(MKSK system), a system of units for thermal quantities in which the fundamental units are the meter, the kilogram (the unit of mass), the second, and the kelvin (the unit of thermodynamic temperature).
The use of the MKSK system of units was established in the USSR by GOST (All-Union State Standard) 8550–61, “Thermal Units” (this standard still uses the former name, “degree Kelvin,” for the unit of thermodynamic temperature, which was changed to “kelvin” in 1967 at the Thirteenth General Conference on Weights and Measures). Two temperature scales are used in the MKSK system of units: the thermodynamic temperature scale and the International Practical Temperature Scale (ITPS-68). In addition to the kelvin, the degree Celsius, designated by °C and equal to the kelvin (K), is used to express the thermodynamic temperature and temperature difference. As a rule, the Kelvin temperature Tis used below 0°C, whereas above 0°C the Celsius temperature t is used (t = T — TO, where T0 = 273.15 K). In ITPS-68 a distinction is also made between the international practical Kelvin temperature (symbol T68) and the international practical Celsius temperature (T68); they are related by the formula t68 = T68 — 273.15 K. The units for T68 and t68 are the kelvin and the degree Celsius, respectively. Both the kelvin and the degree Celsius may appear in the names of derived thermal units. The MKSK system of units is an integral part of the International System of Units.