• Measurement of the thermal conductivity of liquids over a range of 0.1 to 0.7 Wm^-1K^-1
• Transient Hot-Wire technique (2-Ta wires).
• Reliable results with low absolute uncertainty of 1%.
• Portable easy to use instrument.
• Full theoretical model. No approximations!
• Absolute technique, i.e. no calibration or reference sample is required.
• Suitable for users in industry and academia, research and development institutions.
• Ideal for measuring the thermal conductivity of nanofluids.

The technique

The Transient Hot-Wire technique has successfully been applied for the measurement of the thermal conductivity of fluids since 1980. According to the transient hot-wire technique, the thermal conductivity of the sample is determined by observing the temporal temperature rise of a thin wire, when a step voltage is applied to it. In this way, electrical current flows through the wire and heats it up, thus creating in the liquid a line source of essentially uniform heat flux per unit length that depends on the thermal conductivity of the liquid.Finally, to avoid end effects, two wires identical except for their length, are employed. Thus, if arrangements are made to measure the difference of the resistance of the two wires as a function of time, the measurement corresponds to the resistance change of a finite section of an infinite wire (as the end effects being very similar, are subtracted), from which the temperature rise can be determined. This way absolute measurements can be performed (i.e. no calibration or reference sample is required).

An automatic electronic bridge records 500 resistance rise points in time from 0.01 to 1 s. These are converted to temperature rise vs time. Once small correction have been applied (because if the heat capacity of the wire, the variable fluid properties, and the outer boundary of the cell), the thermal conductivity is obtained from the slope of the temperature rise vs time.

Instrument

The instrument is composed of :

1. A sensor with two-Ta wires of 25 μm diameter and a stainless-steel vessel suitable for measurements up to 200 ‘C (Sensor wires can be made according to request, e.g. for polar liquids, gases, or to fit a specific pressure vessel).
2. An automatic electronic bridge that employs an ARM architecture CPU to control output voltage and data processing. It has the ability to begin measurements from 2 ms after the initiation of heating and to obtain a large number of data points (usually 500 in one run).
3. A laptop that controls the experiment setup. It includes a) the AccuMeasurement custom designed software that performs the measurements, and b) the AccuAnalysis custom designed software for analyzing the results and obtaining the thermal conductivity.

Further reading

The instrument was developed through  a Greek-Chinese collaboration. Further information can be obtained in literature:

1. Assael M.J., Antoniadis K.D., Metaxa I.N., Mylona S.K., Assael J.-A.M, Wu J., and Hu M., “
   A Novel Portable Absolute Transient Hot-Wire Instrument for the Measurement of the Thermal Conductivity of Solids”. Int. J. Thermophys. Online first (2015).
2. Assael M.J., Antoniadis K.D., Kakosimos K.E., “An Improved Application of the Transient Hot-Wire Technique for the Absolute Accurate Measurement of the Thermal Conductivity of Pyroceram up to 420 K”, Int. J. Thermophys. 29:445-456 (2008).
3. Assael M.J., Antoniadis K., and Wu J., “New Measurements of the Thermal Conductivity of PMMA, BK7 and Pyrex 7740 up to 450 K”, Int. J. Thermophys. 29:1257-1266 (2008).