Audit-proof self-calibrating thermometer

Temperature is a parameter which has the greatest influence on product quality  – particularly in food and life sciences industries. Only the correct temperature can ensure that cooking oil does not become rancid when it is heated, for example, or that milk remains free of pathogens and cells can multiply optimally in bioreactors during the production of pharmaceuticals. 

To meet the Good Manufacturing Practice (GMP) quality guidelines thermometers need to be recalibrated at certain intervals to ensure accuracy is maintained. Unlike during validation, the value indicated by a measurement device during calibration is always compared to a ‘true’ value – an external reference that can be traced to national or international standards. In the field, thermometers are calibrated in block calibrators or calibration baths in comparison with reference thermometers that are traceable to thermometers that were calibrated directly and with high precision in the laboratory with fixed points of the international temperature scale. These are defined temperatures at which substances change their aggregate or phase state, such as the freezing point or triple point of water. 

A thermometer which is able to continuously recalibrate itself has been introduced by Endress+Hauser. The iTHERM TrustSens is based on fixed point calibration which is only carried out in laboratories and uses the physical Curie temperature phenomenon.

“We succeeded in integrating a phase transition point in a solid state of aggregation in a thermometer for the first time,” explained Dr Marc Schalles, who researched the groundwork for the TrustSens at the Ilmenau University of Technology.

The engineer took advantage of the physical Curie temperature phenomenon to develop the operating principle: this is a temperature that is individual to every pure material and at which the magnetic or electrical characteristics abruptly change in an electrically detectable manner once said temperature has been reached. The Curie temperature remains constant. For example, from 768°C iron will no longer be attracted by magnets. This changes again once the temperature falls below this value. “We have found a material whose characteristics change at temperatures that are crucial to the food and life sciences industry where they are used for cleaning and sterilisation purposes,” said Stahl.  

The reference sensor in the TrustSens consists of this material. It was closely combined with the actual Pt100 temperature sensor, which measures the process temperature with the help of the electrical resistance of platinum. This thermal coupling ensures that both sensors are ideally subjected to the same temperature at all times, for example in a calibration bath. The Pt100 is then automatically calibrated during the process after each cooling down procedure following higher temperatures: the reference sensor provides an electrical signal once it has reached the Curie temperature and therefore the physical fixed point. The Pt100 was successfully calibrated if it simultaneously measures a value within the specified tolerances, conforming to GMP guidelines and US Food and Drug Administration rules. 

The thermometer won the 2018 Hermes Award, which is presented every year at the Hannover Messe, for its contribution to the deployment of Industry 4.0. 

The jury were particularly impressed by this product due to the fact that – despite the existence of various self-monitoring temperature sensors on the market – there has so far been none for process-industry applications that is comparably precise and lends itself to automatic, completely traceable self-calibration based on the Curie temperature.

"Smart sensor technology is a major driver in the next stage of Industry 4.0. With the number of sensors increasing dramatically in a smart factory, audit-proof self-calibration of sensors without system downtime will become a key factor for economic success. This is where Endress+Hauser’s innovative product comes into play, making an important contribution to the continuous process verification and quality control during production that we are aiming for with Industry 4.0," said Dr Wolfgang Wahlster, CEO of the German Research Center for Artificial Intelligence (DFKI), and Hermes Award jury chairman.

“The iTHERM TrustSens especially helps plant operators to minimise the risk of an undetected measurement deviation of the Pt100 sensor,” said Stahl. Conventional Pt100 sensors are subject to aging effects that can lead to erroneous measurements specified recalibration interval of a sensor is set at one year, for example, and any deviation is determined during its manual calibration, then it must be assumed that the thermometer was already measuring an incorrect value following installation. 

Recalibration intervals can be significantly shortened with the iTHERM TrustSens, making monitoring more continuous, as processes that trigger the recalibration of the iTHERM TrustSens – such as steam sterilisation – tend to be carried out on a daily basis. This reduces the risk of undetected errors during operation, while making processes more transparent. If a deviation beyond the specified tolerances is detected, the device will raise an alarm or issue an error message that is also clearly displayed via LED. 

The iTHERM TrustSens also electronically saves all data on the last 350 calibrations. This information can be read using asset management software such as Endress+Hauser’s FieldCare which can simultaneously prepare calibration certificates for audits, which can only be done manually with conventional recalibrations.