Thermal diffusivity measurements of dry and moist sandstone using the transient hot-bridge sensor and the xenon-flash method


F. Mzali1, M. Abid2, F. Albouchi1, U. Hammerschmidt2 and V. Meier2

1National Engineering School (ENIM), University of Monastir, Monastir, Tunisia
2Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany

Keywords: transient hot-bridge method, flash method
property: effective thermal diffusivity
material: sandstone, porous material

In this paper, the effective thermal diffusivity of dry and moist sandstone is measured at ambient temperature using the transient hot-bridge method (THB) and a standard Xenon-Flash method. The THB method [1], developed at Physikalisch-Technische Bundesanstalt (PTB), Germany, uses a novel sensor made of a 55 µm thin printed circuit foil of Nickel sandwiched between polyimide sheets. It has an overall size of 120x60x0.055 mm3 to fit between two sample halves of 100x60x20 mm3 each. The layout of the sensor consists of four parallel tandem strips. Each tandem strip comes in two individual striplets, a short and a long one. Two of the tandems are located very close to each other at the centre of the sensor and one additional tandem on either edge. All eight striplets are symmetrically switched for an equal-resistance Wheatstone bridge. As long as the sample temperature is uniform, the bridge is inherently balanced. An electric current makes the pairwise unequally spaced strips establish a predefined inhomogeneous temperature profile that turns the bridge into an unbalanced condition. The high-sensitivity output-signal of the sensor is practically not affected by ambient electrical noise and allows measuring the thermal conductivity and the thermal diffusivity of the surrounding medium.

When plotting the output-signal UB versus time in a logarithmic scale, ln(t), a straight line of slope m and intercept n is obtained. Classically, the thermal diffusivity is obtained from the knowledge of the slope and the intercept [2]. This, however, can lead to errors due to sensor offset. A new identification method [3], using the instantaneous slope mi and time ti values allows for a sufficiently precise determination of the thermal diffusivity.

State-of-the-art in measuring thermal diffusivity of solids is the flash method. That is why, our THB results on dry and moist sandstone specimens were compared to those from this standard method. Our xenon-flash device consists of a flash source, a cylindrical sample holder in horizontal position and an open junction thermocouple (Bismuth Telluride Bi2Te3). The heat pulse is generated by five xenon lamps. They liberate a 6 ms uniform pulse of variable power. The output of the thermocouple is switched to a conditioning-amplifier (2210A) and the response is recorded by a numerical oscilloscope HM-407 that itself is connected to a PC.

The dry sample halves of sandstone for the THB measurements were obtained by keeping them in a furnace for 24 h at 105 °C. Hereafter, they were weighed. Moist samples were obtained by immersing them in a water-filled vessel for 24 h. Their masses at free saturation were then determined. Finally, the corresponding gravimetric water contents were deduced. For Flash measurements, cylindrical samples of diameter 26mm and thickness 7mm were cut from the same sandstone bulk. A thin silver film was deposited on that face of the sample that is used for temperature measurements.

References
  1. U. Hammerschmidt and V. Meier, patent

  2. U. Hammerschmidt and V. Meier, (2006), Int. J. thermophys., 27(3), pp. 840-865.

  3. U. Hammerschmidt and V. Meier, patent

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