are based on triple points. Examples of other defining points are the triple point of equilibrium hydrogen ( or ) and the freezing point of aluminium ( or ). The defining fixed points of the ITS-90 refer to pure chemical samples with specific isotopic compositions. As a consequence of this, the ITS-90 contains several equations to correct for temperature variations due to impurities and isotopic composition. Thermometers calibrated via the ITS-90 use complex mathematical formulas to interpolate between its defined points. The ITS-90 specifies rigorous control over variables to ensure reproducibility from lab to lab. For instance, the small effect that atmospheric pressure has upon the various melting points is compensated for (an effect that typically amounts to no more than half a millikelvin across the different altitudes and barometric pressures likely to be encountered). The standard also compensates for the pressure effect due to how deeply the temperature probe is immersed into the sample. The ITS-90 also draws a distinction between “freezing” and “melting” points. The distinction depends on whether heat is going into (melting) or out of (freezing) the sample when the measurement is made. Only gallium is measured at its melting points; all other metals with defining fixed points on the ITS-90 are measured at their freezing points. A practical effect of the ITS-90 is that the triple points and the freezing/melting points of its thirteen chemical elements are precisely known for all temperature measurements calibrated per the ITS-90 since these thirteen values are fixed by definition. Limitations There are often small differences between measurements calibrated per ITS-90 and thermodynamic temperature. For instance, precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of thermodynamic temperature. When calibrated to ITS-90, where one must interpolate between the defining points of gallium and indium, the boiling point of VSMOW water is about 10 mK less, about 99.974 °C. The virtue of ITS-90 is that another lab in another part of the world will measure the very same temperature with ease due to the advantages of a comprehensive international calibration standard featuring many conveniently spaced, reproducible, defining points spanning a wide range of temperatures. Although “International Temperature Scale of 1990” has the word “scale” in its title, this is a misnomer that can be misleading. The ITS-90 is not a scale;