Transient hot wire method
The transient hot wire method (THW) is a very popular, accurate and precise technique to measure the thermal conductivity of gases, liquids, solids, nanofluids and refrigerants in a wide temperature and pressure range. The technique is based on recording the transient temperature rise of a thin vertical metal wire with infinite length when a step voltage is applied to it. The wire is immersed in a fluid and can act both as an electrical heating element and a resistance thermometer. The transient hot wire method has advantage over the other thermal conductivity methods, since there is  a fully developed theory and there is no calibration or single-point calibration. Furthermore, because of the very small measuring time (1 s) there is no convection present in the measurements and only the thermal conductivity of the fluid is measured with very high accuracy. The most of the transient hot wire sensors used in academia consist of two identical very thin wires with only difference in the length. Sensors using a single wire are used both in academia and industry with the advantage over the two-wire sensors in the ease of handling of the sensor and change of  the wire. An ASTM standard is published for the measurements of engine coolants using a single-transient hot wire method. History. 200 years ago scientists were using a crude version of this method to make the first ever thermal conductivity measurements on gases.