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New THW device

A new portable absolute Transient Hot-Wire instrument for the measurement of the thermal conductivity of solids is in its final stage of testing. Three novelties characterize the new instrument:



  1. Construction of a Novel Sensor (AUTh)

The sensor is composed of two Pt wires, of 5 and 2 cm length placed one after the other. The wires are placed in the center of a 1-mm thick silicone paste layer of 5 x 10 cm dimensions [1]. The one side of this layer is placed over a Plexiglas piece, while the other is covered by a thin Kapton film for protection. The sensor with its Kapton side down, is placed over the solid whose thermal conductivity is required.

The advantage of this sensor is that it can be tested for liquids before placed in the silicone. This way the absolute uncertainty of the sensor is confirmed to be better than 2%



  1. Novel Electrical circuit (China)

As two wires are employed, an automatic Wheatstone type bridge is used in order to compensate for the end effects of the wires. The traditional, large in size Wheatstone bridge is replaced by a newly designed, compact portable printed electronic board employs an ARM architecture CPU to control output voltage and data processing. The resistances of the Wheatstone type bridge are mounted on this new board. The change of the wires’ resistances over time is obtained by recording the off-equilibrium signal of the bridge, while steady-state values required are acquired through voltage ratios employing a known standard resistance of high accuracy. Times employed range from 0.001 s to 10 s, depending on the thermal conductivity of the solid, and up to 1000 points are recorded in this time.



  1. Results Analysis Novel Software (Tessera)

From the change of the wires’ resistance over time, the corresponding temperature-rise curve is obtained. At very low times, this corresponds to the temperature rise in the wire, following that, in the paste and in longer times, in the solid. The developed software application, simulates the whole experiment, with unknown only the solid’s thermal conductivity, until it reproduces the experimental curve.

The simulation takes place using an advanced open-source Finite Elements Method library FiPy. The library was written by the National Institute of Standards and Technology, NIST (USA) and is based on Python. The Finite Elements Method runs on a predefined optimized mesh formed on the structure of the sensor. 

The complex model that searches for the optimal thermal conductivity value, which reproduces the experimental curve, is based on the idea of Bayesian Optimization with Gaussian Processes, optimizing considerably the number of simulations performed by predicting the next best candidate to be examined. The whole process converges in a few minutes (the previous software required 10 hours!!!)