History.4 – Transient Hot Wire Method
The transient hot wire (THW) method today is used for the measurement of thermal conductivity, thermal diffusivity and specific heat of liquids and pastes. 200 years ago scientists were using a crude version of this method to make the first ever thermal conductivity measurements on gases. Since then, the method has evolved with the contribution of many scientists around the world to the accurate and precise method we have today.
Experiments using a heated wire started in 1781 with Joseph Priestley’s attempts to measure the ability of different gases to conduct heat (at this time, it was uncertain if gases could conduct heat at all). From this time forward, experiments using a heated wire began appearing more and more frequently in literature. Throughout the 19th century various scientists used different versions of the hot wire method to measure the thermal conductivity of gases. Some of these measurements agree surprisingly well with current literature values. This period represents a slow and steady improvement of the hot wire method and its measurement capabilities.
In 1931 the method took a leap forward in the form of the first “transient” hot wire method proposed by Pyk and Stalhane for the measurement of thermal conductivity of solids and powders. Unlike previous methods, the one devised by Pyk and Stalhane used shorter measurement times due to the transient nature of the measurement.
In the mid to late 20th century the transient hot wire method began to resemble the method we know today. New THW apparatus developed during this time had a few characteristic properties not seen in all previous models: a wire with potential leads, a temperature rise measured by the resistance of the wire, test times up to 1 minute and electronic bridges. An especially influential development was made in 1971 by Haarman who introduced the electronic Wheatstone bridge that is a common feature of other modern transient methods. Haarman’s apparatus also used two wires of different length for the first time. The addition of the electronic bridge allowed the measurement time to drop tenfold, and completely eliminated the effects of convection as well as reducing time-dependent errors.
In 1976 Healy et al. published a journal article detailing the theory of the transient hot wire, described by an ideal solution with appropriate corrections to address effects like convection, among others.
The work of Pyk, Stalhane, Haarman and Healy laid the groundwork for the transient hot wire method, and over the 30 years since Healy’s paper the method has changed to some small degree. These small changes relate to improving errors of uncertainty by introducing better electronic components and creating software to run the instrument.
The modern THW apparatus is pictured in Figure 2. This machine is capable of the absolute measurement of thermal conductivity, with thermal diffusivity and specific heat also being measured. The apparatus can measure from 0.01 W/m•K up to 2 W/m•K with reproducibility and accuracy of better than 1% and 5% respectively. This instrument is covered by ASTM standard D7896-14.
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