Radiation Effects in Precision Resistance Thermometry: I. Radiation Losses in Transparent Thermometer Sheaths

Abstract

Standard platinum resistance thermometers that are constructed with transparent sheaths of fused silica or pyrex glass are subject to thermal losses arising from losses of visible and near-infrared radiation up (piping) and through these sheaths. This heat loss may introduce substantial errors in temperature determinations; e.g. 0.084, 0.015, and 0.001 1 °C at the Sb, Zn, and Sn fixed-temperature points for a fused-silica sheath, or 0.000 2 °C at the Zn and Sn points for a pyrex sheath. This effect has been investigated in detail for thermometers of various types at the Sb, Zn, and Sn points, and it has been shown that simple stem radiation-traps, consisting of a blackening or roughening of the smooth outer surface of the thermometer sheaths, will eliminate these errors and greatly improve the immersion characteristics of the thermometers.Suitable radiation-trapping should be incorporated on the stems of every transparent-sheathed standard platinum resistance thermometer; without strong evidence to the contrary, radiation loss errors of the orders of magnitude cited above should be suspected in reported measurements involving resistance thermometers having unprotected transparent fused-silica sheaths.

A numerical and experimental investigation of the heat losses in thermometric fixed-point cells

“…Radiative heat losses at higher temperatures were first observed by McLaren and Murdock in 1966 while performing measurements at the freezing point of antimony (630.553°C) [3]. Using a standard resistance thermometer with a transparent, fused-silica sheath they measured a temperature that was at least 80 mK lower than the equilibrium blackbody temperature.…”

Numerical Modeling of Thermal Effects in Fixed-Point Cells

“…As a compromise, the fused-silica glass is considered to be semitransparent in the range below 4 µm, while in the range above 4 µm, it is considered to be opaque. Other types of glasses may have a narrower semi-transparent range as seen in transmittance graphs in [3], so this may be considered as the worst case in uncertainty estimation.…”

“…This effect streams radiation in a direction parallel to the boundary and the glass tube acts as an optical fiber. The light-piping effect is well known and has been experimentally investigated by many authors [3][4][5]8,9]. The worst-case temperature errors that were experimentally determined were in the range from several mK to several tens of mK, which represent a significant uncertainty component.…”

Standard Reference Materials

“…The outer tube was sawed apart at the top. The thermometer well was then sand-blasted on the outer surface starting from about 3 cm from the tip to nearly the top of the well to obtain a matte finish to reduce light (heat) piping [27] from the aluminum-point cell.…”

“…However, the apparent depth of immersion in the thermometric cell can be increased by suitably tempering the thermometer stem above the cell to a temperature close to that of the cell [13,27,31].…”