibration interval is set to one year and a deviation is detected during manual recalibration, it must be assumed that the thermometer has not been measuring the correct value since installation. "Then, the entire production of the last twelve months must be questioned – and in the worst case, product recalls may be necessary," says Marc Stahl.

With the iTHERM TrustSens, recalibration intervals are significantly shortened in one go, making controls more frequent. Typically, processes that trigger recalibrations of the TrustSens, such as steam sterilization, are performed daily. "This greatly reduces the risk of undetected errors during operation and makes processes more transparent," says Marc Stahl. If a deviation occurs outside the set tolerances, the device raises an alarm or displays an error message, which is also clearly visible on-site via an LED. The iTHERM TrustSens also electronically stores all data from the last 350 calibrations. These can be read via asset management software like Endress+Hauser's FieldCare, which can automatically generate calibration certificates for audits – something that is only manual with conventional recalibrations.

Overall, ten years of research and development are invested in the iTHERM TrustSens. In addition to Endress+Hauser Wetzer GmbH, the sensor specialist Innovative Sensor Technology IST AG, which belongs to the Endress+Hauser group, and TU Ilmenau, plant operators have been involved from the beginning. "The impetus for the new technology came from the Endress+Hauser Life Sciences network. Thanks to our industry focus, we knew the customers' requirements very precisely," says Stahl. Through continuous exchange, questions about the measurement range, construction, or calibration certificates were clarified. The collaboration of all involved parties resulted not only in an innovative technology but also in a genuine Industry 4.0 product. "Thanks to its fully automatic and audit-proof inline self-calibration, the TrustSens enables the production-accompanying quality control and continuous process verification aimed for in Industry 4.0," says Marc Stahl.