A second problem with the data reported since 1994 is wild variation from year to year in the estimates of the percent of helium consumed in each category. This aggregation of the data made it difficult for the committee to follow helium use over time within each category. One problem that the committee confronted in analyzing the trends in helium usage during the 1980s is that the Department of the Interior reduced the number of categories of helium use it reported from 13 to 7-even though it currently collects data on 18 categories. Helium use is estimated using the results of these surveys as a baseline. The last survey of domestic helium wholesalers was in 1995 and the one before that was in 1991. Even less is known about foreign production of helium. Although the gross amount of helium exported is known, there is only sparse information on its end uses. These surveys track domestic use in 18 categories. The domestic helium consumption portion of these reports is based on surveys, conducted every 5 years or so, of helium wholesalers. Department of the Interior releases annual reports that discuss the supply and demand for helium. Helium is radiologically inert (i.e., it does not easily participate in nuclear processes and does not become radioactive). The specific heat and thermal conductivity of helium gas are very high. Liquid 4He undergoes a transition to a superfluid phase at temperatures below 2.18 K (-455.5 ☏) and has extraordinary physical properties, including viscosity-free fluid flow and extraordinarily high thermal conductivity (on the order of a million times greater than its conductivity in the normal phase and greater than that of the best metallic conductors). In its solid form, helium is extremely compressible, permitting volume changes of more than 30 percent. Helium remains liquid at atmospheric pressure down to absolute zero and can be solidified only by applying 25 atm. The boiling point of helium is closer to absolute zero than that of any other element, so liquid helium can provide the lowest operating temperatures of any refrigerant. It has the highest ionization potential (24.587 eV). It is chemically inert, having essentially no tendency to combine with other elements.
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