Accurate Down to the Bean: Mini Temperature Sensor Enables Energy-Saving Coffee Production

Extremely Small and Powerful

The TT-3/100 temperature sensor from HBM is only 50 μm thick, with a measuring surface of 3.1 mm x 3.0 mm. With these dimensions, it comes very close to being a "punctiform" sensor, which is how it was represented in the mathematical modeling for this new method. A wide application temperature range from -50°C to +180°C makes it possible to use the sensor for both freeze-drying and roasting coffee beans. 

Innovative Measurement Method

Thanks to the miniature dimensions of the TT-3/100, the PTB was able to develop an innovative measurement method for determining the thermal properties of coffee beans. The method consists of several steps. The coffee bean is divided in half with the sensor placed in between. The contact surfaces need to be smooth to ensure good thermal contact. The sensor is not glued in place; rather, it is clamped, and acts as both temperature sensor and defined source of heat at the same time. 

With a power supply of 8 to 10 mA, the TT-3/100 heats up during measurement. The more the bean conducts inside heat to the outside, the faster and more intensely the temperature and electrical resistance of the sensor rise. 

The measurement results for thermal diffusivity and thermal conductivity are ready in just a few minutes. It is also possible to calculate the specific thermal capacity of the bean if its density is known. That material property can simply be determined according to Archimedes' principle. Instead of thermal conductivity, the humidity of a bean can also be determined.

Accurate and Reliable Results

The reliability of the method has been demonstrated, starting with reference specimens with precisely known thermal transport properties. The measured values for specific heat of a bean can be compared with those of ground coffee. They match very closely.

Efficient Measurement for Various Applications

Dr. Ulf Hammerschmidt of the PTB is confident that the new method will open up newapplications in diverse industries. The importance of thermal conductivity and thermal diffusivity as material parameters extends well beyond the food industry. The method can also speed uptesting, monitoring, and approval of new insulation materials in the building industry, providing reliable results within minutes, compared to a few days. Assessments of existing building materials could be supported by simple field measurements.

Learn more about PTB, German National Metrology Institute.