New Transient Hot-Bridge Sensor to Measure Thermal Conductivity, Thermal Diffusivity, and Volumetric Specific Heat

Abstract

A high sensitivity thermoelectric sensor to measure all relevant thermal transport properties has been developed. This so-called transient hot bridge (THB) decidedly improves the state of the art for transient measurements of the thermal conductivity, thermal diffusivity, and volumetric specific heat. The new sensor is realized as a printed circuit foil of nickel between two polyimide sheets. Its layout consists of four identical strips arranged in parallel and connected for an equal-ratio Wheatstone bridge. At uniform temperature, the bridge is inherently balanced, i.e., no nulling is required prior to a run. An electric current makes the unequally spaced strips establish an inhomogeneous temperature profile that turns the bridge into an unbalanced condition. From then on, the THB produces an offset-free output signal of high sensitivity as a measure of the properties mentioned of the surrounding specimen. The signal is virtually free of thermal emf's because no external bridge resistors are needed. Each single strip is meander-shaped to give it a higher resistivity and, additionally, segmented into a long and short part to compensate for the end effect. The THB closely meets the specific requirements of industry and research institutes for an easy to handle and accurate low cost sensor. As the key component of an instrument, it allows rapid thermal-conductivity measurements on solid and fluid specimens from 0.02 to 100 W· m −1 ·K −1 at temperatures up to 250 • C. Measurements on some reference materials and thermal insulations are presented. These verify the preliminary estimated uncertainty of 2% in thermal conductivity.

Thermal conductivity of cementitious composites containing microencapsulated phase change materials

“…Therefore, to better understand how PCMs influence the thermal conductiv- [14], hot strip [15], hot wire [16], hot bridge [17], and laser flash methods [18]. The first four methods involve temperature measurements collected over a time period ranging from 10 ns to 100 s during which the sample is heated [15][16][17].…”

“…The first four methods involve temperature measurements collected over a time period ranging from 10 ns to 100 s during which the sample is heated [15][16][17]. A thin sensor is used to generate a pulse of thermal energy dissipated by the sample while simultaneously measuring the associated change in temperature at the sample surface [15][16][17]. The measured rate of thermal dissipation and change in sample temperature are used to calculate the thermal effusivity defined as e = ρc p k where ρ, c p , and k are density, specific heat, and thermal conductivity of the sample, respectively [19].…”

“…The measured rate of thermal dissipation and change in sample temperature are used to calculate the thermal effusivity defined as e = ρc p k where ρ, c p , and k are density, specific heat, and thermal conductivity of the sample, respectively [19]. The hot bridge method offers greater accuracy than the other three methods by using multiple sensors aligned in a Wheatstone bridge to collect sample surface temperature measurements with enhanced sensitivity [17]. Alternatively, the flash method [18] infers the thermal diffusivity α, defined as α = k/ρc p .…”

“…Steady-state methods are also simpler in terms of apparatus design and fabrication, experimental procedure, and data analysis [17]. However, they require longer experimental time than transient methods [14,18,23].…”

Thermal conductivity measurement of liquids in a microfluidic device

“…Most commonly, heating is applied at a constant rate, but in other approaches heating is sinusoidal (Bruce and Cannell, 1976, Cahill, 1990, Choi and Kim, 2008. Further work has applied, refined and developed these approaches to improve accuracy and reliability (Filippov, 1966, Asher et al, 1986, Gustafsson, 1991, Wang and Yang, 1995, Hammerschmidt, 2003, Hammerschmidt and Meier, 2006, Voudouris and Hayakawa, 1994, Zhang et al, 2003and 2005, Xie et al, 2006. Particular issues addressed in the case of liquids include the effect of radiative heat transfer (Cahill, 1990, Gustavsson et al, 2003 and the influence of natural convection associated with heating (De Groot et al, 1974, Asher et al, 1986, Gustavsson et al, 2003.…”