###### Abstract  
The lack of a standard experimental procedure to determine thermal conductivity of fluids is noticeable in heat transfer processes from practical and fundamental perspectives. Since a wide variety of techniques have been used, reported literature data have huge discrepancies. A common practice is using manufactured thermal conductivity meters for nanofluids, which can standardize the measurements but are also somewhat inaccurate. In this study, a new methodology to perform reliable measurements with a recent commercial transient hot-wire device is introduced. Accordingly, some extensively studied fluids in the literature (water, ethylene glycol, ethylene glycol:water mixture 50:50 vol%, propylene glycol, and \(n\)-tetradacane) covering the range 0.100 to 0.700 W m\({}^{-1}\) K\({}^{-1}\) were used to check the device in the temperature range 283.15 to 333.15 K. Deviations between the collected data and the theoretical model, and repestabilities and deviations between reported and literature values, were analyzed. Systematic deviations in raw data were found, and a correction factor depending on the mean thermal conductivity was proposed to operate with nanofluids. Considering all tested effects, the expanded (\(k=2\)) uncertainty of the device was set as 5%. This proposed methodology was also checked with \(n\)-hexadecane and magnesium-oxide-based \(n\)-tetradacane nanofluids. (c) 2  
of methods to determine properties of simple samples and under moderate conditions [1,2]. Later, several techniques appeared that were methodologically innovative and contained a meticulous mathematical description of the experimental method to reach accurate values. Precise measurements allow one to suitably check fluid theories, including liquid and gas phases. However, the current scenario shows that the lack of standardized experimental procedures, techniques, and the preparation or well-recognized definition of complex samples itself largely lead to discrepancies. This issue hinders efforts to describe the real physical situation of such complex samples such as humid air, ionic melts (molten salts and ionic liquids), thermal energy storage materials, or nanofluids [3,4]. Likewise, it is well-established that among the different techniques, the transient hot-wire provides the most reliable absolute technique for the measurement of the thermal conductivity of gases, liquids, solids, melts, and nanofluids [2,5,6]. Nanofluids are an innovative kind of material based on well-dispersed nanoparticles within a bulk base fluid. Therefore, they constitute a two-phase thermodynamic system with a continuous phase (base fluid) and a dispersed phase (nanoparticles). Although the concept of thermal conductivity of a single-phase fluid cannot be extended to these complex thermodynamic systems, researchers are used to treat the effective thermal conductivity of the nanofluid as its own thermal conductivity (in a single-phase sense) [7]. It must be clear that we shall talk about 'thermal conductivity' of the nanofluid instead of referring to apparent thermal conductivity, for simplicity. Therefore, and according to [PERSON] et al. [7] and [PERSON] et al. [8], since the apparent thermal conductivity is not a true thermophysical property dependent only on thermodynamic state variables, special care should be taken to perform reliable experimental measurements of such a property. Concerning studies devoted to nanofluids, researchers use transient hot-wire-based commercial devices to obtain thermal conductivity data [9], which can lead to a misuse of this equipment when the methods are not adequate for the object systems [3]. Therefore, it is necessary to establish some methodologies to avoid ill-determined data when using this kind of devices. Thereby, the determination of the effective thermal conductivity of nanofluids needs to be standardized, allowing for better comparison of data from different laboratories.