At liquid-helium temperatures, the adsorption capacity of porous materials is typically measured using a volumetric method.Because the adsorption-measurement device consists of a low-temperature section and an ambient-temperature section, a temperature gradient is distributed along the gas pipeline connecting the two sections. This distribution is difficult to determine experimentally; therefore, an approximation is required to estimate the amount of gas in the non-isothermal section and subsequently calculate the adsorption capacity. This study reviews four existing estimation methods for the non-isothermal section and proposes a new temperature distribution based on one-dimensional heat conduction that consider variable thermal conductivity. Using this temperature distribution as a reference, the effects of the four estimation methods on the adsorption capacity are analyzed under different experimental conditions.The results show that the errors introduced by the overall ambient-temperature and linear-distribution methods are smaller than those introduced by the overall low-temperature and segmented-treatment methods when estimating the amount of gas contained in the non-isothermal section. However, these two methods may also result in measurement errors of more than 10% under the experimental conditions of a higher measurement temperature, lower ambient temperature, larger non-isothermal-section volume, smaller adsorbent mass loading, and weaker adsorption capacity per unit mass.