14 Temperature 14.5 www.hannainst.com | introduction An ideal surface for IR measurements is a black body or radiator with an emissivity of 1.0. Emissivity is the ratio of the energy radiated by an object at a certain temperature to that emitted by a perfect radiator at the same temperature. The shinier or more polished the surface, the less accurate the measurements. For example, the emissivity of most organic material and rough or painted surfaces is in the 0.95 region and hence, suitable for IR measurements. On the other hand, surfaces of highly polished or shiny material, such as mirrors or aluminum, may not be appropriate for this application without using some form of filtration. This is due to other factors, namely, reflectivity and transmissivity. The former is a measure of an object’s ability to reflect infrared energy while the latter is its ability to transmit it. Another important and practical concern with IR measurements is the field of view. Infrared meters measure the average temperature of all objects in their field of view. To obtain an accurate result, it is important that the object completely fills the instrument’s field of view and there are no obstacles between the meter and the object. The distance-to-target ratio, or the optic coefficient, is therefore an important consideration. Reference Temperatures In 1990, NIST established 17 fixed points of the International Temperature Scale (ITS-90) related to reproducible physical phenomena in nature. The ITS-90 Fixed Points are shown in the chart below: * Given for e-H2 , which is hydrogen at the equilibrium concentration of the ort and para molecular forms. Equilibrium state K °C Vapor pressure point of helium 3 to 5 -270.15 to -268.19 Triple point of hydrogen 13.8033* -259.346* Boiling point of hydrogen at a pressure of 33.330.6 Pa 17.042* -256.108* Boiling point of equilibrium hydrogen 20.28* -252.87* Triple point of neon 27.102 -246.048 Triple point of oxygen 54.361 -218.789 Triple point of argon 83.8058 -189.3442 Triple point of mercury 234.3156 -38.8344 Triple point of water 273.16 0.01 Triple point of gallium 302.9146 29.7646 Melting point of indium 429.7485 156.5985 Melting point of tin 505.078 231.928 Melting point of zinc 692.677 419.527 Melting point of aluminum 933.473 660.323 Melting point of silver 1234.93 961.78 Melting point of gold 1337.33 1064.18 Melting point of copper 1357.77 1084.62 Thermometers Introduction
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