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1-582: The Provisional Low Temperature Scale of 2000 ( PLTS-2000 ) is an equipment calibration standard for making measurements of very low temperatures, in the range of 0.9 mK ( millikelvin ) to 1 K, adopted by the International Committee for Weights and Measures in October 2000. It is based on the melting pressure of solidified helium-3 . At these low temperatures, the melting pressure of helium-3 varies from about 2.9 MPa to nearly 4.0 MPa. At