Leiden scale
The Leiden scale was used to calibrate low-temperature indirect measurements in the early twentieth century, by providing conventional values of helium vapour pressure. It was used below -183°C, the starting point of the International Temperature scale in the 1930s.History
The Leiden scale probably goes back to around 1894, when Heike Kamerlingh Onnes' cryogenic laboratory was established in Leiden, Netherlands.
It has been reported that the scale is the kelvin scale shifted so that the boiling points of hydrogen and oxygen become zero and 70 respectively, but this is unlikely to be true. Oxygen under a standard atmosphere boils at a temperature in the 90.15 to 90.18 K range. For hydrogen, it depends on the molecular variety. The boiling point is 20.390 K for "normal" hydrogen and 20.268 K for pure parahydrogen. Under the purported definition, absolute zero would lie at -20.15 °L.
