What is Transient Hot Bridge (THB)?

The Transient Hot Bridge (THB) method represents an enhanced version of the Hot Wire method (THW) designed for measuring thermal transport properties in various substances, including solids, liquids, pastes, and powders. This encompasses the determination of thermal conductivity, thermal diffusivity, and volumetric specific heat capacity.

The THB method employs thermoelectric sensors consisting of a printed circuit foil made of nickel sandwiched between two polyimide sheets. The sensor layout comprises four tandem strips arranged in parallel. Each tandem strip consists of two individual strips, one short and one long.

The tandem strips are strategically positioned, with two tandems in close proximity at the center and one additional tandem on each edge of the sensor. All eight strips are symmetrically switched to form an equal-resistance Wheatstone bridge. Under uniform temperatures, the bridge is initially balanced, requiring no nulling before a measurement. When an electrical current is applied, the unevenly spaced strips create a predefined inhomogeneous temperature profile, leading to an unbalanced bridge condition. The subsequent voltage rise over time serves as an indicator of the thermal conductivity, thermal diffusivity, and volumetric heat capacity of the specimen under investigation.

THB tandem strip
THB sensor

One notable feature of the THB sensor is its ability to produce an almost offset-free output signal with high sensitivity, minimizing errors caused by thermal electromotive forces (emf) since no external bridge resistors are necessary. The meander-shaped design of each strip contributes to increased electrical resistivity. The segmentation into tandems compensates for the end effect, addressing temperature drops at both ends of a linear or strip-shaped heater.

Unlike bridges with resistors located remotely from the measuring area, a THB sensor is entirely surrounded by the specimen, eliminating errors introduced by wiring or external noise pick-up. This configuration enhances the accuracy and reliability of the measurements conducted using the THB method.

Which thermophysical properties are measured?

  • Thermal conductivity
  • Thermal diffusivity
  • Specific heat capacity

Which standard norms are for hot wire/hot strip methods?

ASTM D5930-01 outlines the standard test method for determining the thermal conductivity of plastics using a transient line source technique. This approach provides a means to assess the thermal transport properties of plastic materials.

ASTM D7896-19 establishes a standard test method for evaluating the thermal conductivity, thermal diffusivity, and volumetric heat capacity of engine coolants and related fluids. The method utilizes the Transient Hot Wire Liquid Thermal Conductivity approach to conduct these measurements.

ISO 22007-2 pertains to the determination of thermal conductivity and thermal diffusivity in plastics. This international standard provides guidelines for assessing the thermal transport properties of plastic materials.

These standards serve as crucial references for conducting accurate and consistent measurements of thermal properties, ensuring reliable assessments of thermal conductivity, thermal diffusivity, and volumetric heat capacity for various materials, including plastics and engine coolants.

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