## 1 Introduction
Although correction methods for temperature compensation of a hot-wire anemometer can be established for the general case where both the air temperature and the wire temperature are varying, and the methods can predict the output from the anemometer in different air temperatures from calibration data taken only at one air temperature [17], one method to overcome the temperature contamination is to calibrate the probes separately at all the flow temperatures under investigation [16]. Another property of the fluid that can change between the calibration rig and the application site is the humidity of the air. Hence, to apply hot-wire anemometry in humid air also requires corrections to the velocity measurement if the wires were calibrated under different humidity conditions. Compared to the number of studies dealing with the effect of a varying fluid temperature on hot-wire measurements, there are few investigations available on humidity corrections, experimental or theoretical. Indeed, data on the thermodynamic and fluid transport properties of a mixture of dry air and water vapor are scarce [18]. When applying hot-wire anemometry to velocity measurements in air, it is standard practice to neglect the effect of humidity. Ref. [18] defined a corrective term to expand formulae designed for dry air to work in a humid environment, and estimated the error in velocity by omitting the influence of humidity in terms of temperature and relative humidity, revealing that the influence of the thermodynamic and transport properties of humid air on hot-wire measurements exists. Then, one solution might be to calibrate the hot-wire anemometers in different given humidity values. Thus, to avoid temperature contamination and humidity impact on hot-wire calibration accuracy, a calibration facility should be able to operate in different specified fluid temperatures and humidity values and be able to satisfy the specification of velocity control accuracy. In other words, a calibration rig should enable velocity, temperature, and humidity to be controlled to the required values with satisfactory accuracy and maintain them at constant values for a long enough period of time. In the past decades, various facilities including wind tunnels, nozzles, laminar pipe flow rigs, and probe-moving-type facilities for hot-wire calibration in extremely low speed regime have been built in the world. Wind tunnels, including nozzles (i.e., jet-type tunnels) and laminar pipe flow rig for hot-wire calibration, can be seen in the National Institute of Standards and Technology (NIST) in the USA [1,19,20], the Centre Technique des Industries Aerauliques et Thermiques (CETIAT) in France [2,21], the Physikalisch Technische Bundesansalt (PTB) in Germany [2], the Instituto Nacional de Tecnica Aeroespacial \"Esteban Terradas\" (INTA) in Spain [2], the Universite Catholique de Louvain (UCL) in Belgium [2], the Nottingham Trent University in the United Kingdom (the air can be heated) [16], the Universidad Nacional de La Plata in Argentina [12], the University of Gavle in Sweden [17], the Iranian Research Organization for Science and Technology (IROST) in Iran (the air can be heated, velocity range is 5 m/s to 30 m/s) [10], the Institute of Atmospheric Physics, Chinese Academy of Sciences [22], the Institute of Metrology, Chinese Academy of Meteorological Sciences (CAMS) [23], the National Institute of Metrology, China(NIMC) [24,25], and the University of Gaziantep in Turkey (laminar pipe flow rig) [6]. The air velocity in these facilities ranges from 0.029 m/s to 75 m/s with a velocity control accuracy of 0.02 m/s (0.5% to 7.4%), a velocity uncertainty of 0.014 m/s to 0.06 m/s (1% to 10%), and a calibration fitting error of 0.018 m/s to 0.03377 m/s (4%) (see Table 1).