Temperature is probably the most frequently measured and controlled physical process parameter. This is because material properties and efficiency of processes are usually strongly temperature dependent. Temperature monitoring is also critical to ensure safety, as an unexpected and uncontrolled temperature excursion can often lead to hazardous situations. Hydrogen production and applications require temperature measurements over a wide range, from cryogenic temperatures, which are necessary for the storage of liquid hydrogen at −250 ◦C, to temperatures as high as ca. 1000 ◦C (high-temperature fuel cells) or even up to 2000 ◦C (chemical separation from water).