## 1 Introduction  
Similar to water permeability, gas permeability in unsaturated soil, which characterizes the capacity of gas transport, is an important hydraulic property. The gas permeability of soils is concerned in various fields, e.g., landfill gas emission and oxidation for waste landfill cover systems [1; 2; 3; 4; 5; 6], soil aeration for plant growth [7; 8], soil moisture distribution for land surface [9; 10], three-phase (water, gas, and oil) permeability in the oil industry [11; 12], and the sealing efficiency of CO\({}_{2}\)[13; 14; 15]. Among these fields, the increase in gas pressure above atmospheric pressure is typically below 10 kPa and falls within the measurement range of the gas supply equipment (20 kPa). In the field of landfill, the gas pressure generated by anaerobic decomposition of organic waste under the cover system is usually less than 10 kPa [16]. Likewise, in the agricultural field, the applied gas pressure for measuring soil permeability is recommended to be lower than 8 kPa (that corresponds to a single water column of 0.80 m high) [7]. The gas pressure driven by the water vapor pressure gradient and dry air density gradient during the migration of moisture in the soil is usually about several kPa [9]. The pressure used to study gas permeability in these fields is only 0.1% to 0.05% of that required for CO\({}_{2}\) sealing and oil extraction. For example, in the field of CO\({}_{2}\) sealing, the gas breakthrough pressure of mudstone in the gas reservoir sealing area can reach several MPa [17]. The gas injection pressure in low-permeability reservoirs in the oil extraction has also reached 10-20 MPa [18]. Therefore, it is necessary to develop a special low-pressure supply equipment suitable for triaxial permeameters. Soil gas permeability and water permeability are the characterizations of the intrinsic permeability of different fluid media under given environmental conditions. Therefore, the study of gas permeability can draw lessons from the study of water permeability. The water permeability is mainly affected by two aspects, namely the change of water viscosity and adsorbed water content [19]. Both of them are greatly affected by temperature, which directly affects the viscosity of water and leads to the transformation of adsorbed water to free water [20]. However, unlike water, the compressibility of gas cannot be ignored. Therefore, when studying gas permeability, it is necessary to fix the gas pressure to accurately study the impact of temperature. In the fields with low gas pressure mentioned above, its temperature will vary within a certain range. Due to the heat generated by degradation in the landfill, the temperature inside the landfill varies dynamically over time and can reach 65 \"C [21; 22]. In agriculture, the temperature of soil and air changes with the change of temperature day and night [23]. Currently, few published studies consider the impact of temperature on the permeability of unsaturated soil. Gas permeameters can be categorized into two types based on the type of gas pressure applied: transient and steady states. The transient test method uses a gas pressure that is not constant, meaning that the density of the gas is in a state of fluctuation, which is relatively inaccurate in determining the gas flow [24]. The gas pressure applied to the soil sample used in the steady-state test method is fixed, so the steady-state test method is more favorable. Table 1 summarizes the technology parameters of air steady-state devices for determining soil air permeability, including solid-wall permeameters, flexible wall permeaters, and triaxial permeability instruments. The homemade devices have a simple structure, but a lack of ability to apply confining and principal pressures, a wide range of gas pressure fluctuations (\(\pm\)15%), and low testing accuracy for the edge gas leakage [25; 26; 27].